atom.xml

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  <title><![CDATA[Jared的Android之路]]></title>
  <link href="http://jaredlam.github.io/atom.xml" rel="self"/>
  <link href="http://jaredlam.github.io/"/>
  <updated>2015-12-27T19:18:25+08:00</updated>
  <id>http://jaredlam.github.io/</id>
  <author>
    <name><![CDATA[Jared Luo]]></name>
    
  </author>
  <generator uri="http://octopress.org/">Octopress</generator>

  
  <entry>
    <title type="html"><![CDATA[Android Fragment Transactions &amp; Activity State Loss(译)]]></title>
    <link href="http://jaredlam.github.io/blog/2015/12/23/android-fragment-transactions-and-activity-state-loss-yi/"/>
    <updated>2015-12-23T14:56:15+08:00</updated>
    <id>http://jaredlam.github.io/blog/2015/12/23/android-fragment-transactions-and-activity-state-loss-yi</id>
    <content type="html"><![CDATA[<p>本文翻译自<a href="http://www.androiddesignpatterns.com/2013/08/fragment-transaction-commit-state-loss.html">Fragment Transactions &amp; Activity State Loss</a>，原作者为<a href="https://plus.google.com/+AlexLockwood">Alex Lockwood</a>, 译者为<a href="https://github.com/jaredlam">Jared Luo</a></p>

<p>自从Honeycomb版本发布以后，下面这一块异常消息就成了StackOverflow上问题的常客：</p>

<pre><code>java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState
at android.support.v4.app.FragmentManagerImpl.checkStateLoss(FragmentManager.java:1341)
at android.support.v4.app.FragmentManagerImpl.enqueueAction(FragmentManager.java:1352)
at android.support.v4.app.BackStackRecord.commitInternal(BackStackRecord.java:595)
at android.support.v4.app.BackStackRecord.commit(BackStackRecord.java:574)
</code></pre>

<p>这篇文章就来给大家解释一下为什么这个异常会被抛出，并且会总结出一些建议，让您的APP再也不受其导致崩溃的困扰。</p>

<h3>异常产生的原因？</h3>

<p>这个异常会被抛出的原因是：<strong>在Activity的状态被保存之后，你才尝试去commit一个FragmentTransaction，这样就会导致Activity的状态丢失</strong>。在深入了解这句话真正的意思之前，让我们先来看看当onSaveInstanceState()被调用时，其内部发生了什么。正如我上一篇文章<a href="http://www.androiddesignpatterns.com/2013/08/binders-death-recipients.html">Binders &amp; Death Recipients</a>所说，Android应用程序在Android运行时当中对其自身的命运，几乎没有什么话语权。Android系统有权利随时杀掉任何一个进程来解放内存空间，这样就导致后台运行的Activity们随时都有可能，在没有一点警告的情况下被杀掉。为了保证这种不可预测的动作不被用户所感受到，系统框架给了Activity一个机会，在可能被杀掉之前，通过调用onSaveInstanceState()方法来保存状态。这样当保存的状态被恢复的时候，无论Activity是否被杀掉，用户都会感觉到好像前后台的Activity进行的是无缝的切换。</p>

<p>当框架调用onSaveInstanceState()的时候，它会向这个方法传递一个Bundle参数，Activity可以用这个参数来保存页面、对话框、Fragments和视图的状态。当onSaveInstanceState返回时，会将一个Bundle对象序列化之后通过Binder接口传递给系统服务进程,并安全的保存起来。当系统晚一点想要重启Activity的时候， 它会把之前的Bundle对象传递回应用，并用来恢复Activity之前的状态。</p>

<p>所以为什么会抛出之前的异常呢？问题的根源在于Bundle这个对象仅仅是Activity在onSaveInstanceState()方法被调用那一刻的快照。这就意味着当你如果在onSaveInstanceState()之后再调用FragmentTransaction.commit()的话，由于这次Transaction没有被作为Activity状态的一部分来保存，自然也就丢失掉了。从用户的角度来说，Transaction的遗失就导致意外的UI状态丢失。那么为了维护良好的用户体验，Android系统会不惜一切代价的避免页面状态的丢失，所以在这种情况发生的时候就直接抛出了IllegalStateException。</p>

<h3>异常产生的时机？</h3>

<p>如果你曾经遇见过这个异常，你可能会注意到在各个版本的Android平台上，这个异常抛出的时机略有不同。举个栗子，你可能会发现相对老的设备上这个异常可能会抛出的没有那么频繁，另外如果你的应用使用了support library的话，可能会比使用官方框架更容易崩溃一点。这个不同的现象可能会让我们假设support library是不是有bug，不能让我们信任呢？然而，并不是这样。</p>

<p>这种各个版本之间的现象不一致源于，在Honeycomb这个Android版本中，Activity的生命周期做了相当的调整。在Honeycomb之前的版本中，Activity在pause以前都是不能被杀掉的，所以onSaveInstanceState() 就会在onPause()之前被立刻调用。但是从Honeycomb开始，Activity变成了在stop之前不能被杀掉。所以自然onSaveInstanceState()就会变成在onStop()之前被立刻调用。下面的表格总结了这个区别：</p>

<p>上面的针对Activity生命周期的改变就导致了，有时候support library需要根据系统版本的不同, 来调整自己的行为。举个栗子，在Honeycomb和其以上版本的设备上，只要在onSaveInstanceState()以后调用commit(), 就会抛出异常来提示开发者页面状态丢失了。但是在Honeycomb之前的版本，因为onSaveInstanceState()被调用的时机提前了很多，导致状态的丢失更容易出现，所以每次都抛出异常就显得有点太严格了。Android开发团队不得不被迫做了一个妥协：为了更好的配合老版本的系统，Honeycomb之前的版本就必须忍受onPause()和onStop()之间可能会发生的状态丢失（而不会抛出异常，译者注）。
如下表格总结了Support library在两类版本的表现：</p>

<h3>怎样避免异常出现？</h3>

<p>一旦您了解了真正的原理，避免Activity状态丢失就会变得容易许多。如果您已经读到了这里，我希望您对support library对这类问题的处理方式，以及避免页面状态丢失的重要性，有了更深入的了解。如果您是来查找怎样能够快速修复这个问题，下面有一些关于使用FragmentTransactions的建议：</p>

<ul>
<li><strong>谨慎的在Activity的生命周期方法中调用transaction的commit方法</strong>。大多数应用只会在第一次调用onCreate()或者响应用户输入的时候去commit transaction， 这样不会有什么问题。但是，
当您把transaction放到其他Activity的生命周期方法中时，比如onActivityResult()， onStart() 和onResume()，事情就会变得有点复杂。举个栗子，您不应该在FragmentActivity#onResume()中去commit transaction，因为在某些情况下，这个方法可能会在Activity状态恢复之前被调用(<a href="http://developer.android.com/reference/android/support/v4/app/FragmentActivity.html#onResume(">看这里</a>))。如果您的应用需要在onCreate()之外的其他生命周期方法中去commit transaction，那就请在 FragmentActivity#onResumeFragments()或者Activity#onPostResume()中去commit。这两个方法保证会在Activity恢复状态之后才会被调用，所以就能完全避免状态丢失的可能性。（作为例子，可以查看我在StackOverflow上，关于怎样在Activity#onActivityResult()中commit FragmentTransactions 的回答）。</li>
<li><p>避免在异步回调中去处理transaction。包括使用AsyncTask#onPostExecute()和 LoaderManager.LoaderCallbacks#onLoadFinished()方法等。这样做的问题在于，您不会知道这些异步方法在调用的时候，当前的Activity究竟处于生命周期的哪一个状态。比如说，下面这一系列事件：</p></li>
<li><p>Activity执行一个AsyncTask。</p></li>
<li>当用户按下Home键，导致Activity的onSaveInstanceState()和onStop()执行。</li>
<li>AsyncTask执行完成并且onPostExecute()被调用，其不知道Activity这个时候已经被stopped。</li>
<li>FragmentTransaction在onPostExecute()中被调用，导致异常被抛出。</li>
</ul>


<p>总体来说，在这种情况下，最好避免异常的方法是不要在异步回调中去commit transaction。Google的工程师们看起来也同意这个观点。根据Android开发者团队邮件组的这篇<a href="https://groups.google.com/forum/#!msg/android-developers/dXZZjhRjkMk/QybqCW5ukDwJ">文章</a>，Android团队认为在异步方法中去调用commit transaction而产生的UI切换，会导致不好的用户体验。 如果您的应用确实需要在异步方法中处理transaction，并且没有简单的方法可以保证回调在onSaveInstanceState()之前被调用的话，你可能只有使用commitAllowingStateLoss()，并且自己处理可能发生的状态丢失了(请参考StackOverflow，<a href="http://stackoverflow.com/questions/8040280/how-to-handle-handler-messages-when-activity-fragment-is-paused">这里</a>，<a href="http://stackoverflow.com/questions/7992496/how-to-handle-asynctask-onpostexecute-when-paused-to-avoid-illegalstateexception">还有这里</a>)。</p>

<ul>
<li>只在不得已的情况下，才使用commitAllowingStateLoss()。commit()和commitAllowingStateLoss()唯一的不同是，后者当状态丢失时，不会抛出异常。通常由于存在状态丢失的可能性，您不会希望使用这个方法。更好的解决方法是，在您的应用中使用commit()，并保证在Activity的状态保存之前调用它，来获取更好的用户体验。除非状态丢失的可能性不能被避免，否则您不应该使用commitAllowingStateLoss()。</li>
</ul>


<p>希望这些提示能够帮您解决这个异常带来的相关问题。如果您仍然有没有解决的疑问，请到<a href="http://stackoverflow.com/">StackOverflow</a>上发帖子，并且把地址发布到下面的评论中，我可以帮您看看:)</p>
]]></content>
  </entry>
  
  <entry>
    <title type="html"><![CDATA[Android中FragmentPagerAdapter对Fragment的缓存(二)]]></title>
    <link href="http://jaredlam.github.io/blog/2015/10/10/androidzhong-fragmentpageradapterdui-fragmentde-huan-cun-er/"/>
    <updated>2015-10-10T11:57:04+08:00</updated>
    <id>http://jaredlam.github.io/blog/2015/10/10/androidzhong-fragmentpageradapterdui-fragmentde-huan-cun-er</id>
    <content type="html"><![CDATA[<blockquote><p>上一篇我们谈到了，当应用程序恢复时，由于FragmentPagerAdapter对Fragment进行了缓存的读取，导致其并未使用在Activity中新创建的Fragment实例。今天我们来看如何解决这种情况。</p></blockquote>

<p>&emsp;根据<a href="http://jaredlam.github.io/blog/2015/10/08/androidzhong-fragmentpageradapterdui-fragmentde-huan-cun/">上篇Blog</a>的描述，我们不难发现，目前需要解决的问题有以下两个：</p>

<p>&emsp;1. 缓存Fragment内部成员变量缺失的问题。</p>

<p>&emsp;2. 新Fragment的创建和缓存Fragment使用之间的矛盾。</p>

<p>&emsp;下面先来解决第一个问题，缓存Fragment内部成员变量缺失。<a href="http://jaredlam.github.io/blog/2015/10/08/androidzhong-fragmentpageradapterdui-fragmentde-huan-cun/">上篇Blog</a>中，Fragment当中，有一个成员变量mText，是通过setter的方式在创建Fragment之初设置进去的。但是在经历了一系列的存储和恢复操作过后，其值在最终却为空，导致了程序展示的异常。那么能不能让mText也在Fragment中同步缓存和恢复呢？</p>

<p>&emsp;最先能想到的方法，就是通过Fragment的onSaveInstanceState方法在进程被杀掉时存储，当恢复时通过onCreateView的savedInstanceState参数取出；代码如下：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
<span class='line-number'>12</span>
<span class='line-number'>13</span>
<span class='line-number'>14</span>
<span class='line-number'>15</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'> <span class="nd">@Override</span>
</span><span class='line'> <span class="kd">public</span> <span class="n">View</span> <span class="nf">onCreateView</span><span class="o">(</span><span class="n">LayoutInflater</span> <span class="n">inflater</span><span class="o">,</span> <span class="n">ViewGroup</span> <span class="n">container</span><span class="o">,</span> <span class="n">Bundle</span> <span class="n">savedInstanceState</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>    <span class="o">...</span>
</span><span class='line'>    <span class="k">if</span> <span class="o">(</span><span class="n">savedInstanceState</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="n">mText</span> <span class="o">=</span> <span class="n">savedInstanceState</span><span class="o">.</span><span class="na">getString</span><span class="o">(</span><span class="n">SAVED_KEY_TEXT</span><span class="o">);</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'>    <span class="o">...</span>
</span><span class='line'><span class="o">}</span>
</span><span class='line'>
</span><span class='line'>
</span><span class='line'><span class="nd">@Override</span>
</span><span class='line'><span class="kd">public</span> <span class="kt">void</span> <span class="nf">onSaveInstanceState</span><span class="o">(</span><span class="n">Bundle</span> <span class="n">outState</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>    <span class="kd">super</span><span class="o">.</span><span class="na">onSaveInstanceState</span><span class="o">(</span><span class="n">outState</span><span class="o">);</span>
</span><span class='line'>    <span class="n">outState</span><span class="o">.</span><span class="na">putString</span><span class="o">(</span><span class="n">SAVED_KEY_TEXT</span><span class="o">,</span> <span class="n">mText</span><span class="o">);</span>
</span><span class='line'><span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;这种Activity和Fragment通用的方法，无疑是应用被杀掉时我们存储数据比较好的选择。不过还有其他方式吗？</p>

<p>&emsp;目前，mText是通过setter向Fragment设置的，这样做从实现来讲没有问题，不过其实并不是Android官方文档推荐的最佳实践; 官方文档上不推荐使用setter或者重写默认构造器的方式来传递参数：</p>

<blockquote><p>It is strongly recommended that subclasses do not have other constructors with parameters, since these constructors will not be called when the fragment is re-instantiated; instead, arguments can be supplied by the caller with setArguments(Bundle) and later retrieved by the Fragment with getArguments().</p></blockquote>

<p>&emsp;原因是，当Fragment重新被恢复时，不会去重新调用这些setter/有参构造方法; 而是会调用onCreateView，我们却可以在其中重新调用getArguments去获取这些参数。这就保证了在恢复过后，我们需要传入的参数可以重新被设置。一番改造之后如下：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
<span class='line-number'>12</span>
<span class='line-number'>13</span>
<span class='line-number'>14</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'>    <span class="n">TestFragment</span> <span class="n">fragmentOne</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">TestFragment</span><span class="o">();</span>
</span><span class='line'>    <span class="n">Bundle</span> <span class="n">bundleOne</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">Bundle</span><span class="o">();</span>
</span><span class='line'>    <span class="n">bundleOne</span><span class="o">.</span><span class="na">putString</span><span class="o">(</span><span class="n">TestFragment</span><span class="o">.</span><span class="na">PARAM_KEY_TEXT</span><span class="o">,</span> <span class="s">&quot;One&quot;</span><span class="o">);</span>
</span><span class='line'>    <span class="n">fragmentOne</span><span class="o">.</span><span class="na">setArguments</span><span class="o">(</span><span class="n">bundleOne</span><span class="o">);</span>
</span><span class='line'>
</span><span class='line'>    <span class="n">TestFragment</span> <span class="n">fragmentTwo</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">TestFragment</span><span class="o">();</span>
</span><span class='line'>    <span class="n">Bundle</span> <span class="n">bundleTwo</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">Bundle</span><span class="o">();</span>
</span><span class='line'>    <span class="n">bundleTwo</span><span class="o">.</span><span class="na">putString</span><span class="o">(</span><span class="n">TestFragment</span><span class="o">.</span><span class="na">PARAM_KEY_TEXT</span><span class="o">,</span> <span class="s">&quot;Two&quot;</span><span class="o">);</span>
</span><span class='line'>    <span class="n">fragmentTwo</span><span class="o">.</span><span class="na">setArguments</span><span class="o">(</span><span class="n">bundleTwo</span><span class="o">);</span>
</span><span class='line'>
</span><span class='line'>    <span class="n">TestFragment</span> <span class="n">fragmentThree</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">TestFragment</span><span class="o">();</span>
</span><span class='line'>    <span class="n">Bundle</span> <span class="n">bundleThree</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">Bundle</span><span class="o">();</span>
</span><span class='line'>    <span class="n">bundleThree</span><span class="o">.</span><span class="na">putString</span><span class="o">(</span><span class="n">TestFragment</span><span class="o">.</span><span class="na">PARAM_KEY_TEXT</span><span class="o">,</span> <span class="s">&quot;Three&quot;</span><span class="o">);</span>
</span><span class='line'>    <span class="n">fragmentThree</span><span class="o">.</span><span class="na">setArguments</span><span class="o">(</span><span class="n">bundleThree</span><span class="o">);</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;这样传入的参数，就不需要在onSaveInstanceState里面去手动保存了。</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'><span class="kd">public</span> <span class="n">View</span> <span class="nf">onCreateView</span><span class="o">(</span><span class="n">LayoutInflater</span> <span class="n">inflater</span><span class="o">,</span> <span class="n">ViewGroup</span> <span class="n">container</span><span class="o">,</span> <span class="n">Bundle</span> <span class="n">savedInstanceState</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="n">View</span> <span class="n">view</span> <span class="o">=</span> <span class="n">inflater</span><span class="o">.</span><span class="na">inflate</span><span class="o">(</span><span class="n">R</span><span class="o">.</span><span class="na">layout</span><span class="o">.</span><span class="na">fragment_test</span><span class="o">,</span> <span class="n">container</span><span class="o">,</span> <span class="kc">false</span><span class="o">);</span>
</span><span class='line'>        <span class="n">TextView</span> <span class="n">textView</span> <span class="o">=</span> <span class="o">(</span><span class="n">TextView</span><span class="o">)</span> <span class="n">view</span><span class="o">.</span><span class="na">findViewById</span><span class="o">(</span><span class="n">R</span><span class="o">.</span><span class="na">id</span><span class="o">.</span><span class="na">center_text_view</span><span class="o">);</span>
</span><span class='line'>        <span class="n">mText</span> <span class="o">=</span> <span class="o">(</span><span class="n">getArguments</span><span class="o">()</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">?</span> <span class="n">getArguments</span><span class="o">().</span><span class="na">getString</span><span class="o">(</span><span class="n">PARAM_KEY_TEXT</span><span class="o">)</span> <span class="o">:</span> <span class="s">&quot;&quot;</span><span class="o">;</span>
</span><span class='line'>        <span class="n">textView</span><span class="o">.</span><span class="na">setText</span><span class="o">(</span><span class="n">mText</span><span class="o">);</span>
</span><span class='line'>        <span class="k">return</span> <span class="n">view</span><span class="o">;</span>
</span><span class='line'>    <span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;第一个问题到这里就处理好了，接下来看看第二个问题：怎样解决onCreate中新实例化的Fragment，与Adapter中FragmentManager中取出的Fragment不一致的冲突。</p>

<p>&emsp;虽然mText找回来了，但是如果我们需要对Activity中实例化的Fragment做一些进一步的操作，比如传入一些Listener之类的事情，就会遇到一些麻烦，因为毕竟我们处理的这些Fragment，实际上并不是当前展示在屏幕上的Fragment。</p>

<p>&emsp;<a href="http://jaredlam.github.io/blog/2015/10/08/androidzhong-fragmentpageradapterdui-fragmentde-huan-cun/">上篇Blog</a>中讲到，FragmentPagerAdapter使用container.getId()与getItemId拼接的字符串作为FragmentManager中缓存的Key，FragmentPagerAdapter代码如下：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'>  <span class="n">String</span> <span class="n">name</span> <span class="o">=</span> <span class="n">makeFragmentName</span><span class="o">(</span><span class="n">container</span><span class="o">.</span><span class="na">getId</span><span class="o">(),</span> <span class="n">itemId</span><span class="o">);</span>
</span><span class='line'>  <span class="n">Fragment</span> <span class="n">fragment</span> <span class="o">=</span> <span class="n">mFragmentManager</span><span class="o">.</span><span class="na">findFragmentByTag</span><span class="o">(</span><span class="n">name</span><span class="o">);</span>
</span><span class='line'>
</span><span class='line'>  <span class="o">...</span>
</span><span class='line'>
</span><span class='line'>  <span class="kd">private</span> <span class="kd">static</span> <span class="n">String</span> <span class="nf">makeFragmentName</span><span class="o">(</span><span class="kt">int</span> <span class="n">viewId</span><span class="o">,</span> <span class="kt">long</span> <span class="n">id</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="k">return</span> <span class="s">&quot;android:switcher:&quot;</span> <span class="o">+</span> <span class="n">viewId</span> <span class="o">+</span> <span class="s">&quot;:&quot;</span> <span class="o">+</span> <span class="n">id</span><span class="o">;</span>
</span><span class='line'>    <span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;从上面的代码来看，其实要避免缓存和新创建的Fragment不一致，最简单的方式是，通过重写getItemId()方法，让每次打开应用返回不同的值(比如随机数之内的)，让FragmentPagerAdapter找不到之前的缓存，就会使用我们新传入的实例了。</p>

<p>&emsp;不过这样做，看起来既不优雅，也不靠谱。毕竟Android官方给我们提供了这样一种缓存机制，那我们还是应该考虑怎样利用才好。</p>

<p>&emsp;1. 既然有缓存，那我们不必在Activity中每次都去新创建Fragment实例了。从源码中可以看出，每次如果FragmentPagerAdapter需要新实例化Fragment的话，都回去调用getItem方法，所以，可以考虑把Fragment的实例化工作放到getItem当中去。</p>

<p>&emsp;2. 考虑到后面我们会使用到这些Fragment实例，可以考虑在instantiateItem当中去获取并存放在数组当中。这里选择到instantiateItem，而不是getItem方法中去取的原因是：如果一旦出现有缓存的情况，FragmentPagerAdapter并不会调用getItem方法，如下：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'>    <span class="n">String</span> <span class="n">name</span> <span class="o">=</span> <span class="n">makeFragmentName</span><span class="o">(</span><span class="n">container</span><span class="o">.</span><span class="na">getId</span><span class="o">(),</span> <span class="n">itemId</span><span class="o">);</span>
</span><span class='line'>    <span class="n">Fragment</span> <span class="n">fragment</span> <span class="o">=</span> <span class="n">mFragmentManager</span><span class="o">.</span><span class="na">findFragmentByTag</span><span class="o">(</span><span class="n">name</span><span class="o">);</span>
</span><span class='line'>    <span class="k">if</span> <span class="o">(</span><span class="n">fragment</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="k">if</span> <span class="o">(</span><span class="n">DEBUG</span><span class="o">)</span> <span class="n">Log</span><span class="o">.</span><span class="na">v</span><span class="o">(</span><span class="n">TAG</span><span class="o">,</span> <span class="s">&quot;Attaching item #&quot;</span> <span class="o">+</span> <span class="n">itemId</span> <span class="o">+</span> <span class="s">&quot;: f=&quot;</span> <span class="o">+</span> <span class="n">fragment</span><span class="o">);</span>
</span><span class='line'>        <span class="n">mCurTransaction</span><span class="o">.</span><span class="na">attach</span><span class="o">(</span><span class="n">fragment</span><span class="o">);</span>
</span><span class='line'>    <span class="o">}</span> <span class="k">else</span> <span class="o">{</span>
</span><span class='line'>        <span class="n">fragment</span> <span class="o">=</span> <span class="n">getItem</span><span class="o">(</span><span class="n">position</span><span class="o">);</span>
</span><span class='line'>        <span class="k">if</span> <span class="o">(</span><span class="n">DEBUG</span><span class="o">)</span> <span class="n">Log</span><span class="o">.</span><span class="na">v</span><span class="o">(</span><span class="n">TAG</span><span class="o">,</span> <span class="s">&quot;Adding item #&quot;</span> <span class="o">+</span> <span class="n">itemId</span> <span class="o">+</span> <span class="s">&quot;: f=&quot;</span> <span class="o">+</span> <span class="n">fragment</span><span class="o">);</span>
</span><span class='line'>        <span class="n">mCurTransaction</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">container</span><span class="o">.</span><span class="na">getId</span><span class="o">(),</span> <span class="n">fragment</span><span class="o">,</span>
</span><span class='line'>                <span class="n">makeFragmentName</span><span class="o">(</span><span class="n">container</span><span class="o">.</span><span class="na">getId</span><span class="o">(),</span> <span class="n">itemId</span><span class="o">));</span>
</span><span class='line'>    <span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;按照上面两点想法，经过改造的Adapter的代码如下：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
<span class='line-number'>12</span>
<span class='line-number'>13</span>
<span class='line-number'>14</span>
<span class='line-number'>15</span>
<span class='line-number'>16</span>
<span class='line-number'>17</span>
<span class='line-number'>18</span>
<span class='line-number'>19</span>
<span class='line-number'>20</span>
<span class='line-number'>21</span>
<span class='line-number'>22</span>
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<span class='line-number'>24</span>
<span class='line-number'>25</span>
<span class='line-number'>26</span>
<span class='line-number'>27</span>
<span class='line-number'>28</span>
<span class='line-number'>29</span>
<span class='line-number'>30</span>
<span class='line-number'>31</span>
<span class='line-number'>32</span>
<span class='line-number'>33</span>
<span class='line-number'>34</span>
<span class='line-number'>35</span>
<span class='line-number'>36</span>
<span class='line-number'>37</span>
<span class='line-number'>38</span>
<span class='line-number'>39</span>
<span class='line-number'>40</span>
<span class='line-number'>41</span>
<span class='line-number'>42</span>
<span class='line-number'>43</span>
<span class='line-number'>44</span>
<span class='line-number'>45</span>
<span class='line-number'>46</span>
<span class='line-number'>47</span>
<span class='line-number'>48</span>
<span class='line-number'>49</span>
<span class='line-number'>50</span>
<span class='line-number'>51</span>
<span class='line-number'>52</span>
<span class='line-number'>53</span>
<span class='line-number'>54</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'><span class="kd">public</span> <span class="kd">class</span> <span class="nc">CustomPagerAdapter</span> <span class="kd">extends</span> <span class="n">FragmentPagerAdapter</span> <span class="o">{</span>
</span><span class='line'>    <span class="kd">private</span> <span class="kd">static</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">COUNT</span> <span class="o">=</span> <span class="mi">3</span><span class="o">;</span>
</span><span class='line'>
</span><span class='line'>    <span class="kd">private</span> <span class="n">Fragment</span><span class="o">[]</span> <span class="n">mFragments</span><span class="o">;</span>
</span><span class='line'>    <span class="kd">private</span> <span class="n">Context</span> <span class="n">mContext</span><span class="o">;</span>
</span><span class='line'>
</span><span class='line'>    <span class="kd">public</span> <span class="nf">CustomPagerAdapter</span><span class="o">(</span><span class="n">Context</span> <span class="n">context</span><span class="o">,</span> <span class="n">FragmentManager</span> <span class="n">fm</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="kd">super</span><span class="o">(</span><span class="n">fm</span><span class="o">);</span>
</span><span class='line'>        <span class="k">this</span><span class="o">.</span><span class="na">mContext</span> <span class="o">=</span> <span class="n">context</span><span class="o">;</span>
</span><span class='line'>        <span class="k">this</span><span class="o">.</span><span class="na">mFragments</span> <span class="o">=</span> <span class="k">new</span> <span class="n">Fragment</span><span class="o">[</span><span class="n">COUNT</span><span class="o">];</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'>
</span><span class='line'>    <span class="nd">@Override</span>
</span><span class='line'>    <span class="kd">public</span> <span class="n">Fragment</span> <span class="nf">getItem</span><span class="o">(</span><span class="kt">int</span> <span class="n">position</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="n">String</span> <span class="n">text</span><span class="o">;</span>
</span><span class='line'>        <span class="k">switch</span> <span class="o">(</span><span class="n">position</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>            <span class="k">case</span> <span class="mi">0</span><span class="o">:</span>
</span><span class='line'>                <span class="n">text</span> <span class="o">=</span> <span class="s">&quot;One&quot;</span><span class="o">;</span>
</span><span class='line'>                <span class="k">break</span><span class="o">;</span>
</span><span class='line'>            <span class="k">case</span> <span class="mi">1</span><span class="o">:</span>
</span><span class='line'>                <span class="n">text</span> <span class="o">=</span> <span class="s">&quot;Two&quot;</span><span class="o">;</span>
</span><span class='line'>                <span class="k">break</span><span class="o">;</span>
</span><span class='line'>            <span class="k">case</span> <span class="mi">2</span><span class="o">:</span>
</span><span class='line'>                <span class="n">text</span> <span class="o">=</span> <span class="s">&quot;Three&quot;</span><span class="o">;</span>
</span><span class='line'>                <span class="k">break</span><span class="o">;</span>
</span><span class='line'>            <span class="k">default</span><span class="o">:</span>
</span><span class='line'>                <span class="n">text</span> <span class="o">=</span> <span class="s">&quot;&quot;</span><span class="o">;</span>
</span><span class='line'>        <span class="o">}</span>
</span><span class='line'>        <span class="n">Bundle</span> <span class="n">bundle</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">Bundle</span><span class="o">();</span>
</span><span class='line'>        <span class="n">bundle</span><span class="o">.</span><span class="na">putString</span><span class="o">(</span><span class="n">TestFragment</span><span class="o">.</span><span class="na">PARAM_KEY_TEXT</span><span class="o">,</span> <span class="n">text</span><span class="o">);</span>
</span><span class='line'>        <span class="k">return</span> <span class="n">Fragment</span><span class="o">.</span><span class="na">instantiate</span><span class="o">(</span><span class="n">mContext</span><span class="o">,</span> <span class="n">TestFragment</span><span class="o">.</span><span class="na">class</span><span class="o">.</span><span class="na">getName</span><span class="o">(),</span> <span class="n">bundle</span><span class="o">);</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'>
</span><span class='line'>    <span class="nd">@Override</span>
</span><span class='line'>    <span class="kd">public</span> <span class="kt">int</span> <span class="nf">getCount</span><span class="o">()</span> <span class="o">{</span>
</span><span class='line'>        <span class="k">return</span> <span class="n">COUNT</span><span class="o">;</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'>
</span><span class='line'>    <span class="nd">@Override</span>
</span><span class='line'>    <span class="kd">public</span> <span class="kt">long</span> <span class="nf">getItemId</span><span class="o">(</span><span class="kt">int</span> <span class="n">position</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="k">return</span> <span class="n">position</span><span class="o">;</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'>
</span><span class='line'>    <span class="nd">@Override</span>
</span><span class='line'>    <span class="kd">public</span> <span class="n">Object</span> <span class="nf">instantiateItem</span><span class="o">(</span><span class="n">ViewGroup</span> <span class="n">container</span><span class="o">,</span> <span class="kt">int</span> <span class="n">position</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="n">Fragment</span> <span class="n">fragment</span> <span class="o">=</span> <span class="o">(</span><span class="n">Fragment</span><span class="o">)</span> <span class="kd">super</span><span class="o">.</span><span class="na">instantiateItem</span><span class="o">(</span><span class="n">container</span><span class="o">,</span> <span class="n">position</span><span class="o">);</span>
</span><span class='line'>        <span class="n">mFragments</span><span class="o">[</span><span class="n">position</span><span class="o">]</span> <span class="o">=</span> <span class="n">fragment</span><span class="o">;</span>
</span><span class='line'>        <span class="k">return</span> <span class="n">fragment</span><span class="o">;</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'>
</span><span class='line'>    <span class="kd">public</span> <span class="n">Fragment</span><span class="o">[]</span> <span class="nf">getFragments</span><span class="o">()</span> <span class="o">{</span>
</span><span class='line'>        <span class="k">return</span> <span class="n">mFragments</span><span class="o">;</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'><span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>有一点需要注意的是，mFragment数组需要在每个页面都实例化好了之后才会填充完成，需要注意调用的时机。</p>

<p>FragmentPagerAdapter对Fragment缓存的分析就是这么多了，欢迎指正。</p>
]]></content>
  </entry>
  
  <entry>
    <title type="html"><![CDATA[Android中FragmentPagerAdapter对Fragment的缓存(一)]]></title>
    <link href="http://jaredlam.github.io/blog/2015/10/08/androidzhong-fragmentpageradapterdui-fragmentde-huan-cun/"/>
    <updated>2015-10-08T21:02:43+08:00</updated>
    <id>http://jaredlam.github.io/blog/2015/10/08/androidzhong-fragmentpageradapterdui-fragmentde-huan-cun</id>
    <content type="html"><![CDATA[<blockquote><p>ViewPager + FragmentPagerAdapter，时我们经常使用的一对搭档，其实际应用的代码也非常简单，但是也有一些容易被忽略的地方，这次我们就来讨论下FragmentPagerAdapter对Fragment的缓存应用。</p></blockquote>

<p>&emsp;我们可以先看看最简单的实现，自定义Adapter如下：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
<span class='line-number'>12</span>
<span class='line-number'>13</span>
<span class='line-number'>14</span>
<span class='line-number'>15</span>
<span class='line-number'>16</span>
<span class='line-number'>17</span>
<span class='line-number'>18</span>
<span class='line-number'>19</span>
<span class='line-number'>20</span>
<span class='line-number'>21</span>
<span class='line-number'>22</span>
<span class='line-number'>23</span>
<span class='line-number'>24</span>
<span class='line-number'>25</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'><span class="kd">public</span> <span class="kd">class</span> <span class="nc">CustomPagerAdapter</span> <span class="kd">extends</span> <span class="n">FragmentPagerAdapter</span><span class="o">{</span>
</span><span class='line'>
</span><span class='line'>    <span class="kd">private</span> <span class="n">List</span><span class="o">&lt;</span><span class="n">Fragment</span><span class="o">&gt;</span> <span class="n">mFragments</span><span class="o">;</span>
</span><span class='line'>
</span><span class='line'>    <span class="kd">public</span> <span class="nf">CustomPagerAdapter</span><span class="o">(</span><span class="n">FragmentManager</span> <span class="n">fm</span><span class="o">,</span> <span class="n">List</span><span class="o">&lt;</span><span class="n">Fragment</span><span class="o">&gt;</span> <span class="n">fragments</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="kd">super</span><span class="o">(</span><span class="n">fm</span><span class="o">);</span>
</span><span class='line'>        <span class="k">this</span><span class="o">.</span><span class="na">mFragments</span> <span class="o">=</span> <span class="n">fragments</span><span class="o">;</span>
</span><span class='line'>        <span class="n">fm</span><span class="o">.</span><span class="na">beginTransaction</span><span class="o">().</span><span class="na">commitAllowingStateLoss</span><span class="o">();</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'>
</span><span class='line'>    <span class="nd">@Override</span>
</span><span class='line'>    <span class="kd">public</span> <span class="n">Fragment</span> <span class="nf">getItem</span><span class="o">(</span><span class="kt">int</span> <span class="n">position</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="k">return</span> <span class="k">this</span><span class="o">.</span><span class="na">mFragments</span><span class="o">.</span><span class="na">get</span><span class="o">(</span><span class="n">position</span><span class="o">);</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'>
</span><span class='line'>    <span class="nd">@Override</span>
</span><span class='line'>    <span class="kd">public</span> <span class="kt">int</span> <span class="nf">getCount</span><span class="o">()</span> <span class="o">{</span>
</span><span class='line'>        <span class="k">return</span> <span class="k">this</span><span class="o">.</span><span class="na">mFragments</span><span class="o">.</span><span class="na">size</span><span class="o">();</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'>
</span><span class='line'>    <span class="nd">@Override</span>
</span><span class='line'>    <span class="kd">public</span> <span class="kt">long</span> <span class="nf">getItemId</span><span class="o">(</span><span class="kt">int</span> <span class="n">position</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="k">return</span> <span class="n">position</span><span class="o">;</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'><span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;代码比较简单，就不解释了，接着在Activity中使用这个Adapter:</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
<span class='line-number'>12</span>
<span class='line-number'>13</span>
<span class='line-number'>14</span>
<span class='line-number'>15</span>
<span class='line-number'>16</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'><span class="n">ViewPager</span> <span class="n">pager</span> <span class="o">=</span> <span class="o">(</span><span class="n">ViewPager</span><span class="o">)</span> <span class="n">findViewById</span><span class="o">(</span><span class="n">R</span><span class="o">.</span><span class="na">id</span><span class="o">.</span><span class="na">view_pager</span><span class="o">);</span>
</span><span class='line'>
</span><span class='line'><span class="n">List</span><span class="o">&lt;</span><span class="n">Fragment</span><span class="o">&gt;</span> <span class="n">fragmentList</span> <span class="o">=</span> <span class="k">new</span> <span class="n">ArrayList</span><span class="o">&lt;&gt;()</span>
</span><span class='line'><span class="n">TestFragment</span> <span class="n">fragmentOne</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">TestFragment</span><span class="o">();</span>
</span><span class='line'><span class="n">fragmentOne</span><span class="o">.</span><span class="na">setText</span><span class="o">(</span><span class="s">&quot;One&quot;</span><span class="o">);</span>
</span><span class='line'><span class="n">TestFragment</span> <span class="n">fragmentTwo</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">TestFragment</span><span class="o">();</span>
</span><span class='line'><span class="n">fragmentTwo</span><span class="o">.</span><span class="na">setText</span><span class="o">(</span><span class="s">&quot;Two&quot;</span><span class="o">);</span>
</span><span class='line'><span class="n">TestFragment</span> <span class="n">fragmentThree</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">TestFragment</span><span class="o">();</span>
</span><span class='line'><span class="n">fragmentThree</span><span class="o">.</span><span class="na">setText</span><span class="o">(</span><span class="s">&quot;Three&quot;</span><span class="o">);</span>
</span><span class='line'>
</span><span class='line'><span class="n">fragmentList</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">fragmentOne</span><span class="o">);</span>
</span><span class='line'><span class="n">fragmentList</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">fragmentTwo</span><span class="o">);</span>
</span><span class='line'><span class="n">fragmentList</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">fragmentThree</span><span class="o">);</span>
</span><span class='line'>
</span><span class='line'><span class="n">CustomPagerAdapter</span> <span class="n">adapter</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">CustomPagerAdapter</span><span class="o">(</span><span class="n">getSupportFragmentManager</span><span class="o">(),</span> <span class="n">fragmentList</span><span class="o">);</span>
</span><span class='line'><span class="n">pager</span><span class="o">.</span><span class="na">setAdapter</span><span class="o">(</span><span class="n">adapter</span><span class="o">);</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;这样就完成了一个FragmentPagerAdapter最基本的应用。现在，看上去一切都如我们所愿，但是真的没有任何问题了吗？</p>

<p><img src="http://jaredlam.github.io/images/20151008_02.png"></p>

<p>&emsp;接下来，我们来模拟一下程序运行在后台时，Android系统由于内存紧张，杀掉我们程序进程的情况：</p>

<blockquote><ol>
<li>首先运行程序至前台</li>
<li>接下来，点击Home键，返回桌面，同时我们的程序退回至后台运行。</li>
<li>进入Android Studio中，点击Android Monitor这个tab，并选择当前Device，并选择我们程序的进程名。</li>
<li>点击Terminal Application这个小红叉按钮，如下图：<img src="http://jaredlam.github.io/images/20151008_01.png"></li>
<li>这个时候后台进程已经被杀掉了，但是应用程序历史里我们的应用还在，所以长按Home键，并选择我们的程序，让其恢复到前台。</li>
<li>这时会看到，程序的确恢复到之前的页面。但奇怪的是，页面上却只有Hello，我们之前传入的Two到哪里去了？</li>
</ol>
</blockquote>

<p><img src="http://jaredlam.github.io/images/20151008_03.png"></p>

<p>&emsp;Fragment代码也比较简单，通过日志，我们发现恢复时，mText字段为空。所以页面上对应的TextView无法显示。</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
<span class='line-number'>12</span>
<span class='line-number'>13</span>
<span class='line-number'>14</span>
<span class='line-number'>15</span>
<span class='line-number'>16</span>
<span class='line-number'>17</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'><span class="kd">public</span> <span class="kd">class</span> <span class="nc">TestFragment</span> <span class="kd">extends</span> <span class="n">Fragment</span> <span class="o">{</span>
</span><span class='line'>
</span><span class='line'>    <span class="kd">private</span> <span class="n">String</span> <span class="n">mText</span><span class="o">;</span>
</span><span class='line'>
</span><span class='line'>    <span class="nd">@Nullable</span>
</span><span class='line'>    <span class="nd">@Override</span>
</span><span class='line'>    <span class="kd">public</span> <span class="n">View</span> <span class="nf">onCreateView</span><span class="o">(</span><span class="n">LayoutInflater</span> <span class="n">inflater</span><span class="o">,</span> <span class="n">ViewGroup</span> <span class="n">container</span><span class="o">,</span> <span class="n">Bundle</span> <span class="n">savedInstanceState</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="n">View</span> <span class="n">view</span> <span class="o">=</span> <span class="n">inflater</span><span class="o">.</span><span class="na">inflate</span><span class="o">(</span><span class="n">R</span><span class="o">.</span><span class="na">layout</span><span class="o">.</span><span class="na">fragment_test</span><span class="o">,</span> <span class="n">container</span><span class="o">,</span> <span class="kc">false</span><span class="o">);</span>
</span><span class='line'>        <span class="n">TextView</span> <span class="n">textView</span> <span class="o">=</span> <span class="o">(</span><span class="n">TextView</span><span class="o">)</span> <span class="n">view</span><span class="o">.</span><span class="na">findViewById</span><span class="o">(</span><span class="n">R</span><span class="o">.</span><span class="na">id</span><span class="o">.</span><span class="na">center_text_view</span><span class="o">);</span>
</span><span class='line'>        <span class="n">textView</span><span class="o">.</span><span class="na">setText</span><span class="o">(</span><span class="n">mText</span><span class="o">);</span>
</span><span class='line'>        <span class="k">return</span> <span class="n">view</span><span class="o">;</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'>
</span><span class='line'>    <span class="kd">public</span> <span class="kt">void</span> <span class="nf">setText</span><span class="o">(</span><span class="n">String</span> <span class="n">text</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="k">this</span><span class="o">.</span><span class="na">mText</span> <span class="o">=</span> <span class="n">text</span><span class="o">;</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'><span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;我们知道，以上是模拟Android系统内存紧张时，杀掉后台应用的流程。另外，当用户安装了类似360安全管家等应用，选择清理内存时，也会触发以上情况。</p>

<p>&emsp;那么当上面的流程发生时，Activity的onSaveInstanceState会被调用，以便我们可以保存当前的用户数据/页面状态等。当恢复时，在onCreate时，我们通过savedInstanceState参数，可以取到之前存储的数据，然后重新绑定到View上。</p>

<p>&emsp;这个过程都可以理解，可是回到我们的Activity代码当中:</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
<span class='line-number'>12</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'><span class="n">TestFragment</span> <span class="n">fragmentOne</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">TestFragment</span><span class="o">();</span>
</span><span class='line'><span class="n">fragmentOne</span><span class="o">.</span><span class="na">setText</span><span class="o">(</span><span class="s">&quot;One&quot;</span><span class="o">);</span>
</span><span class='line'><span class="n">TestFragment</span> <span class="n">fragmentTwo</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">TestFragment</span><span class="o">();</span>
</span><span class='line'><span class="n">fragmentTwo</span><span class="o">.</span><span class="na">setText</span><span class="o">(</span><span class="s">&quot;Two&quot;</span><span class="o">);</span>
</span><span class='line'><span class="n">TestFragment</span> <span class="n">fragmentThree</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">TestFragment</span><span class="o">();</span>
</span><span class='line'><span class="n">fragmentThree</span><span class="o">.</span><span class="na">setText</span><span class="o">(</span><span class="s">&quot;Three&quot;</span><span class="o">);</span>
</span><span class='line'><span class="n">fragmentList</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">fragmentOne</span><span class="o">);</span>
</span><span class='line'><span class="n">fragmentList</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">fragmentTwo</span><span class="o">);</span>
</span><span class='line'><span class="n">fragmentList</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">fragmentThree</span><span class="o">);</span>
</span><span class='line'>
</span><span class='line'><span class="n">CustomPagerAdapter</span> <span class="n">adapter</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">CustomPagerAdapter</span><span class="o">(</span><span class="n">getSupportFragmentManager</span><span class="o">(),</span> <span class="n">fragmentList</span><span class="o">);</span>
</span><span class='line'>        <span class="n">pager</span><span class="o">.</span><span class="na">setAdapter</span><span class="o">(</span><span class="n">adapter</span><span class="o">);</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;这段代码，是在onCreate方法中调用的。应用从后台恢复的时候，这段代码是被完整的执行过的。既然这样，三个Fragment都被重新创建过，并设置过对应的Text值，那么为什么Fragment中mText字段仍然为空呢？</p>

<p>&emsp;难道说，呈现在屏幕上的Fragment，和我们在onCreate中实例化的Fragment，已然不是同一个实例？</p>

<p>&emsp;为了验证这个想法，在OnCreate中加入下面的日志：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'> <span class="n">TestFragment</span> <span class="n">fragmentOne</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">TestFragment</span><span class="o">();</span>
</span><span class='line'> <span class="n">fragmentOne</span><span class="o">.</span><span class="na">setText</span><span class="o">(</span><span class="s">&quot;One&quot;</span><span class="o">);</span>
</span><span class='line'> <span class="n">Log</span><span class="o">.</span><span class="na">i</span><span class="o">(</span><span class="s">&quot;test&quot;</span><span class="o">,</span> <span class="s">&quot;++++fragmentOne++++:&quot;</span> <span class="o">+</span> <span class="n">fragmentOne</span><span class="o">.</span><span class="na">toString</span><span class="o">());</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;同时在TestFragment的onCreateView方法中也记下日志：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'><span class="n">Log</span><span class="o">.</span><span class="na">i</span><span class="o">(</span><span class="s">&quot;test&quot;</span><span class="o">,</span> <span class="s">&quot;++++current fragment++++:&quot;</span> <span class="o">+</span> <span class="k">this</span><span class="o">.</span><span class="na">toString</span><span class="o">());</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;第一次运行，恩，没有问题。创建和运行的都是同一个实例 <strong>534ed278</strong></p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'><span class="n">I</span><span class="o">/</span><span class="nl">test:</span> <span class="o">++++</span><span class="n">fragmentOne</span><span class="o">++++:</span><span class="n">TestFragment</span><span class="o">{</span><span class="mi">534</span><span class="n">ed278</span><span class="o">}</span>
</span><span class='line'><span class="n">I</span><span class="o">/</span><span class="nl">test:</span> <span class="o">++++</span><span class="n">current</span> <span class="n">fragment</span><span class="o">++++:</span><span class="n">TestFragment</span><span class="o">{</span><span class="mi">534</span><span class="n">ed278</span> <span class="err">#</span><span class="mi">0</span> <span class="n">id</span><span class="o">=</span><span class="mh">0x7f0c0066</span> <span class="nl">android:switcher:</span><span class="mi">2131492966</span><span class="o">:</span><span class="mi">0</span><span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;接下来，我们再次进行杀进程并恢复的过程。日志输出为：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'><span class="n">I</span><span class="o">/</span><span class="nl">test:</span> <span class="o">++++</span><span class="n">fragmentOne</span><span class="o">++++:</span><span class="n">TestFragment</span><span class="o">{</span><span class="mi">534</span><span class="n">c5c30</span><span class="o">}</span>
</span><span class='line'><span class="n">I</span><span class="o">/</span><span class="nl">test:</span> <span class="o">++++</span><span class="n">current</span> <span class="n">fragment</span><span class="o">++++:</span><span class="n">TestFragment</span><span class="o">{</span><span class="mi">534</span><span class="n">d10d4</span> <span class="err">#</span><span class="mi">0</span> <span class="n">id</span><span class="o">=</span><span class="mh">0x7f0c0066</span> <span class="nl">android:switcher:</span><span class="mi">2131492966</span><span class="o">:</span><span class="mi">0</span><span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;额。。果然，这次我们创建的Fragment，和实际经过onCreateView的Fragment。并不是同一个（<strong>534c5c30/534d10d4</strong>）。</p>

<p>&emsp;看来，还是要从源码中寻求真相，打开FragmentPagerAdapter的源码，在instantiateItem方法中发现了下面这一段：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
<span class='line-number'>12</span>
<span class='line-number'>13</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'><span class="c1">// Do we already have this fragment?</span>
</span><span class='line'><span class="n">String</span> <span class="n">name</span> <span class="o">=</span> <span class="n">makeFragmentName</span><span class="o">(</span><span class="n">container</span><span class="o">.</span><span class="na">getId</span><span class="o">(),</span> <span class="n">itemId</span><span class="o">);</span>
</span><span class='line'><span class="n">Fragment</span> <span class="n">fragment</span> <span class="o">=</span> <span class="n">mFragmentManager</span><span class="o">.</span><span class="na">findFragmentByTag</span><span class="o">(</span><span class="n">name</span><span class="o">);</span>
</span><span class='line'>
</span><span class='line'><span class="k">if</span> <span class="o">(</span><span class="n">fragment</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>    <span class="k">if</span> <span class="o">(</span><span class="n">DEBUG</span><span class="o">)</span> <span class="n">Log</span><span class="o">.</span><span class="na">v</span><span class="o">(</span><span class="n">TAG</span><span class="o">,</span> <span class="s">&quot;Attaching item #&quot;</span> <span class="o">+</span> <span class="n">itemId</span> <span class="o">+</span> <span class="s">&quot;: f=&quot;</span> <span class="o">+</span> <span class="n">fragment</span><span class="o">);</span>
</span><span class='line'>    <span class="n">mCurTransaction</span><span class="o">.</span><span class="na">attach</span><span class="o">(</span><span class="n">fragment</span><span class="o">);</span>
</span><span class='line'><span class="o">}</span> <span class="k">else</span> <span class="o">{</span>
</span><span class='line'>    <span class="n">fragment</span> <span class="o">=</span> <span class="n">getItem</span><span class="o">(</span><span class="n">position</span><span class="o">);</span>
</span><span class='line'>    <span class="k">if</span> <span class="o">(</span><span class="n">DEBUG</span><span class="o">)</span> <span class="n">Log</span><span class="o">.</span><span class="na">v</span><span class="o">(</span><span class="n">TAG</span><span class="o">,</span> <span class="s">&quot;Adding item #&quot;</span> <span class="o">+</span> <span class="n">itemId</span> <span class="o">+</span> <span class="s">&quot;: f=&quot;</span> <span class="o">+</span> <span class="n">fragment</span><span class="o">);</span>
</span><span class='line'>    <span class="n">mCurTransaction</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">container</span><span class="o">.</span><span class="na">getId</span><span class="o">(),</span> <span class="n">fragment</span><span class="o">,</span>
</span><span class='line'>            <span class="n">makeFragmentName</span><span class="o">(</span><span class="n">container</span><span class="o">.</span><span class="na">getId</span><span class="o">(),</span> <span class="n">itemId</span><span class="o">));</span>
</span><span class='line'><span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;makeFragmentName方法如下：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'><span class="kd">private</span> <span class="kd">static</span> <span class="n">String</span> <span class="nf">makeFragmentName</span><span class="o">(</span><span class="kt">int</span> <span class="n">viewId</span><span class="o">,</span> <span class="kt">long</span> <span class="n">id</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>    <span class="k">return</span> <span class="s">&quot;android:switcher:&quot;</span> <span class="o">+</span> <span class="n">viewId</span> <span class="o">+</span> <span class="s">&quot;:&quot;</span> <span class="o">+</span> <span class="n">id</span><span class="o">;</span>
</span><span class='line'><span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;原来，在实例化Fragment的时候，FragmentPagerAdapter会先通过makeFragmentName返回的tag，到FragmentManager当中进行查找是否有当前Fragment的缓存，如果有的话，就直接将之前的Fragment恢复回来并使用；反之，才会使用我们传入的新实例。</p>

<p>&emsp;而makeFragmentName产生的tag，只受我们重写的getItemId()方法返回值，和当前容器View的Id，container.getId()的影响。</p>

<p>&emsp;到这里，问题就清楚了，由于FragmentPagerAdapter会主动的去取缓存当中的Fragment，所以导致恢复回来之后，Fragment的实例不一样的问题。</p>

<p>&emsp;至于为什么mText字段为空，以及怎样解决这个情况，我们下一篇再来讨论^_^。</p>
]]></content>
  </entry>
  
  <entry>
    <title type="html"><![CDATA[Android ListView中使用多种Type时，getItemViewType返回值不能为负数，特别是-2]]></title>
    <link href="http://jaredlam.github.io/blog/2015/10/02/android-listviewzhong-shi-yong-duo-chong-typeshi-,getitemviewtypefan-hui-zhi-bu-neng-wei-fu-shu-,te-bie-shi-2/"/>
    <updated>2015-10-02T16:03:39+08:00</updated>
    <id>http://jaredlam.github.io/blog/2015/10/02/android-listviewzhong-shi-yong-duo-chong-typeshi-,getitemviewtypefan-hui-zhi-bu-neng-wei-fu-shu-,te-bie-shi-2</id>
    <content type="html"><![CDATA[<h2>背景</h2>

<blockquote><p>大家在Android开发中使用，常常由于业务的需求，会重写ListView的getItemViewType方法，来展示不同种类的Items；比如微博客户端，就有很多类型的Item(比如，普通微博，转发，推荐&hellip;)。 在公司项目的开发中，我们也采用了这种手段。但是没想到却意外的遇到了问题&hellip;&hellip;</p></blockquote>

<p>&emsp; 其实遇到的问题也很简单, 假设我们有6种类型的item需要展示在ListView当中，我们会在Adapter中这样去写</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'>    <span class="kd">public</span> <span class="kd">static</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">VIEW_TYPE_NEWS_SPORT</span> <span class="o">=</span> <span class="mi">1</span><span class="o">;</span>
</span><span class='line'>    <span class="kd">public</span> <span class="kd">static</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">VIEW_TYPE_NEWS_SOCIAL</span> <span class="o">=</span> <span class="mi">2</span><span class="o">;</span>
</span><span class='line'>    <span class="kd">public</span> <span class="kd">static</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">VIEW_TYPE_NEWS_POLITICS</span> <span class="o">=</span> <span class="mi">3</span><span class="o">;</span>
</span><span class='line'>    <span class="kd">public</span> <span class="kd">static</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">VIEW_TYPE_NEWS_SIGNIFICANT</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1</span><span class="o">;</span>
</span><span class='line'>    <span class="kd">public</span> <span class="kd">static</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">VIEW_TYPE_NEWS_LATEST</span> <span class="o">=</span> <span class="o">-</span><span class="mi">2</span><span class="o">;</span>
</span><span class='line'>    <span class="kd">public</span> <span class="kd">static</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">VIEW_TYPE_NEWS_SUGGESTION</span> <span class="o">=</span> <span class="o">-</span><span class="mi">3</span><span class="o">;</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp; 上面我们在Adapter中定义好了6个常量，分别对应我们要展示的6种type。前三种type是我们和服务端约定好的，后面三种是客户端自己加入的，为了良好的扩展性，我们将后三种定义为负数，以免和以后服务端增加的type有所冲突。</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'>    <span class="nd">@Override</span>
</span><span class='line'>    <span class="kd">public</span> <span class="kt">int</span> <span class="nf">getItemViewType</span><span class="o">(</span><span class="kt">int</span> <span class="n">position</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="k">return</span> <span class="nf">getCurrentTypeByPosition</span><span class="o">([</span><span class="n">position</span><span class="o">]);</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'>
</span><span class='line'>    <span class="nd">@Override</span>
</span><span class='line'>    <span class="kd">public</span> <span class="kt">int</span> <span class="nf">getViewTypeCount</span><span class="o">()</span> <span class="o">{</span>
</span><span class='line'>        <span class="k">return</span> <span class="mi">6</span><span class="o">;</span>
</span><span class='line'>    <span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp; 然后就是重写这两个方法，根据数据结构的特征，返回不同的type。</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'>    <span class="nd">@Override</span>
</span><span class='line'>    <span class="n">getView</span><span class="o">(</span><span class="n">View</span> <span class="n">convertView</span><span class="o">){</span>
</span><span class='line'>       <span class="k">switch</span><span class="o">(</span><span class="n">type</span><span class="o">){</span>
</span><span class='line'>             <span class="k">case</span><span class="o">:</span>
</span><span class='line'>                <span class="o">.....</span>
</span><span class='line'>        <span class="o">}</span>
</span><span class='line'>    <span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp; 接下来，当然是在getView中根据不同的类型，返回不同的View。然后代码基本就是这样，APP运行起来也符合我们的预期。目前看起来一切ok。</p>

<p>&emsp; 不过，当ListView数据量大一点过后，我们发现了一个现象：每当ListView滚动到VIEW_TYPE_NEWS_POLITICS这种type的Item的时候，会有一点卡顿掉帧的现象。另外，我们的图片异步加载出来后，有一个渐变的动画，每当这种Item呈现出来时，只要手指放在屏幕上，轻轻移动，这个Item中的图片，就会频繁闪动。</p>

<p>&emsp; 起初遇到这个Bug时，我的心情还是很淡定的。猜想是因为这个item的getView被频繁调用了多次呗，于是在getView方法中每种Type里都记下了日志，一运行，输出是这样的：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
<span class='line-number'>12</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'><span class="o">====</span> <span class="n">VIEW_TYPE_NEWS_SPORT</span> <span class="o">====</span>
</span><span class='line'><span class="o">====</span> <span class="n">VIEW_TYPE_NEWS_POLITICS</span> <span class="o">====</span>
</span><span class='line'><span class="o">====</span> <span class="n">VIEW_TYPE_NEWS_LATEST</span> <span class="o">====</span>
</span><span class='line'><span class="o">====</span> <span class="n">VIEW_TYPE_NEWS_LATEST</span> <span class="o">====</span>
</span><span class='line'><span class="o">====</span> <span class="n">VIEW_TYPE_NEWS_LATEST</span> <span class="o">====</span>
</span><span class='line'><span class="o">====</span> <span class="n">VIEW_TYPE_NEWS_LATEST</span> <span class="o">====</span>
</span><span class='line'><span class="o">====</span> <span class="n">VIEW_TYPE_NEWS_LATEST</span> <span class="o">====</span>
</span><span class='line'><span class="o">....</span>
</span><span class='line'><span class="o">====</span> <span class="n">VIEW_TYPE_NEWS_LATEST</span> <span class="o">====</span>
</span><span class='line'><span class="o">====</span> <span class="n">VIEW_TYPE_NEWS_SIGNIFICANT</span> <span class="o">====</span>
</span><span class='line'><span class="o">====</span> <span class="n">VIEW_TYPE_NEWS_SOCIAL</span> <span class="o">====</span>
</span><span class='line'><span class="o">====</span> <span class="n">VIEW_TYPE_NEWS_SUGGESTION</span> <span class="o">====</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;果然！当Item为VIEW_TYPE_NEWS_LATEST这种type时，手指一放上屏幕，getView就会被调用了很多次！</p>

<p>&emsp;getView这种异常频繁的调用在之前也遇到过，不过通常是因为ListView的android:layout_height被设置成了wrap_content, 导致其需要在Layout时调用getView去计算自身高度导致的。改成match_parent或者固定值就好了。具体可以参考<a href="http://stackoverflow.com/questions/2618272/custom-listview-adapter-getview-method-being-called-multiple-times-and-in-no-co">StackOverFlow上这个问题</a>。</p>

<p>&emsp; 不过，这次似乎有点不同，我检查了ListView的android:layout_height，确实是match_parent。并且这次只有这一个Item类型出现这种状况，另外5种却完全正常。</p>

<p>&emsp; 接下来，我分别尝试了改换VIEW_TYPE_NEWS_LATEST的布局文件，更换这种Item在列表中的位置。可这个问题却和这种Type如影随行，不管怎么改动，只要当前Item的Type为VIEW_TYPE_NEWS_LATEST，getView便会被调用个不停。</p>

<p>&emsp; 此刻，我的心情是崩溃的T<sup>T</sup></p>

<p>&emsp; 冷静下来想了想，这个现象，既然和位置，布局都没有关系，唯一的关系大概也就只剩下VIEW_TYPE_NEWS_LATEST这个Type的值了吧？不管了，试一下，把换VIEW_TYPE_NEWS_LATEST改成了4。</p>

<p>&emsp; 运行&hellip;</p>

<p>&emsp; &hellip;</p>

<p>&emsp; &hellip;</p>

<p>&emsp; 居然好了！！！当手指放到这个type上的时候，一大堆getView的日志居然消失了！！！</p>

<p>&emsp; 于是马上想到，这个特别的值 <strong>-2</strong>，一定在源码中有所应用吧。</p>

<p>&emsp;于是，马上打开ListView源码，搜索使用getItemViewType的地方，果然发现了如下代码：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
<span class='line-number'>12</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'><span class="n">p</span><span class="o">.</span><span class="na">viewType</span> <span class="o">=</span> <span class="n">mAdapter</span><span class="o">.</span><span class="na">getItemViewType</span><span class="o">(</span><span class="n">position</span><span class="o">);</span>
</span><span class='line'>
</span><span class='line'><span class="k">if</span> <span class="o">((</span><span class="n">recycled</span> <span class="o">&amp;&amp;</span> <span class="o">!</span><span class="n">p</span><span class="o">.</span><span class="na">forceAdd</span><span class="o">)</span> <span class="o">||</span> <span class="o">(</span><span class="n">p</span><span class="o">.</span><span class="na">recycledHeaderFooter</span>
</span><span class='line'>        <span class="o">&amp;&amp;</span> <span class="n">p</span><span class="o">.</span><span class="na">viewType</span> <span class="o">==</span> <span class="n">AdapterView</span><span class="o">.</span><span class="na">ITEM_VIEW_TYPE_HEADER_OR_FOOTER</span><span class="o">))</span> <span class="o">{</span>
</span><span class='line'>    <span class="n">attachViewToParent</span><span class="o">(</span><span class="n">child</span><span class="o">,</span> <span class="n">flowDown</span> <span class="o">?</span> <span class="o">-</span><span class="mi">1</span> <span class="o">:</span> <span class="mi">0</span><span class="o">,</span> <span class="n">p</span><span class="o">);</span>
</span><span class='line'><span class="o">}</span> <span class="k">else</span> <span class="o">{</span>
</span><span class='line'>    <span class="n">p</span><span class="o">.</span><span class="na">forceAdd</span> <span class="o">=</span> <span class="kc">false</span><span class="o">;</span>
</span><span class='line'>    <span class="k">if</span> <span class="o">(</span><span class="n">p</span><span class="o">.</span><span class="na">viewType</span> <span class="o">==</span> <span class="n">AdapterView</span><span class="o">.</span><span class="na">ITEM_VIEW_TYPE_HEADER_OR_FOOTER</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="n">p</span><span class="o">.</span><span class="na">recycledHeaderFooter</span> <span class="o">=</span> <span class="kc">true</span><span class="o">;</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'>    <span class="n">addViewInLayout</span><span class="o">(</span><span class="n">child</span><span class="o">,</span> <span class="n">flowDown</span> <span class="o">?</span> <span class="o">-</span><span class="mi">1</span> <span class="o">:</span> <span class="mi">0</span><span class="o">,</span> <span class="n">p</span><span class="o">,</span> <span class="kc">true</span><span class="o">);</span>
</span><span class='line'><span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;注意，AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER 的值为-2，很简单，当item type为-2时，就把当前这种type当成了Header或者Footer。</p>

<p>&emsp;另外，看看ListView是怎么处理这种情况的，当其为Header或者Footer时，会调用attachViewToParent方法；而当前View为普通Item时，则调用addViewInLayout方法，并传入最后一个参数preventRequestLayout为true：</p>

<pre><code>@param preventRequestLayout if true, calling this method will not trigger a layout request on child
</code></pre>

<p>&emsp;这里对Header/Footer与普通View处理方式的不同造成了这种情况。</p>

<p>&emsp;另外，除了-2之外，使用其他负数貌似也会使View无法成功的被缓存，源码如下：</p>

<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
</pre></td><td class='code'><pre><code class='java'><span class='line'>    <span class="k">if</span> <span class="o">(</span><span class="n">recycleOnMeasure</span><span class="o">()</span> <span class="o">&amp;&amp;</span> <span class="n">mRecycler</span><span class="o">.</span><span class="na">shouldRecycleViewType</span><span class="o">(</span>
</span><span class='line'>                <span class="o">((</span><span class="n">LayoutParams</span><span class="o">)</span> <span class="n">child</span><span class="o">.</span><span class="na">getLayoutParams</span><span class="o">()).</span><span class="na">viewType</span><span class="o">))</span> <span class="o">{</span>
</span><span class='line'>            <span class="n">mRecycler</span><span class="o">.</span><span class="na">addScrapView</span><span class="o">(</span><span class="n">child</span><span class="o">,</span> <span class="mi">0</span><span class="o">);</span>
</span><span class='line'>    <span class="o">}</span>
</span><span class='line'>
</span><span class='line'>    <span class="kd">public</span> <span class="kt">boolean</span> <span class="nf">shouldRecycleViewType</span><span class="o">(</span><span class="kt">int</span> <span class="n">viewType</span><span class="o">)</span> <span class="o">{</span>
</span><span class='line'>        <span class="k">return</span> <span class="n">viewType</span> <span class="o">&gt;=</span> <span class="mi">0</span><span class="o">;</span>
</span><span class='line'>    <span class="o">}</span>
</span></code></pre></td></tr></table></div></figure>


<p>&emsp;最后，去看看文档上对getItemViewType的return写的什么：</p>

<pre><code>Note: Integers must be in the range 0 to getViewTypeCount() - 1. IGNORE_ITEM_VIEW_TYPE can also be returned.
</code></pre>

<p>&emsp;话说，自己对文档和源码不熟悉，才导致了这次的问题；编写代码的时候还是要注意细节，要不很容易造成隐含的问题。不过话说回来，Android这API设计的，也太&hellip;</p>
]]></content>
  </entry>
  
</feed>