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<a href="/2022/05/31/%E4%BA%8C%E5%88%86/" class="post-title-link" itemprop="url">二分</a>
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<p>在有序序列中查询/插入一个元素时的使用算法。</p>
<p>相较于遍历，使用二分在平均情况下可以将时间复杂度从 O(n) 减少为 O(√n).</p>
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<p>本人考完试就忘题，虽然面完就开始回想记录问题了，但是题目顺序也都记不清了，所以下面的顺序都是乱的，就看个大概吧。</p>
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<h3 id="什么是回调"><a href="#什么是回调" class="headerlink" title="什么是回调"></a>什么是回调</h3><blockquote>
<p>在计算机程序设计中，回调函数，或简称回调（Callback 即call then back 被主函数调用运算后会返回主函数），是指通过参数将函数传递到其它代码的，某一块可执行代码的引用。这一设计允许了底层代码调用在高层定义的子程序。</p>
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<p>本规范引用自<a target="_blank" rel="noopener" href="https://nvie.com/posts/a-successful-git-branching-model/">A successful Git branching model</a></p>
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<p>以下源码均来自于<a target="_blank" rel="noopener" href="https://github.com/YZune/WakeupSchedule_Kotlin">wakeup课程表</a></p>
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<h3 id="引入"><a href="#引入" class="headerlink" title="引入"></a>引入</h3><h4 id="hook-methods？"><a href="#hook-methods？" class="headerlink" title="hook methods？"></a>hook methods？</h4><p>在翻阅文档的时候，我注意到了一个直译意译都没法理解的词：</p>
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<p>一个帮助实现AppWidget提供程序的便利类。可以使用AppWidgetProvider进行的所有操作，也是使用常规的BroadcastReceiver可以进行的操作。AppWidgetProvider仅从onReceive(Context, Intent)中接收到的Intent中解析出相关字段，然后使用接收到的附加数据调用<strong>hook（钩子）方法</strong>。</p>
<p>扩展此类并重写onUpdate(Context, AppWidgetManager, int [])，onDeleted(Context, int [])，onEnabled(Context)或onDisabled(Context)方法中的一个或多个，以实现自己的AppWidget功能。</p>
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<h3 id="APK的内部结构"><a href="#APK的内部结构" class="headerlink" title="APK的内部结构"></a>APK的内部结构</h3><p>APK是AndroidPackage的缩写，即Android安装包(apk)。apk安装包实际上是一个zip压缩包。我们用解压软件可以将它解压，解压后可以看到如下图的文件结构和目录结构。</p>
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<h3 id="协程是什么"><a href="#协程是什么" class="headerlink" title="协程是什么"></a>协程是什么</h3><blockquote>
<p>协程（英语：coroutine）是计算机程序的一类组件，推广了协作式多任务的子程序，允许执行被挂起与被恢复。相对子例程而言，协程更为一般和灵活，但在实践中使用没有子例程那样广泛。协程更适合于用来实现彼此熟悉的程序组件，如协作式多任务、异常处理、事件循环、迭代器、无限列表和管道。</p>
</blockquote>
<p>协程并不是 Kotlin 提出来的新概念，其他的一些编程语言，例如：Go、Python 等都可以在语言层面上实现协程，甚至是 Java，也可以通过使用扩展库来间接地支持协程。</p>
<p>当我们讨论协程和线程的关系时，很容易<strong>陷入中文的误区</strong>，两者都有一个「程」字，就觉得有关系，其实就英文而言，Coroutines 和 Threads 就是两个概念。</p>
<p>从 Android 开发者的角度去理解它们的关系：</p>
<ul>
<li>我们所有的代码都是跑在线程中的，而线程是跑在进程中的。</li>
<li>协程没有直接和操作系统关联，但它不是空中楼阁，它也是跑在线程中的，可以是单线程，也可以是多线程。</li>
<li>单线程中的协程总的执行时间并不会比不用协程少。</li>
<li>Android 系统上，如果在主线程进行网络请求，会抛出 <code>NetworkOnMainThreadException</code>，对于在主线程上的协程也不例外，这种场景使用协程还是要切线程的。</li>
</ul>
<p>协程设计的初衷是为了解决并发问题，让 「协作式多任务」 实现起来更加方便。</p>
<p>协程就是 Kotlin 提供的一套线程封装的 API，但并不是说协程就是为线程而生的。</p>
<p>不过，我们学习 Kotlin 中的协程，一开始确实可以从线程控制的角度来切入。因为在 Kotlin 中，协程的一个典型的使用场景就是线程控制。就像 Java 中的 <code>Executor</code> 和 Android 中的 <code>AsyncTask</code>，Kotlin 中的协程也有对 Thread API 的封装，让我们可以在写代码时，不用关注多线程就能够很方便地写出并发操作。</p>
<p>在 Java 中要实现并发操作通常需要开启一个 <code>Thread</code> ：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">new</span> <span class="title class_">Thread</span>(<span class="keyword">new</span> <span class="title class_">Runnable</span>() {</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">run</span><span class="params">()</span> {</span><br><span class="line"> ...</span><br><span class="line"> }</span><br><span class="line">}).start();</span><br></pre></td></tr></table></figure>
<p>这里仅仅只是开启了一个新线程，至于它何时结束、执行结果怎么样，我们在主线程中是无法直接知道的。</p>
<p>Kotlin 中同样可以通过线程的方式去写：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">Thread({</span><br><span class="line"> ...</span><br><span class="line">}).start()</span><br></pre></td></tr></table></figure>
<p>可以看到，和 Java 一样也摆脱不了直接使用 <code>Thead</code> 的那些困难和不方便：</p>
<ul>
<li>线程什么时候执行结束</li>
<li>线程间的相互通信</li>
<li>多个线程的管理</li>
</ul>
<p>我们可以用 Java 的 <code>Executor</code> 线程池来进行线程管理：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">val</span> executor = Executors.newCachedThreadPool()</span><br><span class="line">executor.execute({</span><br><span class="line"> ...</span><br><span class="line">})</span><br></pre></td></tr></table></figure>
<p>用 Android 的 <code>AsyncTask</code> 来解决线程间通信：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">object</span> : AsyncTask<T0, T1, T2> { </span><br><span class="line"> <span class="keyword">override</span> <span class="function"><span class="keyword">fun</span> <span class="title">doInBackground</span><span class="params">(<span class="keyword">vararg</span> args: <span class="type">T0</span>)</span></span>: String { ... }</span><br><span class="line"> <span class="keyword">override</span> <span class="function"><span class="keyword">fun</span> <span class="title">onProgressUpdate</span><span class="params">(<span class="keyword">vararg</span> args: <span class="type">T1</span>)</span></span> { ... }</span><br><span class="line"> <span class="keyword">override</span> <span class="function"><span class="keyword">fun</span> <span class="title">onPostExecute</span><span class="params">(t3: <span class="type">T3</span>)</span></span> { ... }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>下面的例子是使用协程进行网络请求获取用户信息并显示到 UI 控件上：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">launch({</span><br><span class="line"> <span class="keyword">val</span> user = api.getUser() <span class="comment">// 👈 网络请求（IO 线程）</span></span><br><span class="line"> nameTv.text = user.name <span class="comment">// 👈 更新 UI（主线程）</span></span><br><span class="line">})</span><br></pre></td></tr></table></figure>
<p>这里只是展示了一个代码片段，<code>launch</code> 并不是一个顶层函数，它必须在一个对象中使用。</p>
<p><code>launch</code> 函数加上实现在 <code>{}</code> 中具体的逻辑，就构成了一个协程。</p>
<p>通常我们做网络请求，要不就传一个 callback，要不就是在 IO 线程里进行阻塞式的同步调用，而在这段代码中，上下两个语句分别工作在两个线程里，但写法上看起来和普通的单线程代码一样。</p>
<p>这里的 <code>api.getUser</code> 是一个<strong>挂起函数</strong>，所以能够保证 <code>nameTv.text</code> 的正确赋值，这就涉及到了协程中最著名的「非阻塞式挂起」。这个名词看起来不是那么容易理解，我们后续的文章会专门对这个概念进行讲解。现在先把这个概念放下，只需要记住协程就是这样写的就行了。</p>
<p>这种「用同步的方式写异步的代码」看起来很方便吧，那么我们来看看协程具体好在哪。</p>
<h2 id="协程好在哪"><a href="#协程好在哪" class="headerlink" title="协程好在哪"></a>协程好在哪</h2><h3 id="基本使用"><a href="#基本使用" class="headerlink" title="基本使用"></a>基本使用</h3><p>前面提到，<code>launch</code> 函数不是顶层函数，是不能直接用的，可以使用下面三种方法来创建协程：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 方法一，使用 runBlocking 顶层函数</span></span><br><span class="line">runBlocking {</span><br><span class="line"> getImage(imageId)</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">// 方法二，使用 GlobalScope 单例对象</span></span><br><span class="line"><span class="comment">// 👇 可以直接调用 launch 开启协程</span></span><br><span class="line">GlobalScope.launch {</span><br><span class="line"> getImage(imageId)</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">// 方法三，自行通过 CoroutineContext 创建一个 CoroutineScope 对象</span></span><br><span class="line"><span class="comment">// 👇 需要一个类型为 CoroutineContext 的参数</span></span><br><span class="line"><span class="keyword">val</span> coroutineScope = CoroutineScope(context)</span><br><span class="line">coroutineScope.launch {</span><br><span class="line"> getImage(imageId)</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<ul>
<li>方法一通常适用于单元测试的场景，而业务开发中不会用到这种方法，因为它是线程阻塞的。</li>
<li>方法二和使用 <code>runBlocking</code> 的区别在于不会阻塞线程。但在 Android 开发中同样不推荐这种用法，因为它的生命周期会和 app 一致，且不能取消（什么是协程的取消后面的文章会讲）。</li>
<li>方法三是比较推荐的使用方法，我们可以通过 <code>context</code> 参数去管理和控制协程的生命周期（这里的 <code>context</code> 和 Android 里的不是一个东西，是一个更通用的概念，会有一个 Android 平台的封装来配合使用）。</li>
</ul>
<p>关于 <code>CoroutineScope</code> 和 <code>CoroutineContext</code> 的更多内容后面的文章再讲。</p>
<p>协程最常用的功能是并发，而并发的典型场景就是多线程。可以使用 <code>Dispatchers.IO</code> 参数把任务切到 IO 线程执行：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">coroutineScope.launch(Dispatchers.IO) {</span><br><span class="line"> ...</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>也可以使用 <code>Dispatchers.Main</code> 参数切换到主线程：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">coroutineScope.launch(Dispatchers.Main) {</span><br><span class="line"> ...</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>所以在「协程是什么」一节中讲到的异步请求的例子完整写出来是这样的：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">coroutineScope.launch(Dispatchers.Main) { <span class="comment">// 在主线程开启协程</span></span><br><span class="line"> <span class="keyword">val</span> user = api.getUser() <span class="comment">// IO 线程执行网络请求</span></span><br><span class="line"> nameTv.text = user.name <span class="comment">// 主线程更新 UI</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Kotlin</p>
<p>而通过 Java 实现以上逻辑，我们通常需要这样写：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line">☕️</span><br><span class="line">api.getUser(<span class="keyword">new</span> <span class="title class_">Callback</span><User>() {</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">success</span><span class="params">(User user)</span> {</span><br><span class="line"> runOnUiThread(<span class="keyword">new</span> <span class="title class_">Runnable</span>() {</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">run</span><span class="params">()</span> {</span><br><span class="line"> nameTv.setText(user.name);</span><br><span class="line"> }</span><br><span class="line"> })</span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">failure</span><span class="params">(Exception e)</span> {</span><br><span class="line"> ...</span><br><span class="line"> }</span><br><span class="line">});</span><br></pre></td></tr></table></figure>
<p>Java</p>
<p>这种回调式的写法，打破了代码的顺序结构和完整性，读起来相当难受。</p>
<h3 id="协程的「1-到-0」"><a href="#协程的「1-到-0」" class="headerlink" title="协程的「1 到 0」"></a>协程的「1 到 0」</h3><p>对于回调式的写法，如果并发场景再复杂一些，代码的嵌套可能会更多，这样的话维护起来就非常麻烦。但如果你使用了 Kotlin 协程，多层网络请求只需要这么写：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">🏝️</span><br><span class="line">coroutineScope.launch(Dispatchers.Main) { <span class="comment">// 开始协程：主线程</span></span><br><span class="line"> <span class="keyword">val</span> token = api.getToken() <span class="comment">// 网络请求：IO 线程</span></span><br><span class="line"> <span class="keyword">val</span> user = api.getUser(token) <span class="comment">// 网络请求：IO 线程</span></span><br><span class="line"> nameTv.text = user.name <span class="comment">// 更新 UI：主线程</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Kotlin</p>
<p>如果遇到的场景是多个网络请求需要等待所有请求结束之后再对 UI 进行更新。比如以下两个请求：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">🏝️</span><br><span class="line">api.getAvatar(user, callback)</span><br><span class="line">api.getCompanyLogo(user, callback)</span><br></pre></td></tr></table></figure>
<p>Kotlin</p>
<p>如果使用回调式的写法，那么代码可能写起来既困难又别扭。于是我们可能会选择妥协，通过先后请求代替同时请求：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">🏝️</span><br><span class="line">api.getAvatar(user) { avatar -></span><br><span class="line"> api.getCompanyLogo(user) { logo -></span><br><span class="line"> show(merge(avatar, logo))</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Kotlin</p>
<p>在实际开发中如果这样写，本来能够并行处理的请求被强制通过串行的方式去实现，可能会导致等待时间长了一倍，也就是性能差了一倍。</p>
<p>而如果使用协程，可以直接把两个并行请求写成上下两行，最后再把结果进行合并即可：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">🏝️</span><br><span class="line">coroutineScope.launch(Dispatchers.Main) {</span><br><span class="line"> <span class="comment">// 👇 async 函数之后再讲</span></span><br><span class="line"> <span class="keyword">val</span> avatar = async { api.getAvatar(user) } <span class="comment">// 获取用户头像</span></span><br><span class="line"> <span class="keyword">val</span> logo = async { api.getCompanyLogo(user) } <span class="comment">// 获取用户所在公司的 logo</span></span><br><span class="line"> <span class="keyword">val</span> merged = suspendingMerge(avatar, logo) <span class="comment">// 合并结果</span></span><br><span class="line"> <span class="comment">// 👆</span></span><br><span class="line"> show(merged) <span class="comment">// 更新 UI</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Kotlin</p>
<p>可以看到，即便是比较复杂的并行网络请求，也能够通过协程写出结构清晰的代码。需要注意的是 <code>suspendingMerge</code> 并不是协程 API 中提供的方法，而是我们自定义的一个可「挂起」的结果合并方法。至于挂起具体是什么，可以看下一篇文章。</p>
<p>让复杂的并发代码，写起来变得简单且清晰，是协程的优势。</p>
<p>这里，两个没有相关性的后台任务，因为用了协程，被安排得明明白白，互相之间配合得很好，也就是我们之前说的「协作式任务」。</p>
<p>本来需要回调，现在直接没有回调了，这种从 1 到 0 的设计思想真的妙哉。</p>
<p>在了解了协程的作用和优势之后，我们再来看看协程是怎么使用的。</p>
<h2 id="协程怎么用"><a href="#协程怎么用" class="headerlink" title="协程怎么用"></a><strong>协程怎么用</strong></h2><h3 id="在项目中配置对-Kotlin-协程的支持"><a href="#在项目中配置对-Kotlin-协程的支持" class="headerlink" title="在项目中配置对 Kotlin 协程的支持"></a>在项目中配置对 Kotlin 协程的支持</h3><p>在使用协程之前，我们需要在 <code>build.gradle</code> 文件中增加对 Kotlin 协程的依赖：</p>
<ul>
<li>项目根目录下的 <code>build.gradle</code> :</li>
</ul>
<figure class="highlight groovy"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">buildscript {</span><br><span class="line"> ...</span><br><span class="line"> <span class="comment">// 👇</span></span><br><span class="line"> ext.kotlin_coroutines = <span class="string">'1.3.1'</span></span><br><span class="line"> ...</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Groovy</p>
<ul>
<li>Module 下的 <code>build.gradle</code> :</li>
</ul>
<figure class="highlight groovy"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">dependencies {</span><br><span class="line"> ...</span><br><span class="line"> <span class="comment">// 👇 依赖协程核心库</span></span><br><span class="line"> implementation <span class="string">"org.jetbrains.kotlinx:kotlinx-coroutines-core:$kotlin_coroutines"</span></span><br><span class="line"> <span class="comment">// 👇 依赖当前平台所对应的平台库</span></span><br><span class="line"> implementation <span class="string">"org.jetbrains.kotlinx:kotlinx-coroutines-android:$kotlin_coroutines"</span></span><br><span class="line"> ...</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Groovy</p>
<p>Kotlin 协程是以官方扩展库的形式进行支持的。而且，我们所使用的「核心库」和 「平台库」的版本应该保持一致。</p>
<ul>
<li>核心库中包含的代码主要是协程的公共 API 部分。有了这一层公共代码，才使得协程在各个平台上的接口得到统一。</li>
<li>平台库中包含的代码主要是协程框架在具体平台的具体实现方式。因为多线程在各个平台的实现方式是有所差异的。</li>
</ul>
<p>完成了以上的准备工作就可以开始使用协程了。</p>
<h3 id="开始使用协程"><a href="#开始使用协程" class="headerlink" title="开始使用协程"></a>开始使用协程</h3><p>协程最简单的使用方法，其实在前面章节就已经看到了。我们可以通过一个 <code>launch</code> 函数实现线程切换的功能：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">🏝️</span><br><span class="line"><span class="comment">// 👇</span></span><br><span class="line">coroutineScope.launch(Dispatchers.IO) {</span><br><span class="line"> ...</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Kotlin</p>
<p>这个 <code>launch</code> 函数，它具体的含义是：我要创建一个新的协程，并在指定的线程上运行它。这个被创建、被运行的所谓「协程」是谁？就是你传给 <code>launch</code> 的那些代码，这一段连续代码叫做一个「协程」。</p>
<p>所以，什么时候用协程？当你需要切线程或者指定线程的时候。你要在后台执行任务？切！</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">🏝️</span><br><span class="line">launch(Dispatchers.IO) {</span><br><span class="line"> <span class="keyword">val</span> image = getImage(imageId)</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Kotlin</p>
<p>然后需要在前台更新界面？再切！</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">🏝️</span><br><span class="line">coroutineScope.launch(Dispatchers.IO) {</span><br><span class="line"> <span class="keyword">val</span> image = getImage(imageId)</span><br><span class="line"> launch(Dispatchers.Main) {</span><br><span class="line"> avatarIv.setImageBitmap(image)</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Kotlin</p>
<p>好像有点不对劲？这不还是有嵌套嘛。</p>
<p>如果只是使用 <code>launch</code> 函数，协程并不能比线程做更多的事。不过协程中却有一个很实用的函数：<code>withContext</code> 。这个函数可以切换到指定的线程，并在闭包内的逻辑执行结束之后，自动把线程切回去继续执行。那么可以将上面的代码写成这样：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">🏝️</span><br><span class="line">coroutineScope.launch(Dispatchers.Main) { <span class="comment">// 👈 在 UI 线程开始</span></span><br><span class="line"> <span class="keyword">val</span> image = withContext(Dispatchers.IO) { <span class="comment">// 👈 切换到 IO 线程，并在执行完成后切回 UI 线程</span></span><br><span class="line"> getImage(imageId) <span class="comment">// 👈 将会运行在 IO 线程</span></span><br><span class="line"> }</span><br><span class="line"> avatarIv.setImageBitmap(image) <span class="comment">// 👈 回到 UI 线程更新 UI</span></span><br><span class="line">} </span><br></pre></td></tr></table></figure>
<p>Kotlin</p>
<p>这种写法看上去好像和刚才那种区别不大，但如果你需要频繁地进行线程切换，这种写法的优势就会体现出来。可以参考下面的对比：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br></pre></td><td class="code"><pre><span class="line">🏝️</span><br><span class="line"><span class="comment">// 第一种写法</span></span><br><span class="line">coroutineScope.launch(Dispatchers.IO) {</span><br><span class="line"> ...</span><br><span class="line"> launch(Dispatchers.Main){</span><br><span class="line"> ...</span><br><span class="line"> launch(Dispatchers.IO) {</span><br><span class="line"> ...</span><br><span class="line"> launch(Dispatchers.Main) {</span><br><span class="line"> ...</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">// 通过第二种写法来实现相同的逻辑</span></span><br><span class="line">coroutineScope.launch(Dispatchers.Main) {</span><br><span class="line"> ...</span><br><span class="line"> withContext(Dispatchers.IO) {</span><br><span class="line"> ...</span><br><span class="line"> }</span><br><span class="line"> ...</span><br><span class="line"> withContext(Dispatchers.IO) {</span><br><span class="line"> ...</span><br><span class="line"> }</span><br><span class="line"> ...</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Kotlin</p>
<p>由于可以”自动切回来”，消除了并发代码在协作时的嵌套。由于消除了嵌套关系，我们甚至可以把 <code>withContext</code> 放进一个单独的函数里面：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">🏝️</span><br><span class="line">launch(Dispatchers.Main) { <span class="comment">// 👈 在 UI 线程开始</span></span><br><span class="line"> <span class="keyword">val</span> image = getImage(imageId)</span><br><span class="line"> avatarIv.setImageBitmap(image) <span class="comment">// 👈 执行结束后，自动切换回 UI 线程</span></span><br><span class="line">}</span><br><span class="line"><span class="comment">// 👇</span></span><br><span class="line"><span class="function"><span class="keyword">fun</span> <span class="title">getImage</span><span class="params">(imageId: <span class="type">Int</span>)</span></span> = withContext(Dispatchers.IO) {</span><br><span class="line"> ...</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Kotlin</p>
<p>这就是之前说的「用同步的方式写异步的代码」了。</p>
<p>不过如果只是这样写，编译器是会报错的：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">🏝️</span><br><span class="line"><span class="function"><span class="keyword">fun</span> <span class="title">getImage</span><span class="params">(imageId: <span class="type">Int</span>)</span></span> = withContext(Dispatchers.IO) {</span><br><span class="line"> <span class="comment">// IDE 报错 Suspend function'withContext' should be called only from a coroutine or another suspend funcion</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Kotlin</p>
<p>意思是说，<code>withContext</code> 是一个 <code>suspend</code> 函数，它需要在协程或者是另一个 <code>suspend</code> 函数中调用。</p>
<h2 id="suspend"><a href="#suspend" class="headerlink" title="suspend"></a><strong>suspend</strong></h2><p><code>suspend</code> 是 Kotlin 协程最核心的关键字，几乎所有介绍 Kotlin 协程的文章和演讲都会提到它。它的中文意思是「暂停」或者「可挂起」。如果你去看一些技术博客或官方文档的时候，大概可以了解到：「代码执行到 <code>suspend</code> 函数的时候会『挂起』，并且这个『挂起』是非阻塞式的，它不会阻塞你当前的线程。」</p>
<p>上面报错的代码，其实只需要在前面加一个 <code>suspend</code> 就能够编译通过：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">🏝️</span><br><span class="line"><span class="comment">//👇</span></span><br><span class="line"><span class="keyword">suspend</span> <span class="function"><span class="keyword">fun</span> <span class="title">getImage</span><span class="params">(imageId: <span class="type">Int</span>)</span></span> = withContext(Dispatchers.IO) {</span><br><span class="line"> ...</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Kotlin</p>
<p>本篇文章到此结束，而 <code>suspend</code> 具体是什么，「非阻塞式」又是怎么回事，函数怎么被挂起，这些疑问的答案，将在下一篇文章全部揭晓。</p>
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