<div id="js_content">
<p>本文分析的源代码基于 android27</p>
<p>Activity的启动流程相当复杂，比如我们在Activity <strong>A </strong>打开Activity <strong>B</strong><strong>。</strong>这一过程开始于<strong>A</strong>.startActivity 经过 Android System 的处理，最终调用 <strong>B</strong> 的生命周期方法结束。</p>
<p style="text-align: center"><img src="https://beijingoptbbs.oss-cn-beijing.aliyuncs.com/cs/5606289-1c8f935bc7ca1e5eb37dfeadef149b08"></p>
<p>上面的过程具体描述的话太过于复杂，我们可以将其拆解成3部分依次突破。</p>
<blockquote>
<ol><li><p>startActivity --> ActivityManagerService</p></li><li><p>ActivityManagerService --> ApplicationThread</p></li><li><p>ApplicationThread --> Activity</p></li></ol>
</blockquote>
<h2>(一) startActivity --> ActivityManagerService</h2>
<p>ActivityManagerService后续简称 AMS。先放一张时序图:<br></p>
<p style="text-align: center"><img src="https://beijingoptbbs.oss-cn-beijing.aliyuncs.com/cs/5606289-447fc5cbb1870de63dfd047d7bb330ca"></p>
<p style="text-align: left">从Activity的startActivity方法到AMS中间的过程并不复杂，下面看下源码中做了哪些操作：</p>
<p style="text-align: left"><strong>Activity的startActivity</strong></p>
<pre class="blockcode"><code class="language-go">1@Override
2public void startActivity(Intent intent, @Nullable Bundle options) {
3 if (options != null) {
4 //第二个参数为-1，表示不需要知道Activity启动的结果
5 startActivityForResult(intent, -1, options);
6 } else {
7 startActivityForResult(intent, -1);
8}
</code></pre>
<p>startActivity中调用startActivityForResult</p>
<p><strong>Activity的startActivityForResult</strong></p>
<pre class="blockcode"><code class="language-go"> 1public void startActivityForResult(@RequiresPermission Intent intent, int requestCode, @Nullable Bundle options) {
2 if (mParent == null) {//mParent表示当前Activity的父类，一般为null
3 options = transferSpringboardActivityOptions(options);
4
5 //调用Instrumentation.execStartActivity()，启动新的Activity。
6 //mMainThread类型为ActivityThread, 在attach()时被回调时被赋值。
7 Instrumentation.ActivityResult ar = mInstrumentation.execStartActivity(this, mMainThread.getApplicationThread(), mToken, this, intent, requestCode, options);
8 if (ar != null) {
9 mMainThread.sendActivityResult(
10 mToken, mEmbeddedID, requestCode, ar.getResultCode(),
11 ar.getResultData());
12 }
13 if (requestCode >= 0) {
14 mStartedActivity = true;
15 }
16
17 cancelInputsAndStartExitTransition(options);
18 } else {
19 ...
20 }
21}
</code></pre>
<p><strong></strong>startActivityForResult也很简单，调用Instrumentation.execStartActivity方法。剩下的交给Instrumentation类去处理。</p>
<blockquote>
<ol><li><p><strong>Instrumentation</strong>类主要用来监控应用程序与系统交互</p></li><li><p>红色字体标明的mMainThread是<strong>ActivityThread</strong>类型，<strong>ActivityThread</strong>可以理解为一个进程，在这就是<strong>A</strong>所在的进程</p></li><li><p>通过mMainThread获取一个<strong>ApplicationThread</strong>的引用，这个引用就是用来实现进程间通信的，具体来说就 AMS 所在的系统进程通知应用程序进程进行一系列操作，后面会用到</p></li></ol>
</blockquote>
<p><strong>Instrumentation的execStartActivity</strong></p>
<pre class="blockcode"><code class="language-go"> 1public ActivityResult execStartActivity(
2 Context who, IBinder contextThread, IBinder token, Activity target,
3 Intent intent, int requestCode, Bundle options) {
4
5 //将contextThread转成ApplicationThread.
6 IApplicationThread whoThread = (IApplicationThread) contextThread;
7 /*
8 * 中间代码省略
9 */
10 try {
11 intent.migrateExtraStreamToClipData();
12 intent.prepareToLeaveProcess(who);
13 //实际上这里是通过AIDL来调用AMS的startActivity方法，下面我们看下ActivityManager.getService()的代码
14 int result = ActivityManager.getService()
15 .startActivity(whoThread, who.getBasePackageName(), intent,
16 intent.resolveTypeIfNeeded(who.getContentResolver()),
17 token, target != null ? target.mEmbeddedID : null,
18 requestCode, 0, null, options);
19 checkStartActivityResult(result, intent);//检查StartActivity的结果
20 } catch (RemoteException e) {
21 throw new RuntimeException("Failure from system", e);
22 }
23 return null;
24}
</code></pre>
<p><strong></strong>熟悉Android Binder机制的同学应该很快就能定位到此处实际调用到了ActivityManagerService的startActivity方法。接下来的工作重心就由<strong>A</strong>进程转移到了AMS所在的系统进程。</p>
<h2>(二) AMS --> ApplicationThread</h2>
<p>接 |
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