[深入frameworks]——startActivity的执行流程

基于Android9.0的代码分析

平时我们从一个Activity启动另外一个Activity，只需要调用startActivity()方法就可以实现，那么实际上是怎样实现的呢？为什么调用startActivity就可以打开另外一个Activity呢？在本文中我们就一起来探讨一下。

在对调用过程的分析中，我也附上了一些调用过程的调试信息，希望能够帮助读者更加清晰的了解这个过程。

1 开始调用context.startActivity

这里我就以stratActivity(intent)为例进行说明。先看下Activity中这个方法的具体实现：

1.1 Activity.java -> startActivity(intent)

/*** Same as {@link #startActivity(Intent, Bundle)} with no options* specified.** @param intent The intent to start.** @throws android.content.ActivityNotFoundException** @see #startActivity(Intent, Bundle)* @see #startActivityForResult*/
@Override
public void startActivity(Intent intent) {this.startActivity(intent, null);
}

首先在FirstActivity调用startActivity去启动SecondActivity。

1.1.1 相应的调用调试图

1.1.2 调试图分析

从上图可以看到，startActivity的参数intent，它的component其实是com.example.activitytest/.SecondActivity，并且还携带参数(has extras)。

并且startActivity(intent)的实现其实是调用它的一个重载方法startActivity(intent, options)，调用真正实现逻辑的方法，将不用的参数传入null，这种实现方式在Java中很常见，比如类的构造方法，有无参的，一个乃至多个参数的构造方法，往往其具体实际实现是在多个参数的构造方法中，至于其他较少参数的构造方法，则是将多出来的参数传入无效值。

1.2 startActivity(intent, options)

/*** Launch a new activity.  You will not receive any information about when* the activity exits.  This implementation overrides the base version,* providing information about* the activity performing the launch.  Because of this additional* information, the {@link Intent#FLAG_ACTIVITY_NEW_TASK} launch flag is not* required; if not specified, the new activity will be added to the* task of the caller.** <p>This method throws {@link android.content.ActivityNotFoundException}* if there was no Activity found to run the given Intent.** @param intent The intent to start.* @param options Additional options for how the Activity should be started.* See {@link android.content.Context#startActivity(Intent, Bundle)}* Context.startActivity(Intent, Bundle)} for more details.** @throws android.content.ActivityNotFoundException** @see #startActivity(Intent)* @see #startActivityForResult*/
@Override
public void startActivity(Intent intent, @Nullable Bundle options) {if (options != null) {startActivityForResult(intent, -1, options);} else {// Note we want to go through this call for compatibility with// applications that may have overridden the method.startActivityForResult(intent, -1);}
}

1.2.1 相应的调用调试图

1.2.2 调试图分析

传入的intent不变，而options传入为null，在这个方法中有对options无效的具体判断，并且调用startActivityForResult(intent, requestCode)。

1.3 startActivityForResult(intent, requestCode)

/*** Same as calling {@link #startActivityForResult(Intent, int, Bundle)}* with no options.** @param intent The intent to start.* @param requestCode If >= 0, this code will be returned in*                    onActivityResult() when the activity exits.** @throws android.content.ActivityNotFoundException** @see #startActivity*/
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode) {startActivityForResult(intent, requestCode, null);
}

1.3.1 相应的调用调试图

1.3.2 调试图分析

继续调用startActivityForResult(intent, requestCode, options)，requestCode和options仍然掺入无效值。

1.4 startActivityForResult(intent, requestCode, options)

/*** Launch an activity for which you would like a result when it finished.* When this activity exits, your* onActivityResult() method will be called with the given requestCode.* Using a negative requestCode is the same as calling* {@link #startActivity} (the activity is not launched as a sub-activity).** <p>Note that this method should only be used with Intent protocols* that are defined to return a result.  In other protocols (such as* {@link Intent#ACTION_MAIN} or {@link Intent#ACTION_VIEW}), you may* not get the result when you expect.  For example, if the activity you* are launching uses {@link Intent#FLAG_ACTIVITY_NEW_TASK}, it will not* run in your task and thus you will immediately receive a cancel result.** <p>As a special case, if you call startActivityForResult() with a requestCode* >= 0 during the initial onCreate(Bundle savedInstanceState)/onResume() of your* activity, then your window will not be displayed until a result is* returned back from the started activity.  This is to avoid visible* flickering when redirecting to another activity.** <p>This method throws {@link android.content.ActivityNotFoundException}* if there was no Activity found to run the given Intent.** @param intent The intent to start.* @param requestCode If >= 0, this code will be returned in*                    onActivityResult() when the activity exits.* @param options Additional options for how the Activity should be started.* See {@link android.content.Context#startActivity(Intent, Bundle)}* Context.startActivity(Intent, Bundle)} for more details.** @throws android.content.ActivityNotFoundException** @see #startActivity*/
public