Android Activity启动流程源码解析

在 Android Launcher启动应用程序流程源码解析一文中从点击应用程序图标startActivity()开始，一直讲解到Application和MainActivity的onCreate()方法被调用。中间流程有些高能，高能部分是两篇博文共有的地方，没办法绕开。本文主要讲述从Activity#onCreate()到Activity#onResume()的流程。想追本溯源的同学建议先阅读 Android Launcher启动应用程序流程源码解析之后，再回头读这篇博文。不啰嗦了，Here we go~

无论显式还是隐式启动Activity，都会进入到ActivityThread#handleLaunchActivity()

源码位置：frameworks/base/core/java/android/app/ActivityThread.java
ActivityThread#handleLaunchActivity()

    private final void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {  
        ......  
        // 加载activity,然后调用onCreate,onStart方法
        Activity a = performLaunchActivity(r, customIntent);  
        if (a != null) {
            // 调用onResume方法
            handleResumeActivity(r.token, false, r.isForward, !r.activity.mFinished && !r.startsNotResumed);
            ...   
        }
        ... 
    }  

注释有点剧透，，今天的重点全在这个方法里。首先看下performLaunchActivity()

ActivityThread#performLaunchActivity()

    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ...
            Activity activity = null;
        try {
            java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);
            ...
            }
            ...
        try {
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);
            if (activity != null) {
                ...
                activity.attach(...)
                ...
                if (r.isPersistable()) {
                   mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                   mInstrumentation.callActivityOnCreate(activity, r.state);
                }
                ...
                if (!r.activity.mFinished) {
                    activity.performStart();
                    r.stopped = false;
                }
                ...
        return activity;
    }

mInstrumentation.callActivityOnCreate这行代码的作用就是调用Activity#onCreate()，在 Android Launcher启动应用程序流程源码解析已经介绍过，这里就不再赘述了。直接看activity.performStart()，这个方法的名字十分吸引人呐～跟进。

源码位置：frameworks/base/core/java/android/app/Activity.java
Activity#performStart()

    final void performStart() {
        mActivityTransitionState.setEnterActivityOptions(this, getActivityOptions());
        mFragments.noteStateNotSaved();
        mCalled = false;
        mFragments.execPendingActions();
        mInstrumentation.callActivityOnStart(this);
        if (!mCalled) {
            throw new SuperNotCalledException(
                "Activity " + mComponent.toShortString() +
                " did not call through to super.onStart()");
        }
        mFragments.dispatchStart();
        mFragments.reportLoaderStart();
        mActivityTransitionState.enterReady(this);
    }

真相已经呼之欲出了～，跟进。

源码位置：frameworks/base/core/java/android/app/Instrumentation.java
Instrumentation#callActivityOnStart()

    public void callActivityOnStart(Activity activity) {
        activity.onStart();
    }

哈哈～又找到一个方法Activity#onStart()。真是轻而易举～
现在回到ActivityThread#performLaunchActivity()中的handleResumeActivity(r.token, false, r.isForward, !r.activity.mFinished && !r.startsNotResumed)，跟进。

源码位置：frameworks/base/core/java/android/app/ActivityThread.java
ActivityThread#handleResumeActivity()
ActivityThread#ActivityClientRecord()

   final void handleResumeActivity(IBinder token,
            boolean clearHide, boolean isForward, boolean reallyResume) {
        ...
        ActivityClientRecord r = performResumeActivity(token, clearHide);
        ...
    }

    public final ActivityClientRecord performResumeActivity(IBinder token,
            boolean clearHide) {
        ...
        r.activity.performResume();
        ...
    }

中间都是各种状态效验，没撒好说的。直接跟进。

源码位置：frameworks/base/core/java/android/app/Activity.java
Activity#performResume()

    final void performResume() {
        performRestart();
        ...
        mInstrumentation.callActivityOnResume(this);
        ...
    }

在这里有两个分支
1. 在执行onResume()前需要先执行onRestart()和onStart()。例如从上个Activity返回的时候，这里不作为重点，但也会稍微看下。
2. 直接执行onResume()

分支1：
Activity#performRestart()

    final void performRestart() {
        ...
        mInstrumentation.callActivityOnRestart(this);
        performStart();
    }

这两个方法在上文都分析过了，不再赘述。

分支2：
源码位置：frameworks/base/core/java/android/app/Instrumentation.java
Instrumentation#callActivityOnStart()

    public void callActivityOnResume(Activity activity) {
        activity.mResumed = true;
        activity.onResume();

        if (mActivityMonitors != null) {
            synchronized (mSync) {
                final int N = mActivityMonitors.size();
                for (int i=0; ifinal ActivityMonitor am = mActivityMonitors.get(i);
                    am.match(activity, activity, activity.getIntent());
                }
            }
        }
    }

直接调用Activity#OnResume()。

结束了。这篇博客结束的比之前任何一篇博文都要快，都要好理解。这一切的前提是已经阅读过我原来的博文 Android Launcher启动应用程序流程源码解析。

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