各位看官好，本文是Android Framework之Activity启动流程的第三篇，本篇将分析Activity生命周期的回调，新世界的大门就在眼前,走起。
第一篇：Android Framework之Activity启动流程(一)
第二篇：Android Framework之Activity启动流程(二)
执行完ApplicationThread# handleBindApplication ()之后，这时候新进程已经启动。
回到AMS# attachApplicationLocked(),代码逻辑来到了上篇文章的注释5处，调用了ActivityStackSupervisor# attachApplicationLocked,在里面又执行了realStartActivityLocked(),这样就回到了第一篇末尾所说到的第二种情况，接下来分析第二种情况。
我们看下ApplicationThread# scheduleLaunchActivity()
public final void scheduleLaunchActivity(/*参数省略*/){
//……省略代码
sendMessage(H.LAUNCH_ACTIVITY, r);
}sendMessage内部使用了Hanlder，发送了一个H.LAUNCH_ACTIVITY类型的消息，然后调用handleLaunchActivity()->performLaunchActivity()方法。
看看这两个方法到底干了什么事。
ActivityThread# handleLaunchActivity()
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
// 当前进程正活跃，避免GC
unscheduleGcIdler();
//确保使用的是最近的配置
handleConfigurationChanged(null, null);
//在创建Activity之前初始化WindowManagerService
if (!ThreadedRenderer.sRendererDisabled) {
GraphicsEnvironment.earlyInitEGL()
}
WindowManagerGlobal.initialize();
//======== 注释9 ========
//执行performLaunchActivity(),并返回Activity对象
Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
reportSizeConfigurations(r);
Bundle oldState = r.state;
//======== 注释10=========
//启动成功后，恢复Activity
handleResumeActivity(r.token, false, r.isForward,!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);
//……省略代码
}
}参数列表中的ActivityClientRecord可以看做是一个Activity的实例，同时还包括了其他一些属性，主要是对Activity状态进行记录。
handleLaunchActivity()主要有两个操作：
1.注释9:performLaunchActivity()
该方法主要是调用Activity的onCreate(),onStart(),onRestoreInstance(),onPostCreate()生命周期
2.注释10:handleResumeActivity()
该方法会调用Activity的onResume()生命周期
先来看看ActivityThread# performLaunchActivity()
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
ActivityInfo aInfo = r.activityInfo;
if (r.packageInfo == null) {
//从PackageManagerService获取应用包信息
r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
Context.CONTEXT_INCLUDE_CODE);
}
ComponentName component = r.intent.getComponent();
if (component == null) {
//获取组件信息
component = r.intent.resolveActivity(
mInitialApplication.getPackageManager());
r.intent.setComponent(component);
}
//如果提前设置好了目标Activity，则重新设置组件信息
if (r.activityInfo.targetActivity != null) {
component = new ComponentName(r.activityInfo.packageName,
r.activityInfo.targetActivity);
}
ContextImpl appContext = createBaseContextForActivity(r);
Activity activity = null;
try {
//利用ClassLoader去加载Activity
java.lang.ClassLoader cl = appContext.getClassLoader();
//利用Instrumentation创建Activity实例
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
r.intent.prepareToEnterProcess();
if (r.state != null) {
r.state.setClassLoader(cl);
}
} catch (Exception e) {
//……省略代码
}
try{
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
if (activity != null) {
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,r.embeddedID, r.lastNonConfigurationInstances, config,r.referrer, r.voiceInteractor, window, r.configCallback);
}
……
//回调Activity的onCreate()方法
//这里回调的重载函数由ActivityInfo的persistableMode参数决定
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
r.activity = activity;
r.stopped = true;
if (!r.activity.mFinished) {
//回调Activity的onStart()方法，同时会改变FragmentManager的状态信息
// mInstrumentation.callActivityOnStart(this);
activity.performStart();
r.stopped = false;
}
//回调Activity的onRestoreInstanceState()方法
//这里的回调方法同样由ActivityInfo的persistableMode参数决定
if (!r.activity.mFinished) {
if (r.isPersistable()) {
if (r.state != null || r.persistentState != null) {
mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state,
r.persistentState);
}
} else if (r.state != null) {
mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
}
}
//回调Activity的OnPostCreate()方法
if (!r.activity.mFinished) {
activity.mCalled = false;
if (r.isPersistable()) {
mInstrumentation.callActivityOnPostCreate(activity, r.state,
r.persistentState);
} else {
mInstrumentation.callActivityOnPostCreate(activity, r.state);
}
if (!activity.mCalled) {
throw new SuperNotCalledException(
"Activity " + r.intent.getComponent().toShortString() +
" did not call through to super.onPostCreate()");
}
}
r.paused = true;
mActivities.put(r.token, r);
} catch(SuperNotCalledException e){
……
}
return activity;
}ActivityThread#handleResumeActivity()
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
ActivityClientRecord r = mActivities.get(token);
//======== 注释11=========
//回调Activity的onResume()方法
r = performResumeActivity(token, clearHide, reason);
if (r != null) {
final Activity a = r.activity;
final int forwardBit = isForward ? WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION : 0;
//显示window
if (r.window == null && !a.mFinished && willBeVisible) {
……
ViewManager wm = a.getWindowManager();
……
//将decorView添加到WindowManager中
wm.addView(decor, l);
}
……
//更新布局
wm.updateViewLayout(decor, l);
……
if (reallyResume) {
try {
//通知AMS已经Resume了
ActivityManager.getService().activityResumed(token);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
} else {
try {
//如果在onResume之前抛出异常了,则通知AMS结束该Activity
ActivityManager.getService()
.finishActivity(token, Activity.RESULT_CANCELED, null,
Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
}
}
}
}该方法主要是回调Activity的onResume()方法(实际上是调用Instrumentation.callActivityOnResume(this)来执行)，并将DecorView添加到WindowManager中，这里的WindowManager是a.getWindowManager()得到的，其实现是WindowManagerImpl，这步操作在onResume()方法之后执行。
至此，Activity的启动流程分析完毕。
看完之后感觉如何，是否恍然大悟？
总结
1、 startActivity()通过Instrumentation向AMS进程发起startActivity()请求
2、 AMS收到启动请求后由ActivityStarter处理Flags和Intent信息,然后再由ActivityStackSupervisor和ActivityStack处理Task和Stack流程
3、 在ActivityStackSupervisor中判断app进程是否存在，若不存在则请求AMS进程创建新进程，app进程存在的情况请直接跳到第7步
4、 AMS进程通过Socket方式请求Zygote fork新进程
5、 在新进程里创建ActivityThread(主线程)并开启Looper循环，同时将ApplicationThread绑定到AMS
6、 AMS回调ApplicationThread的bindApplication()方法将自身与新进程绑定,并在ActivityThread的handleBindApplication()方法中创建应用的Application
7、 Application创建成功后，AMS调用ActivityStackSupervisor# realStartActivityLocked向ActivityThread发送创建Activity请求
8、 ActivityThread利用ClassLoader去加载Activity,并回调其生命周期方法。
