Android 7.0 ActivityManagerService(4) 启动Activity的过程：三

Android 7.0 ActivityManagerService(4)

启动Activity的过程：三

一、startPausingLocked函数

根据启动Activity的过程：一， 我们知道在启动Activity的过程中，当已经将Activity对应的Task移动到前台，同时将待启动的Activity放置到栈顶后，将会调用ActivityStack中的resumeTopActivityInnerLocked函数，在该函数中：

private boolean resumeTopActivityInnerLocked(......) { ...........

//mResumedActivity保存者当前在前台显示的Activity if (mResumedActivity != null) { .............

//调用startPausingLocked函数，中断正在显示的Activity

pausing |= startPausingLocked(userLeaving, false, true, dontWaitForPause); }

if (pausing) { .......... return true; } .......... ............ }

此处，跟进一下startPausingLocked函数：

//Start pausing the currently resumed activity.

final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping, boolean resuming, boolean dontWait) { .................

//mResumedActivity保存着当前正在显示的Activity ActivityRecord prev = mResumedActivity; .................

mResumedActivity = null;

//mPausingActivity保存准备中断的Activity mPausingActivity = prev; ................ //更新状态

prev.state = ActivityState.PAUSING; ................

if (prev.app != null && prev.app.thread != null) { ................ try {

............

//通知该Activity所在的进程，调用schedulePauseActivity函数 prev.app.thread.schedulePauseActivity(prev.appToken, prev.finishing, userLeaving, prev.configChangeFlags, dontWait); } catch (Exception e) { ............... } } else {

.............. }

// If we are not going to sleep, we want to ensure the device is // awake until the next activity is started.

if (!uiSleeping && !mService.isSleepingOrShuttingDownLocked()) { //在启动Activity的第二部分中提过

//该WakeLock防止在Activity切换过程中，系统发生休眠 //当Activity切换成功后，将会释放该WakeLock mStackSupervisor.acquireLaunchWakelock(); }

if (mPausingActivity != null) {

// Have the window manager pause its key dispatching until the new // activity has started. If we're pausing the activity just because // the screen is being turned off and the UI is sleeping, don't interrupt // key dispatch; the same activity will pick it up again on wakeup. if (!uiSleeping) {

//暂停输入事件的派发

prev.pauseKeyDispatchingLocked(); } ........ .........

if (dontWait) {

// If the caller said they don't want to wait for the pause, then complete // the pause now.

completePauseLocked(false); return false; } else {

// Schedule a pause timeout in case the app doesn't respond. // We don't give it much time because this directly impacts the // responsiveness seen by the user.

Message msg = mHandler.obtainMessage(PAUSE_TIMEOUT_MSG); msg.obj = prev;

prev.pauseTime = SystemClock.uptimeMillis(); //延迟时间500ms，当这个消息被处理时，也会调用completePauseLocked函数

mHandler.sendMessageDelayed(msg, PAUSE_TIMEOUT); } } else {

............. } }

容易看出，这段代码最主要的操作是与应用进程的ApplicationThread进行Binder通信，调用其schedulePauseActivity函数。

与之前的流程一样，ApplicationThread仅作为通信接口，它将发送消息触发进程的ActivityThread调用handlePauseActivity进行实际的操作。

二、handlePauseActivity函数

private void handlePauseActivity(IBinder token, boolean finished,

boolean userLeaving, int configChanges, boolean dontReport, int seq) { ActivityClientRecord r = mActivities.get(token); ..............

if (r != null) { ..............

r.activity.mConfigChangeFlags |= configChanges; //执行pause的实际操作

performPauseActivity(token, finished, r.isPreHoneycomb(), \ ..............

// Tell the activity manager we have paused. if (!dontReport) { try {

//通知AMS

ActivityManagerNative.getDefault().activityPaused(token); } catch (RemoteException ex) {

throw ex.rethrowFromSystemServer(); } }

mSomeActivitiesChanged = true; } }

这段代码比较简单，就是调用performPauseActivity完成中断Activity的实际工作，然后再通过Binder通信通知AMS中断完成。 我们稍微看一下performPauseActivity函数：

final Bundle performPauseActivity(IBinder token, boolean finished, boolean saveState, String reason) {

ActivityClientRecord r = mActivities.get(token);

return r != null ? performPauseActivity(r, finished, saveState, reason) : null; }

final Bundle performPauseActivity(ActivityClientRecord r, boolean finished, boolean saveState, String reason) { ................

// Next have the activity save its current state and managed dialogs... if (!r.activity.mFinished && saveState) {

//完成调用Activity的onSaveInstanceState接口等操作 callCallActivityOnSaveInstanceState(r); }

//调用Activity的onPause接口

performPauseActivityIfNeeded(r, reason);

// Notify any outstanding on paused listeners ArrayList listeners; synchronized (mOnPauseListeners) {

//ActivityThread提供了对外接口registerOnActivityPausedListener //可以注册观察者监听某个Activity进入paused状态 listeners = mOnPauseListeners.remove(r.activity); }

int size = (listeners != null ? listeners.size() : 0); for (int i = 0; i < size; i++) { //回调

listeners.get(i).onPaused(r.activity); }

return !r.activity.mFinished && saveState ? r.state : null; }

performPauseActivity整体的逻辑很清晰，就是调用Activity生命周期中对应的接口，同时通知观察者Activity paused。

现在我们将视线移回到AMS，看看定义于其中的activityPaused函数。

public final void activityPaused(IBinder token) { final long origId = Binder.clearCallingIdentity(); synchronized(this) {

ActivityStack stack = ActivityRecord.getStackLocked(token); if (stack != null) {

stack.activityPausedLocked(token, false); }