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(), \"handlePauseActivity\"); ..............
// 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 //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); } } Binder.restoreCallingIdentity(origId); } 从上面的代码可以看出，activityPaused的主要工作将交给ActivityStack的activityPausedLocked函数： final void activityPausedLocked(IBinder token, boolean timeout) { ............... final ActivityRecord r = isInStackLocked(token); if (r != null) { //从消息队列中移除该事件 mHandler.removeMessages(PAUSE_TIMEOUT_MSG, r); if (mPausingActivity == r) { .............. completePauseLocked(true); return; } else { ................ } } ............... } 上述代码中的completePauseLocked函数，将负责完成本次中断Activity的剩余的工作，同时重新进入启动Activity的流程。 //此处resumeNext的值为true private void completePauseLocked(boolean resumeNext) { ActivityRecord prev = mPausingActivity; .................. if (prev != null) { final boolean wasStopping = prev.state == ActivityState.STOPPING; prev.state = ActivityState.PAUSED; if (prev.finishing) { ................ //如果已经进入finishing状态，调用finishCurrentActivityLocked(本流程中，Activity还没有finishing) //此处参数为FINISH_AFTER_VISIBLE，仅将Activity加入到mStoppingActivities中 prev = finishCurrentActivityLocked(prev, FINISH_AFTER_VISIBLE, false); } else if (prev.app != null) { ................. //将Activity从等待可见的Activity中移除 if (mStackSupervisor.mWaitingVisibleActivities.remove(prev)) { ................... } if (prev.deferRelaunchUntilPaused) { ............. } else if (wasStopping) { ............. } else if ((!prev.visible && !hasVisibleBehindActivity()) || mService.isSleepingOrShuttingDownLocked()) { // If we were visible then resumeTopActivities will release resources before // stopping. //如果Activity变为不可见时，才会进入此分支，本流程实际上不会直接进入该分支 //但当Activity真的不见了，ActivityStack将调用makeInvisible，重新对中断Activity调用addToStopping函数 //将暂停的Activity保存mStoppingActivities中 addToStopping(prev, true /* immediate */); } } else { .................. } .......... //将mPausingActivity置为null mPausingActivity = null; } if (resumeNext) { final ActivityStack topStack = mStackSupervisor.getFocusedStack(); if (!mService.isSleepingOrShuttingDownLocked()) { //将重新开始启动前台栈顶的Activity，由于此时mResumedActivity为null，于是进入到启动目标Activity的流程 mStackSupervisor.resumeFocusedStackTopActivityLocked(topStack, prev, null); } else { .............. } } if (prev != null) { //将启动新的Activity，可以恢复事件分发 prev.resumeKeyDispatchingLocked(); .................. } ................ } 这部分代码的主要工作是： 1、若中断的Activity变为不可见时，调用addToStopping函数，将中断的Activity加入到mStoppingActivities； 2、将mPausingActivity置为null后，重新进入启动目标Activity的流程。 虽然本流程不会直接调用addToStopping函数，但我们还是进一步看看，中断之后的Activity变为不可见后将如何被AMS处理。 三、addToStopping函数 //此流程中immediate的值为true private void addToStopping(ActivityRecord r, boolean immediate) { if (!mStackSupervisor.mStoppingActivities.contains(r)) { mStackSupervisor.mStoppingActivities.add(r); } // If we already have a few activities waiting to stop, then give up // on things going idle and start clearing them out. Or if r is the // last of activity of the last task the stack will be empty and must // be cleared immediately. // 原生中MAX_STOPPING_TO_FORCE的值为3 boolean forceIdle = mStackSupervisor.mStoppingActivities.size() > MAX_STOPPING_TO_FORCE || (r.frontOfTask && mTaskHistory.size() <= 1); if (immediate || forceIdle) { ............ //ActivityStackSupervisor发送消息IDLE_NOW_MSG，最终由activityIdleInternalLocked函数处理 mStackSupervisor.scheduleIdleLocked(); } else { ................ } } 从上面的代码，可以看出启动Activity和中断Activity的最后一部均是调用activityIdleInternalLocked函数， 我们看看该函数中处理中断的Activity相关的流程： //中断Activity时，fromTimeout的值为true final ActivityRecord activityIdleInternalLocked(final IBinder token, boolean fromTimeout, Configuration config) { ............... ActivityRecord r = ActivityRecord.forTokenLocked(token); if (r != null) { ......... if (fromTimeout) { //该函数中也会调用mService.notifyAll()，因此会唤醒等待Activity启动的ActivityStarter //但ActivityStarter会检测到目标Activity还未可见，因此会重新进入等待状态 reportActivityLaunchedLocked(fromTimeout, r, -1, -1); } ......... } ......... // Atomically retrieve all of the other things to do. final ArrayList // Stop any activities that are scheduled to do so but have been // waiting for the www.sm136.comnext one to start. for (int i = 0; i < NS; i++) { r = stops.get(i); final ActivityStack stack = r.task.stack; if (stack != null) { if (r.finishing) { stack.finishCurrentActivityLocked(r, ActivityStack.FINISH_IMMEDIATELY, false); } else { //调用ActivityStack的stopActivityLocked函数 stack.stopActivityLocked(r); } } } //AMS处理无用的进程等 ............... } 跟进一下ActivityStack的stopActivityLocked函数： final void stopActivityLocked(ActivityRecord r) { .............. //如果Activity携带了FLAG_ACTIVITY_NO_HISTORY，那么当这个Activity结束时，将从Task中被移除掉 if ((r.intent.getFlags()&Intent.FLAG_ACTIVITY_NO_HISTORY) != 0 || (r.info.flags&ActivityInfo.FLAG_NO_HISTORY) != 0) { if (!r.finishing) { if (!mService.isSleepingLocked()) { ........... //将destroy该Activity if (requestFinishActivityLocked(r.appToken, Activity.RESULT_CANCELED, null, \"stop-no-history\ ............... } } else { .......... } } } if (r.app != null && r.app.thread != null) { ........... try { ................... r.stopped = false; //状态更新为stopping r.state = ActivityState.STOPPING; ................... //调用ApplicationThread的scheduleStopActivity接口 //将由ActivityThread的handleStopActivity函数进行实际的操作 r.app.thread.scheduleStopActivity(r.appToken, r.visible, r.configChangeFlags); ............. //发送超时信息，处理该消息时，将调用activityStoppedLocked函数 Message msg = mHandler.obtainMessage(STOP_TIMEOUT_MSG, r); mHandler.sendMessageDelayed(msg, STOP_TIMEOUT); } catch (Exception e) { ....... } } } 这一部分最终还是需要依赖于进程的ActivityThread的handleStopActivity函数： private void handleStopActivity(IBinder token, boolean show, int configChanges, int seq) { ActivityClientRecord r = mActivities.get(tonc630.comken); ............. //StopInfo是一个Runnable对象 StopInfo info = new StopInfo(); //调用Activity的onStop等接口 performStopActivityInner(r, info, show, true, \"handleStopActivity\"); ............. //更新可见性 updateVisibility(r, show); ............. //StopInfo的run函数将被调用 mH.post(info); mSomeActivitiesChanged = true; } private static class StopInfo implements Runnable { ............ @Override public void run() { // Tell activity manager we have been stopped. try { .......... //通知AMS进行扫尾工作 ActivityManagerNative.getDefault().activityStopped( activity.token, state, persistentState, description); } catch (RemoteException ex) { .......... } } } 最后看一下AMS的activityStopped函数： public final void activityStopped(.....) { ........... synchronized (this) { ActivityRecord r = ActivityRecord.isInStackLocked(token); if (r != null) { //调用ActivityStack的activityStoppedLocked函数 r.task.stack.activityStoppedLocked(r, icicle, persistentState, description); } } //移除无用进程 trimApplications(); ........... } 跟进ActivityStack的activityStoppedLocked函数： final void activityStoppedLocked(........) { ........ if (!r.stopped) { ............ mHandler.removeMessages(STOP_TIMEOUT_MSG, r); r.stopped = true; //变为stopped状态 r.state = ActivityState.STOPPED; ............ } } 由此可见，当中断的Activity变为完全不可见后，它的onStop函数才会被调用。 四、总结 startPausingLocked的分析告一段落，这部分内容在了解整个Activity的启动流程后，还是比较好理解的。 主要就是调用前一个Activity的onPause等接口，使其处于中断状态，然后再进入到启动新Activity的流程中。 此外，当被中断的Activity变为完全不可见，将调用其onStop接口。 因篇幅问题不能全部显示，请点此查看更多更全内容