之前的博客中提到过，在启动一个新Activity时，如果界面还存在其它的Activity，那么必须先中断其它的Activity。
因此，除了第一个启动的Home界面对应的Activity外，其它的Activity均需要进行此操作，详细过程在启动Activity的过程：一的第六部分进行了描述。
现在我们可以分析一下，这段之前略去的代码流程。
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;
} ..........
............
}
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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 {
.............
}
}
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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;
}
}
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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<OnActivityPausedListener> 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;
}
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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);
}
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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 {
................
}
}
...............
}
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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();
..................
}
................
}
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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 {
................
}
}
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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<ActivityRecord> stops = processStoppingActivitiesLocked(true);
NS = stops != null ? stops.size() : 0;
..........
// Stop any activities that are scheduled to do so but have been
// waiting for the next 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
...............
}
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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", false)) {
...............
}
} 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) {
.......
}
}
}
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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(token);
.............
//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) {
..........
}
}
}
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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();
...........
}
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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;
............
}
}
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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。