最近任务栏清除app导致app不执行onDestory

问题

1. 步骤：在计算器面板上出入数字，然后按Menu键进入最近任务界面，右滑清楚计算器，接着再进入计算器
2. 期望结果：再次进入计算器，上面显示的内容和之前的一致
3. 实际结果：没有保存

初步分析

1. 开发计算器的工程师A说保存的代码写在了onDestory内，他说因为计算器的生命周期没有走完酒杯杀掉了，觉得是系统的问题，让系统的工程师看

2. 我看了问题和工程师A的回复，我首先说，保存重要数据，不要保存在onStop和onDestory内，Android开发者文档中也有说明
   

3. 至于为什么onDestory没有走完，我怀疑是并行的原因，需要在代码里去找证据

具体分析

1. removeTaskByIdLocked

   文件：AMS

   private boolean removeTaskByIdLocked(int taskId, boolean killProcess,boolean removeFromRecents) {......if (tr != null) {tr.removeTaskActivitiesLocked();    //清除Task中的activity，见 2cleanUpRemovedTaskLocked(tr, killProcess, removeFromRecents);    //清除相关的进程，见3......}return false;
   }

   在最近任务界面右滑时，会调用上面这个方法，里面有两个很重要的方法，分别是removeTaskActivitiesLocked和cleanUpRemovedTaskLocked，我们先来看看removeTaskActivitiesLocked

   ​

2. removeTaskActivitiesLocked

   文件：TaskRecord.java

   public void removeTaskActivitiesLocked() {// 见2.1performClearTaskAtIndexLocked(0);
   }

   2.1 performClearTaskAtIndexLocked

   文件：TaskRecord.java

   final void performClearTaskAtIndexLocked(int activityNdx) {int numActivities = mActivities.size();for ( ; activityNdx < numActivities; ++activityNdx) {......else if (stack.finishActivityLocked(r, Activity.RESULT_CANCELED, null, "clear-task-index", false)) {);--activityNdx;--numActivities;}}
   }

   上面会调用finishActivityLocked，我们继续往下看

   ​

   2.2 finishActivityLocked

   文件：ActivityStack.java

   final boolean finishActivityLocked(ActivityRecord r, int resultCode, Intent resultData,String reason, boolean oomAdj) {
   ......
   else if (r.state != ActivityState.PAUSING) {......// 见2.3return finishCurrentActivityLocked(r, (r.visible || r.nowVisible) ?FINISH_AFTER_VISIBLE : FINISH_AFTER_PAUSE, oomAdj) == null;}......
   }

   ​

   2.3 finishCurrentActivityLocked

   文件：ActivityStack.java

   final ActivityRecord finishCurrentActivityLocked(ActivityRecord r, int mode, boolean oomAdj) 
   {......if (mode == FINISH_IMMEDIATELY|| (prevState == ActivityState.PAUSED&& (mode == FINISH_AFTER_PAUSE || mode == FINISH_AFTER_VISIBLE|| mStackId == PINNED_STACK_ID))|| prevState == ActivityState.STOPPED|| prevState == ActivityState.INITIALIZING) {r.makeFinishingLocked();// 见2.4boolean activityRemoved = destroyActivityLocked(r, true, "finish-imm");......
   }

   ​

   2.4 destroyActivityLocked

   文件：ActivityStack.java

   final boolean destroyActivityLocked(ActivityRecord r, boolean removeFromApp, String reason) {
   ......try {// 见2.5r.app.thread.scheduleDestroyActivity(r.appToken, r.finishing,r.configChangeFlags);}......
   }

   马上要开始进行Binder调用了，System Sever这边是客户端，所以接下来是着ApplicationThreadProxy.java中的scheduleDestroyActivity

   ​

   2.5 scheduleDestroyActivity

   public final void scheduleDestroyActivity(IBinder token, boolean finishing,int configChanges) throws RemoteException {......mRemote.transact(SCHEDULE_FINISH_ACTIVITY_TRANSACTION, data, null,IBinder.FLAG_ONEWAY);......
   }

   注意上面的IBinder.FLAG_ONEWAY参数，这说明该binder调用是非阻塞的，这样问题可以解释的通了，接下来系统进程这边的代码会继续跑，不会等App进程那边的反馈，就当App进程那边准备处理这个SCHEDULE_FINISH_ACTIVITY_TRANSACTION消息时，App已经被杀掉了，所以计算器写在onDestory内的代码也就没有机会执行了

   ​

3. cleanUpRemovedTaskLocked

   文件：AMS

   private void cleanUpRemovedTaskLocked(TaskRecord tr, boolean killProcess,boolean removeFromRecents) {......// Kill the running processes.for (int i = 0; i < procsToKill.size(); i++) {ProcessRecord pr = procsToKill.get(i);if (pr.setSchedGroup == ProcessList.SCHED_GROUP_BACKGROUND&& pr.curReceiver == null) {pr.kill("remove task", true);} else {// We delay killing processes that are not in the background or running a receiver.pr.waitingToKill = "remove task";}}
   }

   这就是杀进程的方法

   ​

4. 时序图

   

   上面的方法中，除了ATN，ApplicationThread，ActivityThread是跑在App进程，其他都跑在System Sever进程

5. 总结

   其实前面都已经说了，这里再罗嗦一下，不要在onStop，onDestory中保存重要数据；最近任务栏清除app，app的onDestory不会掉用是因为该Binder调用是非阻塞的，导致App被杀死，所以onDestory没来得及调用

   ​