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 2
          cleanUpRemovedTaskLocked(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2.1
      performClearTaskAtIndexLocked(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2.3
              return 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2.4
              boolean activityRemoved = destroyActivityLocked(r, true, "finish-imm");
      ......
   }

   ​

   2.4 destroyActivityLocked

   文件：ActivityStack.java

   final boolean destroyActivityLocked(ActivityRecord r, boolean removeFromApp, String reason) {
   ......
           try {
               // 见2.5
                  r.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

   ​