在安卓系统中，我们经常使用清除最近应用列表，或者是”一键清除“这个功能，最近在系统开发中，需要在一键清除时，保留某个应用不被杀死，需要做一些修改，所以就需要最一键清除这个流程进行了解。
长按HOME键会打开最近的APP列表，打开最近APP列表会执行下面的代码：

  1. IStatusBarService statusBarService = IStatusBarService.Stub.asInterface(
  2.         ServiceManager.getService("statusbar"));
  3. try {
  4.     statusBarService.toggleRecentApps();
  5. } catch (RemoteException e) {
  6.     Slog.e(LOG_TAG, "Error toggling recent apps.");
  7. }

IStatusBarService给了我们一个很明确的方向就是StatusBarService,在StatusBarManagerService类中，toggleRecentApps()的源码如下:

  1. @Override
  2.         public void toggleRecentApps() {
  3.             if (mBar != null) {
  4.                 try {
  5.                     mBar.toggleRecentApps();
  6.                 } catch (RemoteException ex) {}
  7.             }
  8.         }

mBar的赋值在:

  1. @Override
  2.     public void registerStatusBar(IStatusBar bar, List<String> iconSlots,
  3.             List<StatusBarIcon> iconList, int switches[], List<IBinder> binders,
  4.             Rect fullscreenStackBounds, Rect dockedStackBounds) {
  6.         ......
  7.         mBar = bar;
  8.         ......

在BaseStatusBar类的start方法中调用了StatusBarManagerService.registerStatusBar方法，下面是调用这个方法的语句：

  1. mBarService.registerStatusBar(mCommandQueue, iconList, switches, binders);

mCommandQueue被赋值给了StatusBarManagerService.mBar，下面是对这个变量赋值的语句：

  1. mCommandQueue = new CommandQueue(this, iconList);

这是CommandQueue类的对象，这个类的代码在文件”frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/CommandQueue.java”中。
回到StatusBarManagerService.toggleRecentApps方法中，在这个方法中执行了mBar.toggleRecentApps();语句，在这里调用的toggleRecentApps方法是CommandQueue.toggleRecentApps方法，下面来看一下CommandQueue.toggleRecentApps方法的的源码：

  1. public void toggleRecentApps() {
  2.     synchronized (mList) {
  3.         ......
  4.         mHandler.obtainMessage(MSG_TOGGLE_RECENT_APPS, 0, 0, null).sendToTarget();
  5.     }
  6. }

追溯MSG_TOGGLE_RECENT_APPS，从CommandQueue.mHandler到BaseStatusBar.toggleRecentApps到BaseStatusBar.H.handleMessage依次调用，代码如下

  1. //CommandQueue.H.handleMessage:
  2. public void handleMessage(Message msg) {
  3.     if (mPaused) {
  4.         this.sendMessageAtFrontOfQueue(Message.obtain(msg));
  5.         return;
  6.     }
  7.     final int what = msg.what & MSG_MASK;
  8.     switch (what) {
  10.         ......
  12.         case MSG_TOGGLE_RECENT_APPS:
  13.             mCallbacks.toggleRecentApps();
  14.             break;
  16.         ......
  17.     }
  18. }
  1. //BaseStatusBar.toggleRecentApps:
  2. @Override
  3. public void toggleRecentApps() {
  4.     int msg = MSG_TOGGLE_RECENTS_APPS;
  5.     ......
  6.     mHandler.sendEmptyMessage(msg);
  7. }
  1. BaseStatusBar.H.handleMessage
  2. public void handleMessage(Message m) {
  3.     switch (m.what) {
  5.      ......
  7.      case MSG_TOGGLE_RECENTS_APPS:
  8.          if (mDeviceProvisioned) {
  9.              toggleRecents();
  10.          }
  11.          break;
  13.      ......
  15.     }
  16. }

这里简单介绍一下statusbar的细节，BaseStatusBar有三个子类：PhoneStatusBar, CarStatusBar, TvStatusBar,很明显我们需要关注的的是PhoneStatusBar
面是PhoneStatusBar.toggleRecents方法的源码：

  1. @Override
  2. protected void toggleRecents() {
  3.     // Toggle the recents visibility flag
  4.     mSystemUiVisibility ^= View.RECENT_APPS_VISIBLE;
  5.     notifyUiVisibilityChanged(mSystemUiVisibility);
  6.     super.toggleRecents();
  7. }

前两行语句设置了一下状态，然后就调用父类的toggleRecents方法，PhoneStatusBar的父类是BaseStatusBar。
下面是BaseStatusBar.toggleRecents方法的源码：

  1. protected void toggleRecents() {
  2.     if (mRecents != null) {
  3.         sendCloseSystemWindows(mContext, SYSTEM_DIALOG_REASON_RECENT_APPS);
  4.         mRecents.toggleRecents(mDisplay, mLayoutDirection, getStatusBarView());
  5.     }
  6. }

mRecents实际调用的是Recents类的对象。Recents类代码在文件”frameworks/base/packages/SystemUI/src/com/android/systemui/recent/Recents.java”中。 下面是Recents.toggleRecents方法的代码：

  1. @Override
  2. public void toggleRecents(Display display, int layoutDirection, View statusBarView) {
  3.    ......
  5.     try {
  6.         Intent intent = new Intent(RecentsActivity.TOGGLE_RECENTS_INTENT);
  7.         intent.setClassName("com.android.systemui",
  8.                 "com.android.systemui.recent.RecentsActivity");
  9.         intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK
  10.                 | Intent.FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS);
  12.         ......
  14.             startActivitySafely(intent, opts.toBundle());
  15.         }
  16.     } catch (ActivityNotFoundException e) {
  17.         Log.e(TAG, "Failed to launch RecentAppsIntent", e);
  18.     }
  19. }

Recents.toggleRecents方法启动了RecentsActivity。RecentsActivity类的代码在文件”frameworks/base/packages/SystemUI/src/com/android/systemui/recent/RecentsActivity.java”中，RecentsActivity的layout是R.layout.recents 查看xml，其中核心的只有com.android.systemui.recents.views.RecentsView这个控件，这个RecentsView位于”frameworks/base/packages/SystemUI/src/com/android/systemui/recents/views/“ 在RecentView中那个“一键清理”的按钮是

  1. mStackActionButton = (TextView) inflater.inflate(Recents.getConfiguration().isLowRamDevice
  2.                     ? R.layout.recents_low_ram_stack_action_button
  3.                     : R.layout.recents_stack_action_button,
  4.                 this, false);

mStackActionButton的点击功能位于android/systemui/recents/views/RecentsViewTouchHandler.java中,在RecentsView的点击事件通过RecentsViewTouchHandler进行处理。RecentsViewTouchHandler中处理点击事件的代码如下:

  1. private boolean handleTouchEvent(MotionEvent ev) {
  2.         int action = ev.getActionMasked();
  3.         ......
  4.           case MotionEvent.ACTION_UP:
  5.             case MotionEvent.ACTION_CANCEL: {
  6.                 if (mRv.getStackActionButton().isPressed() && isWithinStackActionButton(evX, evY)) {
  7.                     EventBus.getDefault().send(new DismissAllTaskViewsEvent());
  8.                     consumed = true;
  9.                 }
  10.             }
  11.         .......
  12.         return consumed;
  13. }
  14. 追溯DismissAllTaskViewsEvent，可以找到两个地方，一个回到RecentView中
  15. ``` java
  16.  public final void onBusEvent(DismissAllTaskViewsEvent event) {
  17.         SystemServicesProxy ssp = Recents.getSystemServices();
  18.         if (!ssp.hasDockedTask()) {
  19.             // Animate the background away only if we are dismissing Recents to home
  20.             animateBackgroundScrim(0f, DEFAULT_UPDATE_SCRIM_DURATION);
  21.         }

BackgroundScrim是在最近应用视图中，在下方的一个渐变的颜色，这里让它动态消失，另一个是frameworks/base/packages/SystemUI/src/com/android/systemui/recents/views/TaskStackView.java中的

  1. public final void onBusEvent(final DismissAllTaskViewsEvent event) {
  2.         // Keep track of the tasks which will have their data removed
  3.         ArrayList<Task> tasks = new ArrayList<>(mStack.getTasks());
  4.         mAnimationHelper.startDeleteAllTasksAnimation(
  5.                 getTaskViews(), useGridLayout(), event.getAnimationTrigger());
  7.         event.addPostAnimationCallback(new Runnable() {
  8.             @Override
  9.             public void run() {
  10.                 // Remove all tasks and delete the task data for all tasks
  11.                 mStack.removeAllTasks(true /* notifyStackChanges */);
  12.                 for (int i = tasks.size() - 1; i >= 0; i--) {
  13.                     EventBus.getDefault().send(new DeleteTaskDataEvent(tasks.get(i)));
  14.                 }
  15.             }
  16.         });
  17.     }

首先执行

  1. mAnimationHelper.startDeleteAllTasksAnimation(
  2.                 getTaskViews(), useGridLayout(), event.getAnimationTrigger());

这里就是展示动画，把界面上显示的任务显示一个动画后清屏 ，在callback中的重点是mStack.removeAllTasks(true /* notifyStackChanges */);和EventBus.getDefault().send(new DeleteTaskDataEvent(tasks.get(i)));。mStack.removeAllTasks(true /* notifyStackChanges */);调用了frameworks/base/packages/SystemUI/src/com/android/systemui/recents/views/TaskStackView.java的removeAllTasks

  1. public void removeAllTasks(boolean notifyStackChanges) {
  2.         ArrayList<Task> tasks = mStackTaskList.getTasks();
  3.         for (int i = tasks.size() - 1; i >= 0; i--) {
  4.             Task t = tasks.get(i);
  5.             mStackTaskList.remove(t);
  6.             mRawTaskList.remove(t);
  7.         }
  8.         if (mCb != null && notifyStackChanges) {
  9.             // Notify that all tasks have been removed
  10.             mCb.onStackTasksRemoved(this);
  11.         }
  12.     }

mCb是TaskStackCallbacks，TaskStackView实现了这一接口

  1. @Override
  2.     public void onStackTasksRemoved(TaskStack stack) {
  3.         // Reset the focused task
  4.         resetFocusedTask(getFocusedTask());
  5.         // Return all the views to the pool
  6.         List<TaskView> taskViews = new ArrayList<>();
  7.         taskViews.addAll(getTaskViews());
  8.         for (int i = taskViews.size() - 1; i >= 0; i--) {
  9.             mViewPool.returnViewToPool(taskViews.get(i));
  10.         }
  11.         // Remove all the ignore tasks
  12.         mIgnoreTasks.clear();
  13.         // If there are no remaining tasks, then just close recents
  14.         EventBus.getDefault().send(new AllTaskViewsDismissedEvent(
  15.                 R.string.recents_empty_message_dismissed_all));
  16.     }

回到EventBus.getDefault().send(new DeleteTaskDataEvent(tasks.get(i)));追溯执行位置在RecentActivity中，代码如下

  1. public final void onBusEvent(DeleteTaskDataEvent event) {
  2.         // Remove any stored data from the loader
  3.         RecentsTaskLoader loader = Recents.getTaskLoader();
  4.         loader.deleteTaskData(event.task, false);
  5.         // Remove the task from activity manager
  6.         ActivityManagerWrapper.getInstance().removeTask(event.task.key.id);
  7.     }

最终调用了ActivityManagerWrapper.getInstance().removeTask(event.task.key.id);,ActivityManagerWrapper的removeTask代码如下

  1. public void removeTask(int taskId) {
  2.         mBackgroundExecutor.submit(new Runnable() {
  3.             @Override
  4.             public void run() {
  5.                 try {
  6.                     ActivityManager.getService().removeTask(taskId);
  7.                 } catch (RemoteException e) {
  8.                     Log.w(TAG, "Failed to remove task=" + taskId, e);
  9.                 }
  10.             }
  11.         });
  12.     }

最终是调用了ActivityManager.getService().removeTask(taskId);结束了这一task。

总结

结束最近使用APP的过程，是从statusbar，到recentActivity，显示最近应用的列表，在界面中显示RecentView，RecentView包括应用列表和清除所有应用的文字，点击一键清除调用到了RecentsViewTouchHandler,之后通过eventbus和handler调用到了TaskStackView，之后就区分两部分，一部分执行动画，一部分处理taskstack的数据，最终会调用到AMS的removeTask结束task。