把当前任务放入后台

  用Activity.moveTaskToBack()把当前任务放入后台，详细看注释：

    /**
     * Move the task containing this activity to the back of the activity
     * stack.  The activity's order within the task is unchanged.
     * 把该activity所在的task移到栈底，顺序不变
     * @param nonRoot If false then this only works if the activity is the root
     *                of a task; if true it will work for any activity in
     *                a task.
     * nonRoot：false：当该activity是root activity才有用，true：所有的activity都有用
     * @return If the task was moved (or it was already at the
     *         back) true is returned, else false.
     */
    public boolean moveTaskToBack(boolean nonRoot) {
        try {
            return ActivityManagerNative.getDefault().moveActivityTaskToBack(
                    mToken, nonRoot);
        } catch (RemoteException e) {
            // Empty
        }
        return false;
    }

上边的mToken是IBinder类型，代表该Activity与ActivityManagerService进行IPC通信（进程间通信），直接看ActivityManagerService源码，如下

    /**
     * Moves an activity, and all of the other activities within the same task, to the bottom
     * of the history stack.  The activity's order within the task is unchanged.
     *  把该activity所在的task移到栈底，顺序不变
     * @param token A reference to the activity we wish to move 保存activity的引用
     * @param nonRoot If false then this only works if the activity is the root
     *                of a task; if true it will work for any activity in a task.
     * nonRoot：false：当该activity是root activity才有用，true：所有的activity都有用
     * @return Returns true if the move completed, false if not.
     */
    public boolean moveActivityTaskToBack(IBinder token, boolean nonRoot) {
        synchronized(this) {
            final long origId = Binder.clearCallingIdentity();
            int taskId = getTaskForActivityLocked(token, !nonRoot);
            if (taskId >= 0) {
                return mMainStack.moveTaskToBackLocked(taskId, null);
            }
            Binder.restoreCallingIdentity(origId);
        }
        return false;
    }

 

getTaskForActivityLocked：

    
 /**
     *  //找出token（activity）所在的栈
     * @param token 在服务端为ActivityRecord，在客户端为Activity
     * @param onlyRoot true 则只对root Activity有效，false对所有的Activity都有效
     * @return
     */
    int getTaskForActivityLocked(IBinder token, boolean onlyRoot) {
        final int N = mMainStack.mHistory.size();
        TaskRecord lastTask = null;
        for (int i=0; i<N; i++) {
            ActivityRecord r = (ActivityRecord)mMainStack.mHistory.get(i);
            if (r == token) {
                if (!onlyRoot || lastTask != r.task) {//不是root Activity时返回第一次匹配的task，否则查找root Activity对应的task
                    return r.task.taskId;
                }
                return -1;
            }
            lastTask = r.task;
        }

        return -1;
    }

mMainStack为ActivityStack，moveTaskToBackLocked如下：

    /**
     * Worker method for rearranging history stack.  Implements the function of moving all 
     * activities for a specific task (gathering them if disjoint) into a single group at the 
     * bottom of the stack.
     * 把特定的task移到栈底，并且保持顺序不变
     * If a watcher is installed, the action is preflighted and the watcher has an opportunity
     * to premeptively cancel the move.
     * 
     * @param task The taskId to collect and move to the bottom.
     * @return Returns true if the move completed, false if not.
     */
    final boolean moveTaskToBackLocked(int task, ActivityRecord reason) {

	....

 //移动到底部
        while (pos < N) {
            ActivityRecord r = mHistory.get(pos);
            if (localLOGV) Slog.v(
                TAG, "At " + pos + " ckp " + r.task + ": " + r);
            if (r.task.taskId == task) {
                if (localLOGV) Slog.v(TAG, "Removing and adding at " + (N-1));
                if (DEBUG_ADD_REMOVE) {
                    RuntimeException here = new RuntimeException("here");
                    here.fillInStackTrace();
                    Slog.i(TAG, "Removing and adding activity " + r + " to stack at "
                            + bottom, here);
                }
                mHistory.remove(pos);
                mHistory.add(bottom, r);
                moved.add(r);
                bottom++;
            }
            pos++;
        }}

	....

}