android中关于activity的返回键处理方法

	/**
	 * 菜单、返回键响应
	 */
	@Override
	public boolean onKeyDown(int keyCode, KeyEvent event) {
		// TODO Auto-generated method stub
		Utiltools.printE("调用双击退出函数onKeyDown"+keyCode+ KeyEvent.KEYCODE_BACK);
		if (keyCode == KeyEvent.KEYCODE_BACK) {
			exitBy2Click(); // 调用双击退出函数
		}
		return false;
	}
	/**
	 * 双击退出函数
	 */
	private static Boolean isExit = false;

	private void exitBy2Click() {
		Timer tExit = null;
		if (isExit == false) {
			isExit = true; // 准备退出
			Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
			tExit = new Timer();
			tExit.schedule(new TimerTask() {
				@Override
				public void run() {
					isExit = false; // 取消退出
				}
			}, 2000); // 如果2秒钟内没有按下返回键，则启动定时器取消掉刚才执行的任务

		} else {
			//MyApplication.removeAll();
			MyApplication application = (MyApplication) getApplication();
			application.finishAllContext();
			System.exit(0);
		}
	}