Android优雅地退出APP的方式之点击两次返回键

	private static final int MESSAGE_BACK = 1;
	private boolean isFlag = true;

    private Handler handler = new Handler() {
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MESSAGE_BACK:
                    isFlag = true; // 在2s时,恢复isFlag的变量值
                    break;
            }
        }
    };


	@Override
    public boolean onKeyUp(int keyCode, KeyEvent event) {

        if (keyCode == KeyEvent.KEYCODE_BACK && isFlag) {
            isFlag = false;
            Toast.makeText(MainActivity.this, "再点击一次返回键退出应用", Toast.LENGTH_SHORT).show();
            handler.sendEmptyMessageDelayed(MESSAGE_BACK, 2000);
            return true;
        }

        return super.onKeyUp(keyCode, event);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // 保证在activity退出前,移除所有未被执行的消息和回调方法,避免出现内存泄漏!
        handler.removeCallbacksAndMessages(null);
    }