通过连续按两次back键，弹出Toast提示再按一次退出功能。关键代码如下：

第一种方式：

 /**
     * 连续按两次返回键就退出
     */
    private int keyBackClickCount = 0;

    @Override
    protected void onResume() {
        super.onResume();
        keyBackClickCount = 0;
    }


    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        // TODO Auto-generated method stub

        if (keyCode == KeyEvent.KEYCODE_BACK) {
            switch (keyBackClickCount++) {
                case 0:
                    Toast.makeText(this,
                            getResources().getString(R.string.press_again_exit),
                            Toast.LENGTH_SHORT).show();
                    Timer timer = new Timer();
                    timer.schedule(new TimerTask() {
                        @Override
                        public void run() {
                            keyBackClickCount = 0;
                        }
                    }, 3000);
                    break;
                case 1:
                    mFrameTv.setVisibility(View.VISIBLE);
                    mImgTv.setVisibility(View.VISIBLE);
                    Animation anim = AnimationUtils.loadAnimation(
                            MainActivity.this, R.anim.tv_off);
                    anim.setAnimationListener(new tvOffAnimListener());
                    mImgTv.startAnimation(anim);
                    break;
                default:
                    break;
            }
          <span style="color:#ff0000;">  return true;</span>
        } else if (keyCode == KeyEvent.KEYCODE_MENU) {

            if (sm.isMenuShowing()) {
                toggle();
            } else {
                showMenu();
            }
        }
        return super.onKeyDown(keyCode, event);
    }

/**
	 * 连续按两次返回键就退出
	 */
	private boolean isWaitingExit = false;

	@Override
	public boolean onKeyDown(int keyCode, KeyEvent event) {
		// TODO Auto-generated method stub

		if (keyCode == KeyEvent.KEYCODE_BACK) {

			if (isWaitingExit) {
				isWaitingExit = false;
				mFrameTv.setVisibility(0);
				mImgTv.setVisibility(0);
				Animation anim = AnimationUtils.loadAnimation(
						MainActivity.this, R.anim.tv_off);
				anim.setAnimationListener(new tvOffAnimListener());
				mImgTv.startAnimation(anim);

			} else {
				Toast.makeText(this,
						getResources().getString(R.string.press_again_exit),
						Toast.LENGTH_SHORT).show();
				isWaitingExit = true;
				Timer timer = new Timer();
				timer.schedule(new TimerTask() {
					@Override
					public void run() {
						isWaitingExit = false;
					}
				}, 3000);
				return true;
			}
			return true;
		} else if (keyCode == KeyEvent.KEYCODE_MENU) {

			if (sm.isMenuShowing()) {
				toggle();
			} else {
				showMenu();
			}
		}
		return super.onKeyDown(keyCode, event);
	}

android 实现按两次back键提示退出应用界面