back按兩次離開

    private boolean isExit = false;
    private CountDownTimer countDownTimer;
    @Override
    public void onBackPressed() {

        if (webView.canGoBack()) {

            webView.goBack();

        } else {

            if (!isExit) {
                isExit = true;
                Toast.makeText(this, getResources().getString(R.string.dialog_back_exit_app), Toast.LENGTH_SHORT).show();
                countDownTimer = new CountDownTimer(2_000, 2_000) {

                    @Override
                    public void onFinish() {
                        isExit = false;
                    }

                    @Override
                    public void onTick(long millisUntilFinished) {
                    }

                }.start();
            } else {
                countDownTimer.cancel();
                finish();
            }
        }
    }