这个解的独特之处在于它的行为;其中，非双击将显示吐司和成功双击将显示没有吐司，同时关闭应用程序。 唯一的缺点是吐司的显示将有650毫秒的延迟。我相信这是最佳行为的最佳解决方案，因为逻辑表明，如果没有这样的延迟，就不可能有这种行为

//App Closing Vars
private var doubleBackPressedInterval: Long = 650
private var doubleTap = false
private var pressCount = 0
private var timeLimit: Long = 0

override fun onBackPressed() {
    pressCount++
    if(pressCount == 1) {
        timeLimit = System.currentTimeMillis() + doubleBackPressedInterval
        if(!doubleTap) {
            showExitInstructions()
        }
    }
    if(pressCount == 2) {
        if(timeLimit > System.currentTimeMillis()) {
            doubleTap = true
            super.onBackPressed()
        }
        else {
            showExitInstructions()
        }
        pressCount = 1
        timeLimit = System.currentTimeMillis() + doubleBackPressedInterval
    }
}

private fun showExitInstructions() {
    Handler().postDelayed({
        if(!doubleTap) {
            Toast.makeText(this, "Try Agian", Toast.LENGTH_SHORT).show()
        }
    }, doubleBackPressedInterval)
}

在这种情况下，Snackbar是比Toast更好的选择来显示退出动作。下面是小吃店的工作方法。

@Override
        public void onBackPressed() {
            if (doubleBackToExitPressedOnce) {
                super.onBackPressed();
                return;
            }
            this.doubleBackToExitPressedOnce = true;
            Snackbar.make(this.getWindow().getDecorView().findViewById(android.R.id.content), "Please click BACK again to exit", Snackbar.LENGTH_SHORT).show();

            new Handler().postDelayed(new Runnable() {

                @Override
                public void run() {
                    doubleBackToExitPressedOnce=false;
                }
            }, 2000);
        }

最近，我需要在我的一个应用程序中实现这个后退按钮功能。对最初问题的回答是有用的，但我必须考虑到另外两点:

在某些时间点，返回按钮被禁用 主要的活动是将片段与反向堆栈结合使用

根据回答和评论，我创建了以下代码:

private static final long BACK_PRESS_DELAY = 1000;

private boolean mBackPressCancelled = false;
private long mBackPressTimestamp;
private Toast mBackPressToast;

@Override
public void onBackPressed() {
    // Do nothing if the back button is disabled.
    if (!mBackPressCancelled) {
        // Pop fragment if the back stack is not empty.
        if (getSupportFragmentManager().getBackStackEntryCount() > 0) {
            super.onBackPressed();
        } else {
            if (mBackPressToast != null) {
                mBackPressToast.cancel();
            }

            long currentTimestamp = System.currentTimeMillis();

            if (currentTimestamp < mBackPressTimestamp + BACK_PRESS_DELAY) {
                super.onBackPressed();
            } else {
                mBackPressTimestamp = currentTimestamp;

                mBackPressToast = Toast.makeText(this, getString(R.string.warning_exit), Toast.LENGTH_SHORT);
                mBackPressToast.show();
            }
        }
    }
}

上面的代码假设使用了支持库。如果您使用片段而不是支持库，则需要用getFragmentManager()替换getSupportFragmentManager()。

如果后退按钮从未取消，则删除第一个if。删除第二个if，如果你不使用片段或片段返回堆栈

另外，重要的是要知道onBackPressed方法从Android 2.0开始就被支持了。查看本页详细描述。为了使背按功能也适用于旧版本，将以下方法添加到您的活动中:

@Override
public boolean onKeyDown(int keyCode, KeyEvent event)  {
    if (android.os.Build.VERSION.SDK_INT < android.os.Build.VERSION_CODES.ECLAIR
            && keyCode == KeyEvent.KEYCODE_BACK
            && event.getRepeatCount() == 0) {
        // Take care of calling this method on earlier versions of
        // the platform where it doesn't exist.
        onBackPressed();
    }

    return super.onKeyDown(keyCode, event);
}

吐司的最佳解决方案

在Java中

private Toast exitToast;

@Override
public void onBackPressed() {
    if (exitToast == null || exitToast.getView() == null || exitToast.getView().getWindowToken() == null) {
        exitToast = Toast.makeText(this, "Press again to exit", Toast.LENGTH_LONG);
        exitToast.show();
    } else {
        exitToast.cancel();
        super.onBackPressed();
    }
}

在Kotlin

private var exitToast: Toast? = null

override fun onBackPressed() {
    if (exitToast == null || exitToast!!.view == null || exitToast!!.view.windowToken == null) {
        exitToast = Toast.makeText(this, "Press again to exit", Toast.LENGTH_LONG)
        exitToast!!.show()
    } else {
        exitToast!!.cancel()
        super.onBackPressed()
    }
}

为MainActivity类声明一个全局Toast变量。示例:Toast exitToast; 在onCreate视图方法中初始化它。例子: exitToast = Toast.makeText(getApplicationContext()， "Press back again to exit"， Toast.LENGTH_SHORT); 最后创建一个onBackPressedMethod，如下所示: @Override onBackPressed() { if (exitToast.getView(). isshow ()) { exitToast.cancel (); 完成(); }其他{ exitToast.show (); ｝ ｝

这工作正确，我已经测试过了。我认为这样更简单。

在Kotlin的背面按下退出应用程序，你可以使用:

定义一个全局变量:

private var doubleBackToExitPressedOnce = false

覆盖onBackPressed:

override fun onBackPressed() {
        if (doubleBackToExitPressedOnce) {
            super.onBackPressed()
            return
        }

        doubleBackToExitPressedOnce = true
        Toast.makeText(this, "Please click BACK again to exit", Toast.LENGTH_LONG).show()

        Handler().postDelayed({
            doubleBackToExitPressedOnce = false;
        }, 2000)
    }