如何在Android应用中实现一个返回键功能

    @Override  public boolean onKeyDown(int keyCode, KeyEvent event) {    if(keyCode==KeyEvent.KEYCODE_BACK){      boolean result=MiddleManager.getInstance().goBack();      if(!result){        Toast.makeText(MainActivity.this, "是否退出应用", 1).show();      }      return false;    }    return super.onKeyDown(keyCode, event);  }