Android Back返回键

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
        //Back返回键不回退，而是将某个指定组件隐藏
        if(share.getVisibility() != View.GONE){
            share.setVisibility(View.GONE);
            return false;
        }


        //Back返回键控制页面后退
        if ((keyCode == KeyEvent.KEYCODE_BACK) && mWebView.canGoBack()) {
            mWebView.goBack();
            return true;
        }

        // Android实现点击两次返回键退出
        if(keyCode == KeyEvent.KEYCODE_BACK)
        {
            ExitClick();
        }
        return false;
    }
    
private static Boolean isExit = false;     
    private void ExitClick() {  
        Timer tExit = null;  
        if (isExit == false) {  
            isExit = true; // 准备退出  
            Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();  
            tExit = new Timer();  
            tExit.schedule(new TimerTask() {  
                @Override  
                public void run() {  
                    isExit = false; // 取消退出  
                }  
            }, 2000); // 如果2秒钟内没有按下返回键，则启动定时器取消掉刚才执行的任务  
      
        } else {  
            finish();  
            System.exit(0);  
        }  
    }