Android按下back键非退出隐藏到后台

public boolean onKeyDown(int keyCode, KeyEvent event) {    
        if (keyCode == KeyEvent.KEYCODE_BACK) {    
            Intent intent = new Intent(Intent.ACTION_MAIN);    
            intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);    
            intent.addCategory(Intent.CATEGORY_HOME);    
            startActivity(intent);    
            return true;    
        }    
        return super.onKeyDown(keyCode, event);    
    }

界面重绘

invalidate()或者view.postinvalidate()方法