抽屜DrawerLayout onBackPressed事件

@Override
public void onBackPressed() {
    if (mDrawerLayout.isDrawerOpen(GravityCompat.START)) {//判斷
        mDrawerLayout.closeDrawers();
    } else {
        if (System.currentTimeMillis() - exittime < 2000){
            super.onBackPressed();
        }else {
            ToastUtil.showToast(MainActivity.this, "再按一次退出", Toast.LENGTH_SHORT);
            exittime = System.currentTimeMillis();
        }
    } 

}