使用FadingActionBarHelper，为啥设置了actionbar 的背景会没用？？

 @Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    //View view=inflater.inflate(R.layout.jobwanted_jobsearch,null);
    View view=helper.createView(inflater);
    init(view);
    jobInfos.clear();
    return view;
}

@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    helper=new FadingActionBarHelper()
            .actionBarBackground(R.mipmap.background)
            .headerLayout(R.layout.jobhomepagefragment_ad)
            .contentLayout(R.layout.jobwanted_jobsearch);
    helper.initActionBar(activity);
}
设置了ActionBar的背景，但是就是不显示，这该咋怎啊？？？