Android 通过mTitle修改标题 无效的解决办法

我通过在Fragment回调Activtiy中的方法来设置标题,但是设置后还是显示更改前的标题,重新刷新一下才是更改后的标题,不清楚为什么..

以下代码

MainActivity.java

class MainActivity{    private CharSequence mTitle;    onCreate(){        mTitle = getTitle();    }    public void onSectionAttached(int number) {        mTitle = getString(R.string.title_detail);    }}

DetailFragment.java

class DetailFragment{@Overridepublic void onAttach(Activity activity) {super.onAttach(activity);if (activity instanceof MainActivity) {((MainActivity) activity).onSectionAttached(C.FRAGMENT_DETAIL);}}}

调用onAttach时,标题没有变,

求各位大神帮忙..先mark