协助访问Fragment中的一些私有属性，这个类必须在android.support.v4.app包下

FragmentAssist.java

package android.support.v4.app;

import android.app.Activity;

public class FragmentAssist {
    public static Activity getActivity(FragmentManager fragmentManager) {
        if (fragmentManager instanceof FragmentManagerImpl) {
            return ((FragmentManagerImpl) fragmentManager).mActivity;
        } else {
            return null;
        }
    }

    public static FragmentManager getChildFragmentManager(Fragment fragment) {
        return fragment.mChildFragmentManager;
    }

    public static void cleanChildFragmentManager(Fragment fragment){
        fragment.mChildFragmentManager = null;
    }
}