onbackpressed的嵌套片段转到MainActivity而不是后片段

Question

I move From Activity to FagmentA then FragmentA to FragmentB. When Press back Button while staying in FragmentB it goes to MainActivity(It should move to a Back fragment which is FragmentA). what's wrong with it?

Moving from Activity to FragmentA

   Fragment fragment = new FragmentA() ;
            FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
            fragmentTransaction.setCustomAnimations(android.R.anim.fade_in,
                    android.R.anim.fade_out);
            fragmentTransaction.replace(R.id.frame, fragment, CURRENT_TAG);
            fragmentTransaction.addToBackStack(null);
            fragmentTransaction.commitAllowingStateLoss();

Moving FramgentA to FragmentB

 layout.removeAllViewsInLayout();
            Fragment fragment =new FragmentB ;
            FragmentManager fragmentManager = getActivity().getSupportFragmentManager();
            FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
            fragmentTransaction.replace(R.id.frame, fragment);
            fragmentTransaction.addToBackStack(null);
            fragmentTransaction.commit();

Onbackpressed MainActivity

 @Override
public void onBackPressed() {
    android.app.FragmentManager fm = getFragmentManager();
    if(fm.getBackStackEntryCount()>0){
        fm.popBackStack();
    }else{
        super.onBackPressed(); 
    }

Answer 1

Your problem is with your imports of FragmentManager you use both :

android.support.v4.app.FragmentManager and android.app.FragmentManager

change this :

@Override
public void onBackPressed() {
    android.app.FragmentManager fm = getFragmentManager();
    if(fm.getBackStackEntryCount()>0){
        fm.popBackStack();
    }else{
        super.onBackPressed(); 
    }

to:

@Override
public void onBackPressed() {
    FragmentManager fm = getSupportFragmentManager();
    if(fm.getBackStackEntryCount()>0){
        fm.popBackStack();
    }else{
        super.onBackPressed(); 
    }

You only add Fragment s using the support FragmentManager meaning your existing code will always return 0 here -> fm.getBackStackEntryCount() until you use correct support FragmentManager