Android use backstack with fragments

Question

Today I got another problem using android.

First of all i have an app which has a login activity. so when my credentials are correct, i start a new activity using an intent. so now you see a tabbar. when i call my onTabSelected() method i remove the current fragment and show for example a gridview. if i tap an element in this gridview the fragment is replaced and my app dives deeper into another fragment (an expandable list with another details fragment).

So my problem is if I tap the back button now, the app returns to my very first activity (the login activity) and not to the last fragment which should be the grid view.

public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {
    FragmentManager fm = getFragmentManager();
    Fragment frag = fm.findFragmentById(R.id.container);
    FragmentTransaction tr = fm.beginTransaction();


    switch (tab.getPosition()) {
    case 1: {//Produkte
        ProdukteGrid pg = new ProdukteGrid();           

        if(frag != null)
            tr.remove(frag);

        tr.add(R.id.container, pg,pg.getTAG());
        tr.addToBackStack(pg.getTAG());
    }

... my grid:

public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {

    View v = inflater.inflate(R.layout.produkte_gridmain, container, false);

    mGV = (GridView) v.findViewById(R.id.gridView1);
    mGV.setAdapter(new ImageAdapter(getActivity(), CATEGORY));

    mGV.setOnItemClickListener(new OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View v,
                int position, long id) {
            FragmentManager fm = getFragmentManager();
            Fragment frag = fm.findFragmentById(R.id.container);
            FragmentTransaction tr = fm.beginTransaction();
            switch (position) {
            case 0: {
                // startActivity(new Intent(MyGrid.this,
                // MainActivity.class));
                ProdukteList f = new ProdukteList();

                tr.remove(frag);
                tr.add(R.id.container, f,f.getTAG());
                tr.addToBackStack(f.getTAG());
            }
                break;
            }
            tr.commit();
        }
    });
    return v;
}

My main layout

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity" >

</FrameLayout>

I hope you can help me..:)

EDIT

for better understanding i added a picture:

Answer 1

You should use replace instead of remove and add fragment

Instead of this :

tr.remove(frag);
tr.add(R.id.container, f,f.getTAG());

Use this :

tr.replace(R.id.container, f,f.getTAG());

And also check if you override onBackPressed event on your activity. Check code there make sure you are not finishing activity manually.

Answer 2

If it can helps you, usually i do my transitions like this :

AlbumGridFragment fragment = new AlbumGridFragment();
getSupportFragmentManager().beginTransaction().setCustomAnimations(R.animator.anim_back_right, R.animator.anim_back_left, R.animator.anim_left, R.animator.anim_right)
                .add(R.id.album_fragment, fragment).addToBackStack(MainFragmentActivity.ALBUM_GRID_TAG).commit();

and the BackTrack for example :

        FragmentManager fragmentManager = getSupportFragmentManager();
        int stackSize = fragmentManager.getBackStackEntryCount();
        if (stackSize == 0) {
            finish();
        }
        else {
            String fragmentTag = fragmentManager.getBackStackEntryAt(stackSize - 1).getName();
            fragmentManager.popBackStack(fragmentTag, FragmentManager.POP_BACK_STACK_INCLUSIVE);
        }

Answer 3

Instead of remove and add try replacing a fragment:

   fragmentTransaction.replace(..............);
    transaction.addToBackStack(null);
    fragmentTransaction.commit();