android - GetFragmentManager.findFragmentByTag() returns null

Question

        getFragmentManager().beginTransaction()
                .replace(R.id.graph_fragment_holder, new GraphFragment(), "GRAPH_FRAGMENT")
                .commit();

        getFragmentManager().beginTransaction()
                .replace(R.id.list_fragment_holder, new ListFragment(), "LIST_FRAGMENT")
                .commit();

        //getFragmentManager().executePendingTransactions();

        GraphFragment graphFragment = (GraphFragment) getFragmentManager().findFragmentByTag("GRAPH_FRAGMENT");
        graphFragment.setData(data);

        ListFragment listFragment = (ListFragment) getFragmentManager().findFragmentByTag("LIST_FRAGMENT");
        listFragment.setData(data);

I've supplied a tag so I'm not sure why findFragmentByTag() returns null.

What I've tried from reading other questions:

1. this.setRetainInstance(true) in the oncreate of both fragments.

2. Both fragment constructors are empty public fragmentName(){}.

3. tried executePendingTransactions after adding the fragments.

4. tried add instead of replace on the fragments (edited)

Answer 1

I was confused about this for a long time. First, you need to save the fragment you are replacing by pushing it onto the back stack. The tag you supply is put on the fragment you are adding, not the one you are pushing onto the back stack. Later, when you do push it onto the back stack, that tag goes with it. Here's code with objects broken out to make it easier to trace. You must call 'addToBackStack' before 'commit'.

GraphFragment grFrag = new GraphFragment();
FragmentTransaction tr = getSupportFragmentManager().beginTransaction();
tr.replace(R.id.fragment_container, grFrag, "GRAPH_FRAGMENT");
// grFrag is about to become the current fragment, with the tag "GRAPH_FRAGMENT"
tr.addToBackStack(null);
// 'addToBackStack' also takes a string, which can be null, but this is not the tag
tr.commit();
// any previous fragment has now been pushed to the back stack, with it's tag

ListFragment liFrag = new ListFragment();
FragmentTransaction tr = getSupportFragmentManager().beginTransaction();
tr.replace(R.id.fragment_container, liFrag, "LIST_FRAGMENT");
// liFrag is is about to become the current fragment, with the tag "LIST_FRAGMENT"
tr.addToBackStack(null);
tr.commit();
// 'grFrag' has now been pushed to the back stack, with it's tag being "GRAPH_FRAGMENT"