How can I control the activity's up button from a contained fragment?

Question

I'm building a pretty simple app. At it's core are 2 screens:
1) list-screen: a list of items
2) detail-screen: a detailed view of an item

I used one Activity (which extends AppCompatActivity) with an Action-Bar, a Navigation-Drawer and a main-content part (a FrameLayout).
I used 2 different fragments for the 2 screens:

When opening the app I inflate the list-fragment into the main-content part.
When an item in the list is clicked I inflate the detail-fragment into the main-content part and it all works well.
On the detail-screen I want the Action-Bar to display an up-button that goes back to the list-screen.
Considering the fact that I am using fragments, rather than separate activites, how can I achieve that?

Answer 1

You can enable the up button each time the Detail Fragment loads, and disable it whenever any of the other Fragments load.

First define these methods in your Activity, which you can call from the Fragments in order to show/hide the up button:

public void showUpButton() {
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
}

public void hideUpButton() {
    getSupportActionBar().setDisplayHomeAsUpEnabled(false);
}

You will probably want to just enable the up button in on Resume() of the detail Fragment (MainActivity is just an example, change to the name of your Activity) .....

@Override
public void onResume() {
    super.onResume();
    MainActivity activity = (MainActivity)getActivity();
    if (activity != null) {
      activity.showUpButton();
    }

}

Then in the other Fragments:

@Override
public void onResume() {
    super.onResume();
    MainActivity activity = (MainActivity)getActivity();
    if (activity != null) {
      activity.hideUpButton();
    }

}

The next thing is to make the up button actually go back. First ensure that you're adding the Fragment with the up button to the back stack, and then add this to that Fragment.

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    case android.R.id.home:
        ((MainActivity)getActivity()).onBackPressed();
        return true;
    default:
        return super.onOptionsItemSelected(item);
    }
}

Then in the Activity, override onBackPressed() and pop from the back stack if the FragmentManager has any entries:

@Override
public void onBackPressed() {

    FragmentManager fragmentManager = getSupportFragmentManager();
    if (fragmentManager.getBackStackEntryCount() > 1) {
        fragmentManager.popBackStackImmediate();
    } else {
        super.onBackPressed();
    }
}