Question

I am creating a Fragment FragParent displaying several input-fields, one of which is a sub-Fragment frag, with specialized audio controls.

The layouts of both fragments are loaded from XML in their onCreateView() methods.
Code I am using is below, but I have some questions about it:

A) is it OK to replace() the sub-Fragment if it is already returned by findFragmentByTag(), or is that an unnecessary step?

B) can the sub-Fragment be instantiated and replaced in the layout, before the parent Fragment completes the inflate() call?

<!-- language: lang-java  -->
// Load sub-Fragment with audio UI.
String fragStr = "fragment_audio_str";
int fragView = R.id.frag_audio_frame_layout;
FragmentManager fm = getChildFragmentManager();
FragmentTransaction fmt = fm.beginTransaction();
Fragment frag = (Fragment)  fm.findFragmentByTag(fragStr);
if (null == frag) {
    frag = new Fragment();
}
fmt.replace(fragView, frag, fragStr); 
fmt.commit();

return inflater.inflate(R.layout.fragment_parent, container, false);
} // END onCreateView() of FragParent

Answer 1

A) is it OK to replace() the sub-Fragment if it is already returned by findFragmentByTag(), or is that an unnecessary step?

No, it shouldn’t be necessary to replace the fragment if it is already there. However you need to replace your child fragment if it isn’t there.

B) can the sub-Fragment be instantiated and replaced in the layout, before the parent Fragment completes the inflate() call?

No, you need to inflate the parent Fragment layout and then add your child Fragment to it before returning the parent’s View.

Try something like this:

        View parentFragLayout = inflater.inflate(R.layout.fragment_parent, container, false);
        FrameLayout childFragContainer = (FrameLayout) parentFragLayout.findViewById(R.id.frag_audio_frame_layout);

        // Load sub-Fragment with audio UI.
        String fragStr = "fragment_audio_str";
        FragmentManager fm = getChildFragmentManager();
        Fragment frag = fm.findFragmentByTag(fragStr);

        // Child fragment isn't there, so add it
        if (frag == null) {
            frag = new Fragment();
            FragmentTransaction fmt = fm.beginTransaction();
            fmt.replace(childFragContainer.getId(), frag, fragStr);
            fmt.commit();
        }
        return parentFragLayout;