Android: how to notify Activity when Fragments views are ready?

If you want to do it without any listener:

Add Fragment with a TAG

 supportFragmentManager
                .beginTransaction()
                .add(R.id.pagerContainer, UniversalWebViewFragment.newInstance(UniversalWebViewFragment.YOUTUBE_SERACH_URL+"HD trailers"), 
                "UniversalWebView")
                .disallowAddToBackStack()
                .commit()

Create a public method in Hosting Activity class which you want to call after fragment is loaded. Here I am calling back a method of my fragment for example

public fun loadURL() {
        val webViewFragment = supportFragmentManager
                              .findFragmentByTag("UniversalWebView") 
                               as UniversalWebViewFragment

        webViewFragment.searchOnYoutube("Crysis Warhead")
    }

Now inside onViewCreated Method of your fragment you can simply call that public method of Host activity like this:

    (activity as HomeActivity ).loadURL()

In addition to Kirk's answer: since public void onAttach(Activity activity) is deprecated you can now simply use:

@Override
public void onAttach(Context context) {
    super.onAttach(context);

    Activity activity;

    if (context instanceof Activity){

        activity=(Activity) context;

        try {
            this.mListener = (OnCompleteListener)activity;
        } catch (final ClassCastException e) {
            throw new ClassCastException(activity.toString() + " must implement OnCompleteListener");
        }
    }
}

The rest remains the same... One might want to use (Fragment sender) as a parameter though and always pass this.

My preferred method is to use a callback to get a signal from a Fragment. Also, this is the recommended method proposed by Android at Communicating with the Activity

For your example, in your Fragment, add an interface and register it.

public static interface OnCompleteListener {
    public abstract void onComplete();
}

private OnCompleteListener mListener;

public void onAttach(Context context) {
    super.onAttach(context);
    try {
        this.mListener = (OnCompleteListener)context;
    }
    catch (final ClassCastException e) {
        throw new ClassCastException(context.toString() + " must implement OnCompleteListener");
    }
}

Now implement this interface in your Activity

public class MyActivity extends FragmentActivity implements MyFragment.OnCompleteListener {
    //...

    public void onComplete() {
        // After the fragment completes, it calls this callback.
        // setup the rest of your layout now
        mMapFragment.getMap()
    }
}

Now, whatever in your Fragment signifies that it's loaded, notify your Activity that it's ready.

@Override
protected void onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    // create your fragment
    //...

    // signal that you're done and tell the Actvity to call getMap()
    mListener.onComplete();
}

EDIT 2017-12-05 onAttach(Activity activity) is deprecated, use onAttach(Context context) instead. Code above adjusted.