Base fragment to ensure the parent activity implements a contract interface.

ContractFragment.java

/* Base fragment to ensure the parent activity implements a contract interface. */
public abstract class ContractFragment<T> extends Fragment {
    private T mContract;

    @Override
    public void onAttach(Activity activity) {
        try {
            mContract = (T)activity;
        } catch (ClassCastException e) {
            throw new IllegalStateException(activity.getClass().getSimpleName()
                + " does not implement " + getClass().getSimpleName() + "'s contract interface.", e);
        }
        super.onAttach(activity);
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mContract = null;
    }

    public final T getContract() {
        return mContract;
    }
}

/* Example fragment showing usage of ContractFragment. */
public class MyCoolFragment extends ContractFragment<MyCoolFragment.Contract> {
    public interface Contract {
        void sayHi(String message);
    }
    
    public void somethingHappened() {
        getContract().sayHi("Hi, Mom!");
    }
}

Isn't it easier and more straightforward to use Otto? Could you comment on the use cases where to use this instead of an event bus?

An example is in the EU4U sample code from The Busy Coder's Guide to Android Development
https://commonsware.com/Android

onAttach(Activity activity) is now deprecated in favour of onAttach(Context context). Does that impact this pattern? Especially considering the use of ContextWrapper.

Hey guys! I use this concept so heavily that I packaged my base ContractFragmet and ContractDialogFragment implementations into a small library. I also extended this pattern to support parent fragment implementations of the contract as well. You can find instructions on how to add this in gradle here:

https://github.com/marksalpeter/contract-fragment

Let me know what you think :)

Shocked to see mContract in @JakeWharton's code. I thought he is totally against Hungarian notation..

Oh I see, that was 7 years ago. Forgivable! :--)