How To Check Typed Callback Type Inside Fragment.onAttach()

I'm trying to implement abstract fragment with typed callback to use it in several subclasses. How can I check if Context is instance of appropriate class? My code of abstact Callb

Solution 1:

mCallback = (C) context; //this line not seems to throw any exception

this call will never throw an Exception. During Runtime, your C is replaced with Object(that's called Type-Erasure) - and everything is an Object. Therefore you can assign anything at this point.

To have the exception (or at least error-determination) at the point, where you need it, you can use:

public abstract class CallbackFragment<C> extends Fragment {

    protected C mCallback;
    protected Class<C> callbackClass;

    public CallbackFragment(Class<C> clazz) {
       this.callbackClass = clazz;
    }

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

        //just in case
        if(context == null)
            throw new NullPointerException();

        if (clazz.isAssignableFrom(context.getClass()){
            mCallback = (C) context;  
        }else{
           //oops
        }
    }
}

ofc. then your FragmentCreation would change from

 CallbackFragment<Something> fragment = new CallbackFragment<Something>();

to

CallbackFragment<Something> fragment = new CallbackFragment<Something>(Something.class);

It's a little different, but allows you to keep track of the actual type at any time, bypassing the Type-Erasure.

ps.: For Inherited classes, you can do it more generic:

public abstract class CallbackFragment<C> extends Fragment {
    protected Class<C> callbackClass;

    public CallbackFragment() {
          this.callbackClass = (Class<C>) ((ParameterizedType) getClass()
                        .getGenericSuperclass()).getActualTypeArguments()[0];;
     }
}

public class CallbackFragmentOfSomething extends <CallbackFragment<Something>>{

}

This only fails, if your actual class is not defined due to inheritance, but "on the fly":

CallbackFragment<Something> fragment = new CallbackFragment<Something>();

(Everything untested / no copy paste, but should be somewhat accurate)