MVP

interface View {
    void displayPhotos(List<Photo> photos);
    void displayError();
}

interface Presenter {
    void onViewCreated();
    void loadPhotosAtPage(Integer page);
    void onPhotosLoaded(List<Photo> photos);
    void onLoadFailure();
    void onViewDestroyed();
}

interface Model {
    void loadPhotos(Integer page);
}


class PhotosPresenter implements Presenter {

    private Model model;
    private WeakReference<View> view;

    private Integer currentPage = 1;

    PhotosPresenter(View view) {
        this.view = new WeakReference(view);
        this.model = new UnsplashService(this);
    }

    void onViewCreated() {
        model.loadPhotos(currentPage);
    }

    void loadPhotosAtPage(Integer page) {
        model.loadPhotos(page);
    }

    void onPhotosLoaded(List<Photo> photos) {
        if (viewExists()) {
            view.get().displayPhotos(photos);
        }
    }

    void onLoadFailure() {
        if (viewExists()) {
            view.get().displayError();
        }
    }

    void onViewDestroyed() {
        view = null;
    }

    private boolean viewExists() {
        return view != null && view.get() != null;
    }

}


class WPFRagment extends Fragment implements View {

    private PhotoAdapter adapter;
    private RecyclerView recyclerView;
    private Presenter presenter;

    ...

    @Override
    void onCreateView(...) {
        ...
        presenter = new PhotosPresenter(this);
        // configure recyclerView and adapter here
        ...
        presenter.onViewCreated();
    }

    @Override
    void onDestroy() { // или как там фрагменты убиваются
        presenter.onViewDestroyed();
    }

    void displayPhotos(List<Photo> photos) {
        adapter.addPhotos(photos);
        adapter.notifyDatasetChanged();
    }

    void displayError() {
        // handle error UI here
    }

}

class UnsplashService implements Model {

    private Presenter presenter;
    private PhotosEndPoint photosEndPoint;

    UnsplashService(Presenter presenter) {
        this.presenter = presenter;
        ...
    }

    void loadPhotos(Integer page) {
        Call<List<Photo>> call = photosEndPoint.getPhotos(USER_ID, page, ORIENTATION);
        call.enqueue(new Callback<List<Photo>>() {

            @Override
            public void onResponse(Call<List<Photo>> call, Response<List<Photo>> response) {
                if (response.isSuccessful()) {
                    presenter.onPhotosLoaded(response.body()); // или как там получать данные из ответа
                } else {
                    // something went wrong
                    presenter.onLoadFailure();
                }
            }

            @Override
            public void onFailure(Call<List<Photo>> call, Throwable t) {
                // handle failure here
                presenter.onLoadFailure();
            }

        });
    }

}