Your Presenter Might Know View Too Early

When MVP(Model-View-Presenter) is applied in Android, the code is commonly written like below. Contract.Presenter is created in onCreate() and pass the Activity or Fragment implementing the Contract.View.

public interface Contract {
  interface View {
    showLoading();
    showData(List<Data> list);
  }
  interface Presenter {
  }
}

public class SearchActivity extends AppCompatActivity implements Contract.View {
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate();
    mPresenter = new SearchPresenter(this);
  }
}

You can consider this code doesn’t have any problem. However, it depends. According to the Android life cycle, Android View is not created yet. After onViewCreated() is called, we can sure it is created. If Contract.Presenter calls showLoading() before onViewCreated(), it could make a crash. So, I would recommend avoiding passing the Contract.View as a constructor parameter of Contract.Presenter.

Then, how can we set a Contract.View for Contract.Presenter? In the lifecycle, View is alive between onStart() and onStop(). By setting and unsetting the Contract.View at that time, we can avoid accessing View before created and after destroyed.

public interface Contract {
  interface View {
    showLoading();
    showData(List<Data> list);
  }
  interface Presenter {
    void onViewAvailable(Contract.View view);
    void onViewUnavailable();
  }
}

public class SearchActivity extends AppCompatActivity implements Contract.View {
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate();
    mPresenter = new SearchPresenter();
  }

  @Override
  public void onStart() {
    super.onStart();
    mPresenter.onViewAttached(this);
  }

  @Override
  public void onStop() {
    super.onStop();
    mPresenter.onViewDetached();
  }
}

If you are using Dagger, you can do like below.

public class SearchActivity extends AppCompatActivity implements Contract.View {
  @Inject SearchPresenter mPresenter;

  @Override
  public void onStart() {
    super.onStart();
    mPresenter.onViewAttached(this);
  }

  @Override
  public void onStop() {
    super.onStop();
    mPresenter.onViewDetached();
  }
}