android - 如何创建 ViewModel 并使用 dagger 2 向其注入(inject)存储库？

Question

我尝试理解 ViewModel。我创建 ViewModel:

public class UsersViewModel extends ViewModel {

    private final UsersRepository usersRepository;

    public UsersViewModel(UsersRepository usersRepository) {
        this.usersRepository = usersRepository;
    }

    public LiveData<List<User>> loadAll() {
        return usersRepository.getAll();
    }

}

但我不明白两件事:

1. 如何将 UsersRepository 注入(inject)此 VievModel？当我使用 Presenter 时，我可以像这样用 Dagger 2 创建它:

@Module
public class PresentersModule {

    @Singleton
    @Provides
    UsersPresenter provideUsersPresenter(UsersRepository usersRepository) {
        return new UsersPresenter(usersRepository);
    }
}

但是我如何使用 ViewModel 来做到这一点？像这样？

@Module
public class ViewModelsModule {

    @Singleton
    @Provides
    UsersViewModel provideUsersViewModel(UsersRepository usersRepository) {
        return new UsersViewModel(usersRepository);
    }
}

1. 如何在 fragment 中获取此 ViewModel？有了 Presenter，我可以做到这一点:

   presenter = MyApplication.get().getAppComponent().getUsersPresenter();

Answer 1

ViewModel 是通过 ViewModelProvider 创建的，它使用 ViewModelFactory 来创建实例。您不能直接注入(inject) ViewModel，而应该使用如下所示的自定义工厂

@Singleton
public class DaggerViewModelFactory implements ViewModelProvider.Factory {
private final Map<Class<? extends ViewModel>, Provider<ViewModel>> creators;

@Inject
public DaggerViewModelFactory(Map<Class<? extends ViewModel>, Provider<ViewModel>> creators) {
    this.creators = creators;
}

@SuppressWarnings("unchecked")
@Override
public <T extends ViewModel> T create(Class<T> modelClass) {
    Provider<? extends ViewModel> creator = creators.get(modelClass);
    if (creator == null) {
        for (Map.Entry<Class<? extends ViewModel>, Provider<ViewModel>> entry : creators.entrySet()) {
            if (modelClass.isAssignableFrom(entry.getKey())) {
                creator = entry.getValue();
                break;
            }
        }
    }
    if (creator == null) {
        throw new IllegalArgumentException("unknown model class " + modelClass);
    }
    try {
        return (T) creator.get();
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}
}

然后您需要一个用于创建 View 模型工厂和 View 模型本身的 Dagger 模块。

@Module
abstract class ViewModelModule {
@Binds
abstract ViewModelProvider.Factory bindViewModelFactory(DaggerViewModelFactory factory);

@Binds
@IntoMap
@ViewModelKey(VideoListViewModel.class)
abstract ViewModel provideVideoListViewModel(VideoListViewModel videoListViewModel);

@Binds
@IntoMap
@ViewModelKey(PlayerViewModel.class)
abstract ViewModel providePlayerViewModel(PlayerViewModel playerViewModel);

@Binds
@IntoMap
@ViewModelKey(PlaylistViewModel.class)
abstract ViewModel providePlaylistViewModel(PlaylistViewModel playlistViewModel);

@Binds
@IntoMap
@ViewModelKey(PlaylistDetailViewModel.class)
abstract ViewModel providePlaylistDetailViewModel(PlaylistDetailViewModel playlistDetailViewModel);
}

ViewModelKey文件是这样的

@Target({ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@MapKey
@interface ViewModelKey {
    Class<? extends ViewModel> value();
}

现在要在 Activity 或 fragment 中获取 View 模型，只需注入(inject) View 模型工厂，然后使用该工厂创建 View 模型实例

public class PlayerActivity extends BaseActivity {
     @Inject DaggerViewModelFactory viewModelFactory;
     PlayerViewModel viewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        AndroidInjection.inject(this);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_player);
        viewModel = ViewModelProviders.of(this,viewModelFactory).get(PlayerViewModel.class);

}

要向您的 ViewModel 注入(inject)任何东西，例如存储库，只需使用构造函数注入(inject)。

public class PlayerViewModel extends ViewModel {
    private VideoRepository videoRepository;
    private AudioManager audioManager;


    @Inject
    public PlayerViewModel(VideoRepository videoRepository, AudioManager audioManager) {
         this.videoRepository = videoRepository;
         this.audioManager = audioManager;

    }
}

从这里查看完整的工作示例 https://github.com/alzahm/VideoPlayer ，我还从谷歌样本中学到了很多 Dagger 的东西，你也可以检查一下。