使用 Dagger 2的Android生命周期库ViewModel

Question

我有一个 ViewModel 类，就像 Architecture guide 的 连接 ViewModel 和存储库 部分中定义的类。 .当我运行我的应用程序时，我得到一个运行时异常。有谁知道如何解决这个问题？我不应该注入(inject) ViewModel 吗？有没有办法告诉 ViewModelProvider 使用 Dagger 来创建模型？

public class DispatchActivityModel extends ViewModel {

    private final API api;

    @Inject
    public DispatchActivityModel(API api) {
        this.api = api;
    }
}

Caused by: java.lang.InstantiationException: java.lang.Class has no zero argument constructor at java.lang.Class.newInstance(Native Method) at android.arch.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:143) at android.arch.lifecycle.ViewModelProviders$DefaultFactory.create(ViewModelProviders.java:143)  at android.arch.lifecycle.ViewModelProvider.get(ViewModelProvider.java:128)  at android.arch.lifecycle.ViewModelProvider.get(ViewModelProvider.java:96)  at com.example.base.BaseActivity.onCreate(BaseActivity.java:65)  at com.example.dispatch.DispatchActivity.onCreate(DispatchActivity.java:53)  at android.app.Activity.performCreate(Activity.java:6682)  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2619) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2727)  at android.app.ActivityThread.-wrap12(ActivityThread.java)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1478)  at android.os.Handler.dispatchMessage(Handler.java:102)  at android.os.Looper.loop(Looper.java:154)  at android.app.ActivityThread.main(ActivityThread.java:6121)

Answer 1

您需要实现自己的ViewModelProvider.Factory。 Google 创建了一个示例应用程序，演示了如何将 Dagger 2 与 ViewModels 连接起来。 LINK .你需要这 5 样东西:

在 ViewModel 中:

@Inject
public UserViewModel(UserRepository userRepository, RepoRepository repoRepository) {

定义注释:

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

在 ViewModelModule 中:

@Module
abstract class ViewModelModule {
    @Binds
    @IntoMap
    @ViewModelKey(UserViewModel.class)
    abstract ViewModel bindUserViewModel(UserViewModel userViewModel);

在 fragment 中:

@Inject
ViewModelProvider.Factory viewModelFactory;

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
            userViewModel = ViewModelProviders.of(this, viewModelFactory).get(UserViewModel.class);

工厂:

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

    @Inject
    public GithubViewModelFactory(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);
        }
    }
}