AndroidViewModel没有零参数构造函数

我正在关注此文档以了解LiveData和ViewModel.在doc中,ViewModel类具有构造函数,

public class UserModel extends ViewModel {
  private MutableLiveData user;

  @Inject UserModel(MutableLiveData user) {
    this.user = user;
  }

  public void init() {
    if (this.user != null) {
      return;
    }
    this.user = new MutableLiveData<>();
  }

  public MutableLiveData getUser() {
    return user;
  }
}

但是,当我运行代码时,我得到异常:

final UserViewModelviewModel = ViewModelProviders.of(this).get(UserViewModel.class);

引起:java.lang.RuntimeException:无法创建类UserViewModel的实例引起:java.lang.InstantiationException:java.lang.Class没有零参数构造函数

Shahbaz Ahme.. 44

在初始化ViewModelusing的子类时ViewModelProviders,默认情况下它期望您的UserModel类具有零参数构造函数.在你的情况下,你的构造函数有参数MutableLiveData user

解决此问题的一种方法是为您设置默认的无arg构造函数 UserModel

否则,如果您希望ViewModel类具有非零参数构造函数,则可能必须创建一个自定义ViewModelFactory类来初始化ViewModel实例,该实例将实现ViewModelProvider.Factory接口.

我还没有尝试过这个,但是这里有链接到google的优秀样本:github.com/googlesamples/android-architecture-components.具体来说,结帐此类GithubViewModelFactory.java用于Java代码,此类GithubViewModelFactory.kt用于相应的Kotlin代码

在初始化ViewModelusing的子类时ViewModelProviders,默认情况下它期望您的UserModel类具有零参数构造函数.在你的情况下,你的构造函数有参数MutableLiveData user

解决此问题的一种方法是为您设置默认的无arg构造函数 UserModel

否则,如果您希望ViewModel类具有非零参数构造函数,则可能必须创建一个自定义ViewModelFactory类来初始化ViewModel实例,该实例将实现ViewModelProvider.Factory接口.

我还没有尝试过这个,但是这里有链接到google的优秀样本:github.com/googlesamples/android-architecture-components.具体来说,结帐此类GithubViewModelFactory.java用于Java代码,此类GithubViewModelFactory.kt用于相应的Kotlin代码

ViewModelFactory 这将为我们提供一个正确的ViewModel ViewModelModule

public class ViewModelFactory implements ViewModelProvider.Factory {
    private final Map, Provider> viewModels;

    @Inject
    public ViewModelFactory(Map, Provider> viewModels) {
        this.viewModels = viewModels;
    }

    @Override
    public  T create(Class modelClass) {
        Provider viewModelProvider = viewModels.get(modelClass);

        if (viewModelProvider == null) {
            throw new IllegalArgumentException("model class " + modelClass + " not found");
        }

        return (T) viewModelProvider.get();
    }
}

ViewModelModule 负责将所有ViewModel类绑定到
Map, Provider> viewModels

@Module
public abstract class ViewModelModule {

    @Binds
    abstract ViewModelProvider.Factory bindViewModelFactory(ViewModelFactory viewModelFactory); 
    //You are able to declare ViewModelProvider.Factory dependency in another module. For example in ApplicationModule.

    @Binds
    @IntoMap
    @ViewModelKey(UserViewModel.class)
    abstract ViewModel userViewModel(UserViewModel userViewModel);

    //Others ViewModels
}

ViewModelKey 是一个注释,用于在地图中使用作为键,看起来像

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@MapKey
@interface ViewModelKey {
    Class value();
}

现在,您可以创建ViewModel并从图中满足所有必需的依赖项

public class UserViewModel extends ViewModel {
    private UserFacade userFacade;

    @Inject
    public UserViewModel(UserFacade userFacade) { // UserFacade should be defined in one of dagger modules
        this.userFacade = userFacade;
    }
} 

实例化ViewModel

public class MainActivity extends AppCompatActivity {

    @Inject
    ViewModelFactory viewModelFactory;
    UserViewModel userViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ((App) getApplication()).getAppComponent().inject(this);

        userViewModel = ViewModelProviders.of(this, viewModelFactory).get(UserViewModel.class);

    }
}

并且不要伪造添加ViewModelModule到modules列表中

@Singleton
@Component(modules = {ApplicationModule.class, ViewModelModule.class})
public interface ApplicationComponent {
    //
}