依赖注入的实现

Question

有没有办法将其他参数传递给我的自定义AndroidViewModel构造函数，除了Application上下文。例如：

public class MyViewModel extends AndroidViewModel {
    private final LiveData<List<MyObject>> myObjectList;
    private AppDatabase appDatabase;

    public MyViewModel(Application application, String param) {
        super(application);
        appDatabase = AppDatabase.getDatabase(this.getApplication());

        myObjectList = appDatabase.myOjectModel().getMyObjectByParam(param);
    }
}

当我想要使用我的自定义ViewModel类时，我在我的片段中使用此代码：

MyViewModel myViewModel = ViewModelProvider.of(this).get(MyViewModel.class)

因此，我不知道如何将其他参数String param传递到我的自定义ViewModel。我只能传递Application上下文，但不能传递其他参数。我真的很感激任何帮助。谢谢。

编辑：我添加了一些代码。我希望现在好多了。

Answer 1

您需要为ViewModel设置工厂类。

public class MyViewModelFactory implements ViewModelProvider.Factory {
    private Application mApplication;
    private String mParam;


    public MyViewModelFactory(Application application, String param) {
        mApplication = application;
        mParam = param;
    }


    @Override
    public <T extends ViewModel> T create(Class<T> modelClass) {
        return (T) new MyViewModel(mApplication, mParam);
    }
}

在实例化视图模型时，你会这样做：

MyViewModel myViewModel = ViewModelProviders.of(this, new MyViewModelFactory(this.getApplication(), "my awesome param")).get(MyViewModel.class);

Answer 2

依赖注入的实现

此功能更先进，更适合生产代码。

Dagger2 ，Square的AssistedInject提供了ViewModels的生产就绪实现，该ViewModels可以注入必要的组件，例如处理网络和数据库请求的存储库。它还允许在活动/片段中手动插入参数/参数。这是steps to implement的简要概述，其中的代码Gists基于Gabor Varadi的详细帖子Dagger Tips。

Dagger Hilt 是下一代解决方案，自20年7月12日开始使用alpha版本，一旦库处于发布状态，即可通过更简单的设置提供相同的用例。< / p>

在Kotlin中使用Lifecycle 2.2.0

传递参数/参数

// Override ViewModelProvider.NewInstanceFactory to create the ViewModel (VM).
class SomeViewModelFactory(private val someString: String): ViewModelProvider.NewInstanceFactory() {
    override fun <T : ViewModel?> create(modelClass: Class<T>): T = SomeViewModel(someString) as T
} 

class SomeViewModel(private val someString: String) : ViewModel() {
    init {
        //TODO: Use 'someString' to init process when VM is created. i.e. Get data request.
    }
}

class Fragment: Fragment() {
    // Create VM in activity/fragment with VM factory.
    val someViewModel: SomeViewModel by viewModels { SomeViewModelFactory("someString") } 
}

启用带有参数/参数的SavedState

class SomeViewModelFactory(
        private val owner: SavedStateRegistryOwner,
        private val someString: String) : AbstractSavedStateViewModelFactory(owner, null) {
    override fun <T : ViewModel?> create(key: String, modelClass: Class<T>, state: SavedStateHandle) =
            SomeViewModel(state, someString) as T
}

class SomeViewModel(private val state: SavedStateHandle, private val someString: String) : ViewModel() {
    val feedPosition = state.get<Int>(FEED_POSITION_KEY).let { position ->
        if (position == null) 0 else position
    }
        
    init {
        //TODO: Use 'someString' to init process when VM is created. i.e. Get data request.
    }
        
     fun saveFeedPosition(position: Int) {
        state.set(FEED_POSITION_KEY, position)
    }
}

class Fragment: Fragment() {
    // Create VM in activity/fragment with VM factory.
    val someViewModel: SomeViewModel by viewModels { SomeViewModelFactory(this, "someString") } 
    private var feedPosition: Int = 0
     
    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        someViewModel.saveFeedPosition((contentRecyclerView.layoutManager as LinearLayoutManager)
                .findFirstVisibleItemPosition())
    }    
        
    override fun onViewStateRestored(savedInstanceState: Bundle?) {
        super.onViewStateRestored(savedInstanceState)
        feedPosition = someViewModel.feedPosition
    }
}

Answer 3

对于在多个不同视图模型之间共享的一个工厂，我会像这样扩展mlyko的答案：

public class MyViewModelFactory extends ViewModelProvider.NewInstanceFactory {
    private Application mApplication;
    private Object[] mParams;

    public MyViewModelFactory(Application application, Object... params) {
        mApplication = application;
        mParams = params;
    }

    @Override
    public <T extends ViewModel> T create(Class<T> modelClass) {
        if (modelClass == ViewModel1.class) {
            return (T) new ViewModel1(mApplication, (String) mParams[0]);
        } else if (modelClass == ViewModel2.class) {
            return (T) new ViewModel2(mApplication, (Integer) mParams[0]);
        } else if (modelClass == ViewModel3.class) {
            return (T) new ViewModel3(mApplication, (Integer) mParams[0], (String) mParams[1]);
        } else {
            return super.create(modelClass);
        }
    }
}

实例化视图模型：

ViewModel1 vm1 = ViewModelProviders.of(this, new MyViewModelFactory(getApplication(), "something")).get(ViewModel1.class);
ViewModel2 vm2 = ViewModelProviders.of(this, new MyViewModelFactory(getApplication(), 123)).get(ViewModel2.class);
ViewModel3 vm3 = ViewModelProviders.of(this, new MyViewModelFactory(getApplication(), 123, "something")).get(ViewModel3.class);

不同的视图模型具有不同的构造函数。

Answer 4

基于@ vilpe89的上述针对AndroidViewModel案例的Kotlin解决方案

class ExtraParamsViewModelFactory(
    private val application: Application,
    private val myExtraParam: String
): ViewModelProvider.NewInstanceFactory() {
    override fun <T : ViewModel?> create(modelClass: Class<T>): T = 
            SomeViewModel(application, myExtraParam) as T
}

然后片段可以将viewModel初始化为

class SomeFragment : Fragment() {
    
    // ...

    private val myViewModel: SomeViewModel by viewModels {
        ExtraParamsViewModelFactory(this.requireActivity().application, "some string value")
    }

    // ...

}

然后是实际的ViewModel类

class SomeViewModel(application: Application, val myExtraParam:String) : AndroidViewModel(application) {
    // ...
}

或者以某种合适的方法...

override fun onActivityCreated(...){
    // ...
    val myViewModel = ViewModelProvider(this, ExtraParamsViewModelFactory(this.requireActivity().application, "some string value")).get(SomeViewModel::class.java)
    // ...
}

Answer 5

class UserViewModelFactory(private val context: Context) : ViewModelProvider.NewInstanceFactory() {
 
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        return UserViewModel(context) as T
    }
 
}

class UserViewModel(private val context: Context) : ViewModel() {
 
    private var listData = MutableLiveData<ArrayList<User>>()
 
    init{
        val userRepository : UserRepository by lazy {
            UserRepository
        }
        if(context.isInternetAvailable()) {
            listData = userRepository.getMutableLiveData(context)
        }
    }
 
    fun getData() : MutableLiveData<ArrayList<User>>{
        return listData
    }

在“活动”中调用Viewmodel

val userViewModel = ViewModelProviders.of(this,UserViewModelFactory(this)).get(UserViewModel::class.java)

更多参考：Android MVVM Kotlin Example

Answer 6

我写了一个库，该库应该使此操作更直接，更简洁，不需要多重绑定或工厂样板，同时与可以由Dagger作为依赖项提供的ViewModel参数无缝地一起工作： https://github.com/radutopor/ViewModelFactory

Option[Unit]

在视图中：

@ViewModelFactory
class UserViewModel(@Provided repository: Repository, userId: Int) : ViewModel() {

    val greeting = MutableLiveData<String>()

    init {
        val user = repository.getUser(userId)
        greeting.value = "Hello, $user.name"
    }    
}

Answer 7

（科特琳）我的解决方案很少使用反射功能。

让我们说您不想每次创建需要某些参数的新ViewModel类时都创建外观相同的Factory类。您可以通过反射来实现。

例如，您将有两个不同的活动：

class Activity1 : FragmentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val args = Bundle().apply { putString("NAME_KEY", "Vilpe89") }
        val viewModel = ViewModelProviders.of(this, ViewModelWithArgumentsFactory(args))
            .get(ViewModel1::class.java)
    }
}

class Activity2 : FragmentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val args = Bundle().apply { putInt("AGE_KEY", 29) }
        val viewModel = ViewModelProviders.of(this, ViewModelWithArgumentsFactory(args))
            .get(ViewModel2::class.java)
    }
}

这些活动的ViewModel：

class ViewModel1(private val args: Bundle) : ViewModel()

class ViewModel2(private val args: Bundle) : ViewModel()

然后是工厂部分的实现的魔术部分：

class ViewModelWithArgumentsFactory(private val args: Bundle) : NewInstanceFactory() {
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        try {
            val constructor: Constructor<T> = modelClass.getDeclaredConstructor(Bundle::class.java)
            return constructor.newInstance(args)
        } catch (e: Exception) {
            Timber.e(e, "Could not create new instance of class %s", modelClass.canonicalName)
            throw e
        }
    }
}

Answer 8

为什么不这样做：

public class MyViewModel extends AndroidViewModel {
    private final LiveData<List<MyObject>> myObjectList;
    private AppDatabase appDatabase;
    private boolean initialized = false;

    public MyViewModel(Application application) {
        super(application);
    }

    public initialize(String param){
      synchronized ("justInCase") {
         if(! initialized){
          initialized = true;
          appDatabase = AppDatabase.getDatabase(this.getApplication());
          myObjectList = appDatabase.myOjectModel().getMyObjectByParam(param);
    }
   }
  }
}

，然后分两步使用它：

MyViewModel myViewModel = ViewModelProvider.of(this).get(MyViewModel.class)
myViewModel.initialize(param)

Answer 9

我将其作为已创建对象的传递类。

private Map<String, ViewModel> viewModelMap;

public ViewModelFactory() {
    this.viewModelMap = new HashMap<>();
}

public void create(ViewModel viewModel) {
    viewModelMap.put(viewModel.getClass().getSimpleName(), viewModel);
    create(viewModel.getClass());
}

@NonNull
@Override
public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
    for (Map.Entry<String, ViewModel> viewModel : viewModelMap.entrySet()) {
        if (viewModel.getKey().equals(modelClass.getSimpleName())) {
            return (T) viewModel.getValue();
        }
    }
    return null;
}

然后

ViewModelFactory viewModelFactory = new ViewModelFactory();
viewModelFactory.create(new SampleViewModel(Args1, Args2));
SampleViewModel sampleViewModel = ViewModelProviders.of(this, viewModelFactory).get(SampleViewModel.class);

Android ViewModel附加参数

9 个答案:

依赖注入的实现

在Kotlin中使用Lifecycle 2.2.0

传递参数/参数

启用带有参数/参数的SavedState