Android ViewModel 附加参数

Question

除了应用程序上下文之外，是否可以将其他参数传递给我的自定义 AndroidViewModel 构造函数。示例：

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);
    }
}

当我想使用我的自定义 ViewModel class 我在我的片段中使用此代码：

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

所以我不知道如何将附加参数 String param 传递到我的自定义 ViewModel 中。我只能传递应用程序上下文，但不能传递其他参数。我真的很感激任何帮助。谢谢你。

编辑：我添加了一些代码。希望现在好点了。

Answer 1

您的 ViewModel 需要一个工厂 class。

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，您可以这样做：

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

对于 kotlin，你可以使用委托属性:

val viewModel: MyViewModel by viewModels { MyViewModelFactory(getApplication(), "my awesome param") }

还有另一个新选项 - 实现 HasDefaultViewModelProviderFactory 并用你的工厂实例化覆盖 getDefaultViewModelProviderFactory() 然后你可以在没有工厂的情况下调用 ViewModelProvider(this) 或 by viewModels() .

Answer 2

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);
        }
    }
}

并实例化视图模型：

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 3

我写了一个库，它应该使这样做更直接、更清晰，不需要多重绑定或工厂样板，同时与 ViewModel 参数无缝工作，这些参数可以由 Dagger 作为依赖项提供： https://github.com/radutopor/ViewModelFactory

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

    val greeting = MutableLiveData<String>()

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

在视图中：

class UserActivity : AppCompatActivity() {
    @Inject
    lateinit var userViewModelFactory2: UserViewModelFactory2

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_user)
        appComponent.inject(this)

        val userId = intent.getIntExtra("USER_ID", -1)
        val viewModel = ViewModelProviders.of(this, userViewModelFactory2.create(userId))
            .get(UserViewModel::class.java)

        viewModel.greeting.observe(this, Observer { greetingText ->
            greetingTextView.text = greetingText
        })
    }
}

Answer 4

(KOTLIN) 我的解决方案使用了一点反射。

假设您不想在每次创建需要一些参数的新 ViewModel class 时都创建外观相同的工厂 class。您可以通过反射来完成此操作。

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)
    }
}

以及这些活动的 ViewModel：

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

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

然后是神奇的部分，工厂 class 的实现：

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 5

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：

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

Answer 6

我把它设为class，其中传递了已创建的对象。

private Map<String, ViewModel> viewModelMap;

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

public void add(ViewModel viewModel) {
    viewModelMap.put(viewModel.getClass().getCanonicalName(), viewModel);
}

@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.getCanonicalName())) {
            return (T) viewModel.getValue();
        }
    }
    return null;
}

然后

ViewModelFactory viewModelFactory = new ViewModelFactory();
viewModelFactory.add(new SampleViewModel(arg1, arg2));
SampleViewModel sampleViewModel = ViewModelProviders.of(this, viewModelFactory).get(SampleViewModel.class);

Answer 7

使用依赖注入实现

这更高级，更适合生产代码。

Dagger2,方块AssistedInject offers a production-ready implementation for ViewModels that can inject necessary components such as a repository that handles network and database requests. It also allows for the manual injection of arguments/parameters in the activity/fragment. Here's a concise outline of the steps to implement with code Gists based on Gabor Varadi's detailed post, Dagger Tips.

Dagger Hilt 是下一代解决方案，自 2020 年 7 月 12 日起处于 alpha 状态，一旦库可用，便会提供相同的用例和更简单的设置处于发布状态。

在 Kotlin

中使用 Lifecycle 2.2.0 实现

路过Arguments/Parameters

// 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") } 
}

使用 Arguments/Parameters

启用 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 8

基于@vilpe89 AndroidViewModel案例的上述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
}

然后片段可以启动 viewModel 为

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

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

    // ...

}

然后是实际的 ViewModel class

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 9

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Activity

中调用Viewmodel

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

更多参考：Android MVVM Kotlin Example

Answer 10

在Kotlin中，由于ViewModel的调用者和ViewModel本身运行在不同的协程中，所以使用[=13在它们之间传递数据更自然方便=]:

class NewViewModel : ViewModel() {
    private val newData: MutableLiveData<Service.DataEntry?> by lazy {
        MutableLiveData<Service.DataEntry?>().also {
            viewModelScope.launch {
                val channel = Service.ParamChannel   // type Channel<Params>
                val params = channel.receive()
                it.value = Service.postSomething(params)
            }
        }
    }

    fun getData(): LiveData<Service.DataEntry?> {
        return newData
    }
}

// Calling code:
val model: NewViewModel by viewModels()
model.getData().observe(this) { newData ->
    if (newData != null) {
        ...
    }
    else
    {
        ...
    }
}
runBlocking {
    Service.ParamChannel.send(theParams)
}

这是工作代码的一部分，出于演示目的我将其匿名化。

Answer 11

正确的方法是使用依赖注入框架，例如Dagger hilt。如果不使用 DI 框架，则使用 ViewModelFactory。

带匕首柄：

带参数的 ViewModel

@HiltViewModel
class MyViewModel @Inject constructor(
    private val myRepository: MyRepository,
    private val savedStateHandle: SavedStateHandle
) : ViewModel() { ... }

一个存储库

class MyRepository @Inject constructor(
    private val myRemoteDataSource: MyDataSource,
    private val ioDispatcher: CoroutineDispatcher = Dispatchers.IO
) { ... }

一个提供 dependencies/parameters 的模块，因此可以将它们注入存储库和 ViewModel。

@InstallIn(ViewModelComponent::class)
@Module
object MyProvideModule {
    @Provides
    fun provideMyDataSource(@ApplicationContext context: Context): MyDataSource {
        //code to create MyDataSource... 
        return MyDataSource(context)
    }

    @Provides
    fun provideCoroutineDispatcher(): CoroutineDispatcher {
        return Dispatchers.IO
    }
}

用于绑定存储库的模块

@Module
@InstallIn(ViewModelComponent::class)
interface RepositoryModules {
    @Binds
    fun provideMyRepository(repository: MyRepository): MyRepository
}

使用带有@HiltAndroidApp 注释的应用程序启动 Dagger 刀柄。

@HiltAndroidApp
class MainApplication : Application() {

    override fun onCreate() {
        super.onCreate()
    }

}

在活动中获取 ViewModel

@AndroidEntryPoint
class MainActivity : AppCompatActivity() {
  private val myViewModel: MyViewModel by viewModels()
  // Other code...
}

在片段中获取 ViewModel

@AndroidEntryPoint
class MyFragment : Fragment() {
  private val myViewModel: MyViewModel by activityViewModels()
  // Other code...
}

与 ViewModelFactory:

带有参数 messageDataStore 的 ViewModel，其中 MessageDataStore 是一个 DataStore class 或者它可以是您想要传递给 ViewModel 的任何其他内容。

class MyViewModel(
    private val messageDataStore: MessageDataStore,
): ViewModel() { ... }

用于创建 ViewModel 的 ViewModel 工厂 class

/**
 * Factory for all ViewModels.
 */
@Suppress("UNCHECKED_CAST")
class ViewModelFactory constructor(
    private val messageDataStore: MessageDataStore,
    owner: SavedStateRegistryOwner,
    defaultArgs: Bundle? = null
) : AbstractSavedStateViewModelFactory(owner, defaultArgs) {
    override fun <T : ViewModel> create(
        key: String,
        modelClass: Class<T>,
        handle: SavedStateHandle
    ) = with(modelClass) {
        when {
            isAssignableFrom(MyViewModel::class.java) ->
                MyViewModel(messageDataStore)
            else ->
                throw IllegalArgumentException("Unknown ViewModel class: ${modelClass.name}")
        }
    } as T
}

用于创建 dependencies/parameters

的应用程序 class

class MyApp : Application() {
    val messageDataStore: MessageDataStore
        get() = MessageDataStore.getInstance(this)

}

在活动和片段中获取工厂 class 的扩展函数，MyExt.kt

fun AppCompatActivity.getViewModelFactory(savedInstanceState: Bundle?): ViewModelFactory {
    val messageDataStore = (applicationContext as MyApp).messageDataStore
    return ViewModelFactory(messageDataStore, this, savedInstanceState)
}

fun Fragment.getViewModelFactory(savedInstanceState: Bundle?): ViewModelFactory {
    val messageDataStore = (requireContext().applicationContext as MyApp).messageDataStore
    return ViewModelFactory(messageDataStore, this.requireActivity(), savedInstanceState)
}

在活动中获取ViewMode

class MainActivity : AppCompatActivity() {

  private lateinit var myViewModel: MyViewModel
  // Other code...

  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    val vm by viewModels<MyViewModel> { getViewModelFactory(savedInstanceState) }
    myViewModel = vm
    // Other code...
  }
}

在片段中获取 ViewModel。

class MyFragment : Fragment() {
    private lateinit var myViewModel: MyViewModel
    //Other code...

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
      val vm by activityViewModels<MyViewModel> { getViewModelFactory(savedInstanceState) }
      myViewModel = vm
      //Other code...
  }
}

Android ViewModel 附加参数

Android ViewModel additional arguments

android

mvvm

viewmodel

android-components

使用依赖注入实现

在 Kotlin

路过Arguments/Parameters

使用 Arguments/Parameters