创建带有依赖项的 ViewModel   属于 Android Jetpack 的一部分。

遵循依赖注入的最佳实践，ViewModel 可以将其依赖项作为参数在其构造函数中接收。这些类型主要来自域或数据层。因为框架提供 ViewModel，所以需要一个特殊的机制来创建它们的实例。这个机制就是ViewModelProvider.Factory接口。只有**此接口的实现才能在正确的范围内实例化 ViewModel**。

带有 CreationExtras 的 ViewModel

如果ViewModel类在其构造函数中接收依赖项，请提供一个实现ViewModelProvider.Factory接口的工厂。重写create(Class<T>, CreationExtras)函数以提供 ViewModel 的新实例。

CreationExtras允许您访问有助于实例化 ViewModel 的相关信息。以下是可从 extras 访问的键列表

键 功能
ViewModelProvider.NewInstanceFactory.VIEW_MODEL_KEY 提供对传递给ViewModelProvider.get()的自定义键的访问。
ViewModelProvider.AndroidViewModelFactory.APPLICATION_KEY 提供对Application类的实例的访问。
SavedStateHandleSupport.DEFAULT_ARGS_KEY 提供对您应该用于构建SavedStateHandle的参数 Bundle 的访问。
SavedStateHandleSupport.SAVED_STATE_REGISTRY_OWNER_KEY 提供对用于构建ViewModel的SavedStateRegistryOwner的访问。
SavedStateHandleSupport.VIEW_MODEL_STORE_OWNER_KEY 提供对用于构建ViewModel的ViewModelStoreOwner的访问。

要创建SavedStateHandle的新实例，请使用CreationExtras.createSavedStateHandle()函数并将其传递给 ViewModel。

带有 APPLICATION_KEY 的 CreationExtras

以下是提供ViewModel实例的示例，该实例采用对Application类和SavedStateHandle进行范围限定的存储库作为依赖项

Kotlin

    import androidx.lifecycle.SavedStateHandle
    import androidx.lifecycle.ViewModel
    import androidx.lifecycle.ViewModelProvider
    import androidx.lifecycle.ViewModelProvider.AndroidViewModelFactory.Companion.APPLICATION_KEY
    import androidx.lifecycle.createSavedStateHandle
    import androidx.lifecycle.viewmodel.CreationExtras

    class MyViewModel(
        private val myRepository: MyRepository,
        private val savedStateHandle: SavedStateHandle
    ) : ViewModel() {

        // ViewModel logic
        // ...

        // Define ViewModel factory in a companion object
        companion object {

            val Factory: ViewModelProvider.Factory = object : ViewModelProvider.Factory {
                @Suppress("UNCHECKED_CAST")
                override fun <T : ViewModel> create(
                    modelClass: Class<T>,
                    extras: CreationExtras
                ): T {
                    // Get the Application object from extras
                    val application = checkNotNull(extras[APPLICATION_KEY])
                    // Create a SavedStateHandle for this ViewModel from extras
                    val savedStateHandle = extras.createSavedStateHandle()

                    return MyViewModel(
                        (application as MyApplication).myRepository,
                        savedStateHandle
                    ) as T
                }
            }
        }
    }

Java

import static androidx.lifecycle.SavedStateHandleSupport.createSavedStateHandle;
import static androidx.lifecycle.ViewModelProvider.AndroidViewModelFactory.APPLICATION_KEY;

import androidx.lifecycle.SavedStateHandle;
import androidx.lifecycle.ViewModel;
import androidx.lifecycle.viewmodel.ViewModelInitializer;

public class MyViewModel extends ViewModel {

    public MyViewModel(
        MyRepository myRepository,
        SavedStateHandle savedStateHandle
    ) { /* Init ViewModel here */ }

    static final ViewModelInitializer<MyViewModel> initializer = new ViewModelInitializer<>(
        MyViewModel.class,
        creationExtras -> {
            MyApplication app = (MyApplication) creationExtras.get(APPLICATION_KEY);
            assert app != null;
            SavedStateHandle savedStateHandle = createSavedStateHandle(creationExtras);

            return new MyViewModel(app.getMyRepository(), savedStateHandle);
        }
    );
}

然后，您可以在检索 ViewModel 实例时使用此工厂

Kotlin

import androidx.activity.viewModels

class MyActivity : AppCompatActivity() {

    private val viewModel: MyViewModel by viewModels { MyViewModel.Factory }

    // Rest of Activity code
}

Java

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.ViewModelProvider;

public class MyActivity extends AppCompatActivity {

MyViewModel myViewModel = new ViewModelProvider(
    this,
    ViewModelProvider.Factory.from(MyViewModel.initializer)
).get(MyViewModel.class);

// Rest of Activity code
}

Jetpack Compose

import androidx.lifecycle.viewmodel.compose.viewModel

@Composable
fun MyScreen(
    modifier: Modifier = Modifier,
    viewModel: MyViewModel = viewModel(factory = MyViewModel.Factory)
) {
    // ...
}

或者，使用ViewModel工厂 DSL 使用更惯用的 Kotlin API 创建工厂

import androidx.lifecycle.SavedStateHandle
import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProvider
import androidx.lifecycle.ViewModelProvider.AndroidViewModelFactory.Companion.APPLICATION_KEY
import androidx.lifecycle.createSavedStateHandle
import androidx.lifecycle.viewmodel.initializer
import androidx.lifecycle.viewmodel.viewModelFactory

class MyViewModel(
    private val myRepository: MyRepository,
    private val savedStateHandle: SavedStateHandle
) : ViewModel() {
    // ViewModel logic

    // Define ViewModel factory in a companion object
    companion object {
        val Factory: ViewModelProvider.Factory = viewModelFactory {
            initializer {
                val savedStateHandle = createSavedStateHandle()
                val myRepository = (this[APPLICATION_KEY] as MyApplication).myRepository
                MyViewModel(
                    myRepository = myRepository,
                    savedStateHandle = savedStateHandle
                )
            }
        }
    }
}

将自定义参数作为 CreationExtras 传递

您可以通过创建自定义键，通过CreationExtras将依赖项传递给您的ViewModel。如果您的ViewModel依赖于无法通过Application类和APPLICATION_KEY访问的对象，这将非常有用。一个示例是当您的ViewModel在 Kotlin 多平台模块内创建时，因此无法访问 Android 依赖项。

在此示例中，ViewModel定义了一个自定义键，并在ViewModelProvider.Factory中使用它。

import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProvider
import androidx.lifecycle.viewModelScope
import androidx.lifecycle.viewmodel.CreationExtras
import androidx.lifecycle.viewmodel.initializer
import androidx.lifecycle.viewmodel.viewModelFactory

class MyViewModel(
    private val myRepository: MyRepository,
) : ViewModel() {
    // ViewModel logic

    // Define ViewModel factory in a companion object
    companion object {

        // Define a custom key for your dependency
        val MY_REPOSITORY_KEY = object : CreationExtras.Key<MyRepository> {}

        val Factory: ViewModelProvider.Factory = viewModelFactory {
            initializer {
                // Get the dependency in your factory
                val myRepository = this[MY_REPOSITORY_KEY] as MyRepository
                MyViewModel(
                    myRepository = myRepository,
                )
            }
        }
    }
}

您可以使用来自 ViewModelStoreOwner（例如 ComponentActivity、Fragment 或 NavBackStackEntry）的 CreationExtras.Key 实例化一个 ViewModel，也可以使用 Jetpack Compose。

Kotlin

import androidx.lifecycle.ViewModelProvider
import androidx.lifecycle.ViewModelStoreOwner
import androidx.lifecycle.viewmodel.CreationExtras
import androidx.lifecycle.viewmodel.MutableCreationExtras
// ...
    // Use from ComponentActivity, Fragment, NavBackStackEntry,
    // or another ViewModelStoreOwner.
    val viewModelStoreOwner: ViewModelStoreOwner = this
    val myViewModel: MyViewModel = ViewModelProvider.create(
        viewModelStoreOwner,
        factory = MyViewModel.Factory,
        extras = MutableCreationExtras().apply {
            set(MyViewModel.MY_REPOSITORY_KEY, myRepository)
        },
    )[MyViewModel::class]

Jetpack Compose

import androidx.lifecycle.viewmodel.MutableCreationExtras
import androidx.lifecycle.viewmodel.compose.viewModel
// ...
@Composable
fun MyApp(myRepository: MyRepository) {
    val extras = MutableCreationExtras().apply {
        set(MyViewModel.MY_REPOSITORY_KEY, myRepository)
    }
    val viewModel: MyViewModel = viewModel(
        factory = MyViewModel.Factory,
        extras = extras,
    )
}

2.5.0 之前的 ViewModel 版本的工厂

如果您使用的是低于 2.5.0 版本的 ViewModel，则需要从扩展 ViewModelProvider.Factory 并实现 create(Class<T>) 函数的子集类中提供工厂。根据 ViewModel 需要哪些依赖项，需要从不同的类扩展：

如果不需要 Application 或 SavedStateHandle，只需从 ViewModelProvider.Factory 扩展即可。

以下示例使用 AbstractSavedStateViewModelFactory 为一个 ViewModel 创建工厂，该 ViewModel 将仓库和 SavedStateHandle 类型作为依赖项。

Kotlin

class MyViewModel(
private val myRepository: MyRepository,
private val savedStateHandle: SavedStateHandle
) : ViewModel() {

// ViewModel logic ...

// Define ViewModel factory in a companion object
companion object {
    fun provideFactory(
        myRepository: MyRepository,
        owner: SavedStateRegistryOwner,
        defaultArgs: Bundle? = null,
    ): AbstractSavedStateViewModelFactory =
        object : AbstractSavedStateViewModelFactory(owner, defaultArgs) {
            @Suppress("UNCHECKED_CAST")
            override fun <T : ViewModel> create(
                key: String,
                modelClass: Class<T>,
                handle: SavedStateHandle
            ): T {
                return MyViewModel(myRepository, handle) as T
            }
        }
    }
}

Java

import androidx.annotation.NonNull;
import androidx.lifecycle.AbstractSavedStateViewModelFactory;
import androidx.lifecycle.SavedStateHandle;
import androidx.lifecycle.ViewModel;

public class MyViewModel extends ViewModel {
    public MyViewModel(
        MyRepository myRepository,
        SavedStateHandle savedStateHandle
    ) { /* Init ViewModel here */ }
}

public class MyViewModelFactory extends AbstractSavedStateViewModelFactory {

    private final MyRepository myRepository;

    public MyViewModelFactory(
        MyRepository myRepository
    ) {
        this.myRepository = myRepository;
    }

    @SuppressWarnings("unchecked")
    @NonNull
    @Override
    protected <T extends ViewModel> T create(
        @NonNull String key, @NonNull Class<T> modelClass, @NonNull SavedStateHandle handle
    ) {
        return (T) new MyViewModel(myRepository, handle);
    }
}

然后，您可以使用工厂来检索您的 ViewModel。

Kotlin

import androidx.activity.viewModels

class MyActivity : AppCompatActivity() {

    private val viewModel: MyViewModel by viewModels {
        MyViewModel.provideFactory((application as MyApplication).myRepository, this)
    }

    // Rest of Activity code
}

Java

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.ViewModelProvider;

public class MyActivity extends AppCompatActivity {

    MyViewModel myViewModel = new ViewModelProvider(
        this,
        ViewModelProvider.Factory.from(MyViewModel.initializer)
    ).get(MyViewModel.class);

    // Rest of Activity code
}

Jetpack Compose

import androidx.lifecycle.viewmodel.compose.viewModel

@Composable
fun MyScreen(
    modifier: Modifier = Modifier,
    viewModel: MyViewModel = viewModel(
        factory = MyViewModel.provideFactory(
            (LocalContext.current.applicationContext as MyApplication).myRepository,
            owner = LocalSavedStateRegistryOwner.current
        )
    )
) {
    // ...
}