创建带依赖项的 ViewModel   Android Jetpack 的一部分。

遵循依赖注入的最佳实践，ViewModel 可以在其构造函数中将依赖项作为参数。这些依赖项大多来自领域层或数据层的类型。由于框架提供 ViewModel，因此需要一种特殊机制来创建它们的实例。该机制就是 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 CreationExtras 的 ViewModel

如果 ViewModel 类在其构造函数中接收依赖项，请提供实现 ViewModelProvider.Factory 接口的工厂。覆盖 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 提供对您传递给 ViewModelProvider.get() 的自定义键的访问权限。
ViewModelProvider.AndroidViewModelFactory.APPLICATION_KEY 提供对 Application 类实例的访问权限。
SavedStateHandleSupport.DEFAULT_ARGS_KEY 提供对您应该用于构建 SavedStateHandle 的参数 Bundle 的访问权限。
SavedStateHandleSupport.SAVED_STATE_REGISTRY_OWNER_KEY 提供对用于构建 ViewModel 的 SavedStateRegistryOwner 的访问权限。
SavedStateHandleSupport.VIEW_MODEL_STORE_OWNER_KEY 提供对用于构建 ViewModel 的 ViewModelStoreOwner 的访问权限。

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

带 APPLICATION_KEY 的 CreationExtras

以下示例说明了如何提供一个 ViewModel 实例，该实例将作用域限定为 Application 类的仓库和 SavedStateHandle 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)
) {
    // ...
}

另外，您也可以使用 ViewModel 工厂 DSL，通过更符合 Kotlin 习惯的 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，将依赖项通过 CreationExtras 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。例如，当您的 ViewModel 是在 Kotlin Multiplatform 模块中创建的，因此无法访问 Android 依赖项时，就可以使用此方法。

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 CreationExtras.Key 从 ViewModelStoreOwner（例如 ComponentActivity、Fragment 或 NavBackStackEntry）或通过 Jetpack Compose 实例化 ViewModel。

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,
    )
}

适用于 2.5.0 版之前 ViewModel 的工厂

如果您使用的是 2.5.0 版之前的 ViewModel，您需要从一组扩展 ViewModelProvider.Factory 并实现 create(Class<T>) 函数的类中提供工厂。根据 ViewModel 需要哪些依赖项，需要从不同的类进行扩展：

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

以下示例使用 AbstractSavedStateViewModelFactory，适用于将仓库和 SavedStateHandle 类型作为依赖项的 ViewModel。

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 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
        )
    )
) {
    // ...
}