使用依赖项创建 ViewModel   作为 Android Jetpack 的一部分。

遵循 依赖项注入 的最佳实践，ViewModel 可以在其构造函数中将依赖项作为参数。这些大多来自 域 或 数据 层。由于框架提供了 ViewModel，因此需要一种特殊的机制来创建它们的实例。该机制是 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。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 中访问的键列表

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。

以下是如何提供 ViewModel 实例的示例，该实例采用作用域为 Application 类和 SavedStateHandle 作为依赖项的 存储库。

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
                )
            }
        }
    }
}

2.5.0 之前的 ViewModel 版本的工厂

如果您使用的是 2.5.0 之前的 ViewModel 版本，则需要从扩展 ViewModelProvider.Factory 并实现 create(Class<T>) 函数的类子集中提供工厂。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 ViewModel 创建工厂，该 ViewModel 将存储库和 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
        )
    )
) {
    // ...
}