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建立含依附元件的 ViewModel (Android Jetpack 的一部分)。

遵循依附元件插入最佳做法，ViewModel 可將依附元件做為其建構函式中的參數。這些類型主要來自網域或資料層。由於架構提供 ViewModel，因此必須採用特殊的機制來建立其執行個體。這種機制即為 ViewModelProvider.Factory 介面。只有實作這個介面才能在適當範圍內將 ViewModel 執行個體化。

ViewModels 搭配 CreationExtras

如果 ViewModel 類別在其建構函式中接收依附元件，請提供實作 ViewModelProvider.Factory 介面的工廠。覆寫 create(Class<T>, CreationExtras) 函式來提供 ViewModel 的新執行個體。

CreationExtras 允許您存取有助於對 ViewModel 執行個體化的相關資訊。下面是可透過額外項目存取的金鑰清單：

金鑰	功能
`ViewModelProvider.NewInstanceFactory.VIEW_MODEL_KEY`	提供傳遞至 `ViewModelProvider.get()` 的自訂金鑰存取權。
`ViewModelProvider.AndroidViewModelFactory.APPLICATION_KEY`	提供 `Application` 類別的執行個體存取權。
`SavedStateHandleSupport.DEFAULT_ARGS_KEY`	提供應當用於建構 `SavedStateHandle` 的引數套件存取權。
`SavedStateHandleSupport.SAVED_STATE_REGISTRY_OWNER_KEY`	提供用於建構 `ViewModel` 的 `SavedStateRegistryOwner` 存取權。
`SavedStateHandleSupport.VIEW_MODEL_STORE_OWNER_KEY`	提供用於建構 `ViewModel` 的 `ViewModelStoreOwner` 存取權。

如要建立新的 SavedStateHandle 執行個體，請使用 CreationExtras.createSavedStateHandle().createSavedStateHandle()) 函式，並將其傳遞至 ViewModel。

以下範例說明如何提供 ViewModel 執行個體，該執行個體以範圍限定為 Application 類別和 SavedStateHandle 的存放區做為依附元件：

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);
        }
    );
}

之後您就可以在擷取 ViewModel 執行個體時使用這個工廠：

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，您也可以使用 ViewModel 工廠 DSL，以更符合語言習慣的 Kotlin API 建立工廠：

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
                )
            }
        }
    }
}

ViewModel 2.5.0 以下版本的工廠

如果您使用的是 2.5.0 以下版本的 ViewModel，則必須提供延伸 ViewModelProvider.Factory 並實作 create(Class<T>) 函式的類別子集中的工廠。根據 ViewModel 所需的依附元件，必須從中延伸不同類別：

AndroidViewModelFactory (如果需要 Application 類別)。
AbstractSavedStateViewModelFactory (如果需要做為依附元件傳遞 SavedStateHandle)。

如果不需要 Application 或 SavedStateHandle，則從 ViewModelProvider.Factory 擴充即可。

下列範例使用 ViewModel 的 AbstractSavedStateViewModelFactory，並將存放區和 SavedStateHandle 類型做為依附元件：

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);
    }
}

然後使用工廠來擷取 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
        )
    )
) {
    // ...
}

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