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ViewModel 範圍設定 API Android Jetpack 的一部分。

範圍是有效使用 ViewModel 的關鍵。每個 ViewModel 的範圍都是限定為實作 ViewModelStoreOwner 介面的物件。您可以運用幾種 API 更輕鬆管理 ViewModel 的範圍。本文件將概述您應瞭解的一些主要技巧。

ViewModelProvider.get() 方法可讓您取得 ViewModel 的執行個體範圍限定為任何 ViewModelStoreOwner。Kotlin 使用者最常見的用途是擴充功能函式。所有 Kotlin 擴充功能函式實作項目會使用 ViewModelProvider API 進階功能。

將 ViewModel 範圍限定為最接近的 ViewModelStoreOwner

您可以將 ViewModel 的範圍限定為活動、片段或目的地導覽圖。 viewModels() 擴充功能函式，以及 Compose 中的 viewModel() 函式可讓您取得 ViewModel 執行個體範圍限定為最接近的 ViewModelStoreOwner。

View

import androidx.activity.viewModels

class MyActivity : AppCompatActivity() {
    // ViewModel API available in activity.activity-ktx
    // The ViewModel is scoped to `this` Activity
    val viewModel: MyViewModel by viewModels()
}

import androidx.fragment.app.viewModels

class MyFragment : Fragment() {
    // ViewModel API available in fragment.fragment-ktx
    // The ViewModel is scoped to `this` Fragment
    val viewModel: MyViewModel by viewModels()
}

檢視畫面

import androidx.lifecycle.ViewModelProvider;

public class MyActivity extends AppCompatActivity {
    // The ViewModel is scoped to `this` Activity
    MyViewModel viewModel = new ViewModelProvider(this).get(MyViewModel.class);
}

public class MyFragment extends Fragment {
    // The ViewModel is scoped to `this` Fragment
    MyViewModel viewModel = new ViewModelProvider(this).get(MyViewModel.class);
}

Compose

import androidx.lifecycle.viewmodel.compose.viewModel

@Composable
fun MyScreen(
    modifier: Modifier = Modifier,
    // ViewModel API available in lifecycle.lifecycle-viewmodel-compose
    // The ViewModel is scoped to the closest ViewModelStoreOwner provided
    // via the LocalViewModelStoreOwner CompositionLocal. In order of proximity,
    // this could be the destination of a Navigation graph, the host Fragment,
    // or the host Activity.
    viewModel: MyViewModel = viewModel()
) { /* ... */ }

範圍限定為任何 ViewModelStoreOwner 的 ViewModel

ComponentActivity.viewModels() 和 Fragment.viewModels() 函式，位於 View 系統和 Compose 中的 viewModel() 函式 ownerProducer 參數，可用來指定 ViewModelStoreOwner ViewModel 的例項範圍。以下範例說明如何取得 ViewModel 範圍限定為父項片段：

View

import androidx.fragment.app.viewModels

class MyFragment : Fragment() {

    // ViewModel API available in fragment.fragment-ktx
    // The ViewModel is scoped to the parent of `this` Fragment
    val viewModel: SharedViewModel by viewModels(
        ownerProducer = { requireParentFragment() }
    )
}

檢視畫面

import androidx.lifecycle.ViewModelProvider;

public class MyFragment extends Fragment {

    SharedViewModel viewModel;

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        // The ViewModel is scoped to the parent of `this` Fragment
        viewModel = new ViewModelProvider(requireParentFragment())
            .get(SharedViewModel.class);
    }
}

Compose

import androidx.lifecycle.viewmodel.compose.viewModel

@Composable
fun MyScreen(
    context: Context = LocalContext.current,
    // ViewModel API available in lifecycle.lifecycle-viewmodel-compose
    // The ViewModel is scoped to the parent of the host Fragment
    // where this composable function is called
    viewModel: SharedViewModel = viewModel(
        viewModelStoreOwner = (context as Fragment).requireParentFragment()
    )
) { /* ... */ }

從片段中取得範圍限定為活動的 ViewModel 是一種常見的用途。時間方法是使用 activityViewModels() 您可以使用 View 擴充功能函式。如果未使用 View 和 Kotlin 您也能使用相同的 API，並傳遞正確的擁有者。

View

import androidx.fragment.app.activityViewModels

class MyFragment : Fragment() {

    // ViewModel API available in fragment.fragment-ktx
    // The ViewModel is scoped to the host Activity
    val viewModel: SharedViewModel by activityViewModels()
}

檢視畫面

import androidx.lifecycle.ViewModelProvider;

public class MyFragment extends Fragment {

    SharedViewModel viewModel;

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        // The ViewModel is scoped to the host Activity
        viewModel = new ViewModelProvider(requireActivity())
            .get(SharedViewModel.class);
    }
}

Compose

import androidx.lifecycle.viewmodel.compose.viewModel

@Composable
fun MyScreen(
    context: Context = LocalContext.current,
    // ViewModel API available in lifecycle.lifecycle-viewmodel-compose
    // The ViewModel is scoped to the Activity of the host Fragment
    // where this composable function is called
    viewModel: SharedViewModel = viewModel(
        viewModelStoreOwner = (context as Fragment).requireActivity()
    )
) { /* ... */ }

範圍限定為 Navigation 圖形的 ViewModel

Navigation 圖表也是 ViewModel 儲存區擁有者。如果使用 Navigation Fragment 或 Navigation Compose 提供 ViewModel 範圍限定為 Navigation 圖表， navGraphViewModels(graphId) View 擴充功能函式。

View

import androidx.navigation.navGraphViewModels

class MyFragment : Fragment() {

    // ViewModel API available in navigation.navigation-fragment
    // The ViewModel is scoped to the `nav_graph` Navigation graph
    val viewModel: SharedViewModel by navGraphViewModels(R.id.nav_graph)

    // Equivalent navGraphViewModels code using the viewModels API
    val viewModel: SharedViewModel by viewModels(
        { findNavController().getBackStackEntry(R.id.nav_graph) }
    )
}

檢視畫面

import androidx.lifecycle.ViewModelProvider;

public class MyFragment extends Fragment {

    SharedViewModel viewModel;

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        NavController navController = NavHostFragment.findNavController(this);
        NavBackStackEntry backStackEntry = navController.getBackStackEntry(R.id.my_graph);

        // The ViewModel is scoped to the `nav_graph` Navigation graph
        viewModel = new ViewModelProvider(backStackEntry).get(SharedViewModel.class);
    }
}

Compose

import androidx.lifecycle.viewmodel.compose.viewModel

@Composable
fun MyAppNavHost() {
    // ...
    composable("myScreen") { backStackEntry ->
        // Retrieve the NavBackStackEntry of "parentNavigationRoute"
        val parentEntry = remember(backStackEntry) {
            navController.getBackStackEntry("parentNavigationRoute")
        }
        // Get the ViewModel scoped to the `parentNavigationRoute` Nav graph
        val parentViewModel: SharedViewModel = viewModel(parentEntry)
        // ...
    }
}

如果除了 Jetpack Navigation 之外，還使用 Hilt，則可使用 hiltNavGraphViewModels(graphId)敬上 API，如下所示：

View

import androidx.hilt.navigation.fragment.hiltNavGraphViewModels

class MyFragment : Fragment() {

    // ViewModel API available in hilt.hilt-navigation-fragment
    // The ViewModel is scoped to the `nav_graph` Navigation graph
    // and is provided using the Hilt-generated ViewModel factory
    val viewModel: SharedViewModel by hiltNavGraphViewModels(R.id.nav_graph)
}

檢視畫面

import androidx.hilt.navigation.HiltViewModelFactory;
import androidx.lifecycle.ViewModelProvider;

public class MyFragment extends Fragment {

    SharedViewModel viewModel;

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        NavController navController = NavHostFragment.findNavController(this);
        NavBackStackEntry backStackEntry = navController.getBackStackEntry(R.id.my_graph);

        // The ViewModel is scoped to the `nav_graph` Navigation graph
        // and is provided using the Hilt-generated ViewModel factory
        viewModel = new ViewModelProvider(
            backStackEntry,
            HiltViewModelFactory.create(getContext(), backStackEntry)
        ).get(SharedViewModel.class);
    }
}

Compose

import androidx.hilt.navigation.compose.hiltViewModel

@Composable
fun MyAppNavHost() {
    // ...
    composable("myScreen") { backStackEntry ->
        val parentEntry = remember(backStackEntry) {
            navController.getBackStackEntry("parentNavigationRoute")
        }

        // ViewModel API available in hilt.hilt-navigation-compose
        // The ViewModel is scoped to the `parentNavigationRoute` Navigation graph
        // and is provided using the Hilt-generated ViewModel factory
        val parentViewModel: SharedViewModel = hiltViewModel(parentEntry)
        // ...
    }
}

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