在 Jetpack Compose Navigation 中共享 viewModel

Question

谁能建议如何在 Jetpack Compose Navigation 的不同部分共享 ViewModel？

根据文档，viewModel 通常应该使用 activity 范围在不同的组合函数中共享，但如果在导航内部则不会。

这是我要修复的代码。看起来我在导航内的两个部分中获得了两个不同的 viewModels：

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        setContent {
            NavigationSystem()
        }
    }
}

@Composable
fun NavigationSystem() {
    val navController = rememberNavController()

    NavHost(navController = navController, startDestination = "home") {
        composable("home") { HomeScreen(navController) }
        composable("result") { ResultScreen(navController) }
    }
}

@Composable
fun HomeScreen(navController: NavController) {
    val viewModel: ConversionViewModel = viewModel()
    
    var temp by remember { mutableStateOf("") }
    val fahrenheit = temp.toIntOrNull() ?: 0

    Column(
        modifier = Modifier
            .padding(16.dp)
            .fillMaxSize(),
        horizontalAlignment = Alignment.CenterHorizontally,
        verticalArrangement = Arrangement.Center
    ) {
        Column {
            OutlinedTextField(
                value = temp,
                onValueChange = { temp = it },
                label = { Text("Fahrenheit") },
                modifier = Modifier.fillMaxWidth(0.85f)
            )

            Spacer(modifier = Modifier.padding(top = 16.dp))

            Button(onClick = {
                Log.d("HomeScreen", fahrenheit.toString())
                if (fahrenheit !in 1..160) return@Button
                viewModel.onCalculate(fahrenheit)
                navController.navigate("result")
            }) {
                Text("Calculate")
            }
        }
    }
}

@Composable
fun ResultScreen(navController: NavController) {
    val viewModel: ConversionViewModel = viewModel()

    Column(
        modifier = Modifier
            .padding(16.dp)
            .fillMaxSize(),
        horizontalAlignment = Alignment.CenterHorizontally,
        verticalArrangement = Arrangement.Center
    ) {
        Log.d("ResultScreenDebug", "celsius: ${ viewModel.celsius.value.toString()}")
        Text(
            viewModel.celsius.value.toString(),
            style = MaterialTheme.typography.h6
        )

        Spacer(modifier = Modifier.padding(top = 24.dp))

        Button(onClick = { navController.navigate("home") }) {
            Text(text = "Calculate again")
        }
    }
}

调试日志：

2021-07-27 22:01:52.542 27113-27113/com.example.navigation D/ViewModelDebug: fh: 65, cs: 18, celcius: 18.0
2021-07-27 22:01:52.569 27113-27113/com.example.navigation D/ResultScreenDebug: celsius: 0.0

谢谢！

Answer 1

您可以创建一个 viewModel 并将其传递给

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        setContent {
            NavigationSystem()
        }
    }
}

@Composable
fun NavigationSystem() {
    val navController = rememberNavController()

    val viewModel: ConversionViewModel = viewModel()

    NavHost(navController = navController, startDestination = "home") {
        composable("home") { HomeScreen(navController, viewModel) }
        composable("result") { ResultScreen(navController, viewModel) }
    }
}

@Composable
fun HomeScreen(navController: NavController, viewModel: ConversionViewModel) {
    var temp by remember { mutableStateOf("") }
    val fahrenheit = temp.toIntOrNull() ?: 0

    Column(
        modifier = Modifier
            .padding(16.dp)
            .fillMaxSize(),
        horizontalAlignment = Alignment.CenterHorizontally,
        verticalArrangement = Arrangement.Center
    ) {
        Column {
            OutlinedTextField(
                value = temp,
                onValueChange = { temp = it },
                label = { Text("Fahrenheit") },
                modifier = Modifier.fillMaxWidth(0.85f)
            )

            Spacer(modifier = Modifier.padding(top = 16.dp))

            Button(onClick = {
                Log.d("HomeScreen", fahrenheit.toString())
                if (fahrenheit !in 1..160) return@Button
                viewModel.onCalculate(fahrenheit)
                navController.navigate("result")
            }) {
                Text("Calculate")
            }
        }
    }
}

@Composable
fun ResultScreen(navController: NavController, viewModel: ConversionViewModel) {
    Column(
        modifier = Modifier
            .padding(16.dp)
            .fillMaxSize(),
        horizontalAlignment = Alignment.CenterHorizontally,
        verticalArrangement = Arrangement.Center
    ) {
        Log.d("ResultScreenDebug", "celsius: ${ viewModel.celsius.value.toString()}")
        Text(
            viewModel.celsius.value.toString(),
            style = MaterialTheme.typography.h6
        )

        Spacer(modifier = Modifier.padding(top = 24.dp))

        Button(onClick = { navController.navigate("home") }) {
            Text(text = "Calculate again")
        }
    }
}

Answer 2

考虑将您的 activity 作为 viewModelStoreOwner 参数传递给 viewModel() fun，因为 ComponentActivity 实现了 ViewModelStoreOwner 接口：

val viewModel: ConversionViewModel = viewModel(LocalContext.current as ComponentActivity)

此代码将 return 在您的所有目的地中使用相同的 ConversionViewModel 实例。

Answer 3

我认为比将 ViewModel 范围限定到整个 NavGraph 更好的解决方案是在 Home 路由中构建 ViewModel，然后从 Result 路由（路由范围）：

//extensions
@Composable
inline fun <reified T : ViewModel> NavBackStackEntry?.viewModel(): T? = this?.let {
    viewModel(viewModelStoreOwner = it)
}

@Composable
inline fun <reified T : ViewModel> NavBackStackEntry.viewModel(
    viewModelStoreOwner: ViewModelStoreOwner = checkNotNull(LocalViewModelStoreOwner.current) {
        "No ViewModelStoreOwner was provided via LocalViewModelStoreOwner"
    }
): T {
    return androidx.lifecycle.viewmodel.compose.viewModel(
        viewModelStoreOwner = viewModelStoreOwner, key = T::class.java.name
    )
}

//use-case
@Composable
fun HomeScreen(navController: NavController) {
    val viewModel: ConversionViewModel = viewModel()
    ...
}
@Composable
fun ResultScreen(navController: NavController) {
    val viewModel: ConversionViewModel? = navController.previousBackStackEntry.viewModel()
    ...
}

但是如果你必须将它的范围限定到整个 NavGraph，你可以像 @akhris 所说的那样做一些事情，但是在某种程度上你可以将 ViewModelStoreOwner 与 [=20] 分开=]:

//composable store-owner builder
@Composable
fun rememberViewModelStoreOwner(): ViewModelStoreOwner {
    val context = LocalContext.current
    return remember(context) { context as ViewModelStoreOwner }
}

通过这种方式，您可以将 Activity 与 ViewModelStoreOwner 分离，并可以执行以下操作：

val LocalNavGraphViewModelStoreOwner =
    staticCompositionLocalOf<ViewModelStoreOwner> {
        TODO("Undefined")
    }

@Composable
fun NavigationSystem() {
    val navController = rememberNavController()
    val vmStoreOwner = rememberViewModelStoreOwner()

    CompositionLocalProvider(
        LocalNavGraphViewModelStoreOwner provides vmStoreOwner
    ) {
        NavHost(navController = navController, startDestination = "home") {
            composable("home") { HomeScreen(navController) }
            composable("result") { ResultScreen(navController) }
        }
    }
}

@Composable
fun HomeScreen(navController: NavController) {
    val viewModel: ConversionViewModel = viewModel(viewModelStoreOwner = LocalNavGraphViewModelStoreOwner.current)
    ...
}

@Composable
fun ResultScreen(navController: NavController) {
    val viewModel: ConversionViewModel = viewModel(viewModelStoreOwner = LocalNavGraphViewModelStoreOwner.current)
    ...
}

Answer 4

这还不够好吗？

获取顶部MainScreen 的sharedViewModel 并显式传递。它似乎不会导致任何内存泄漏。

// MainActivity.kt
class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MainScreen()
        }
    }
}

@Composable
fun MainScreen(sharedViewModel: SharedViewModel = viewModel()) {
    val navController = rememberNavController()
    Navigation(navController, sharedViewModel)
}

@Composable
// Navigation.kt
fun Navigation(navController: NavHostController, sharedViewModel: SharedViewModel) {
    NavHost(navController, startDestination = "Home") {
        composable("Home") {
            EpisodeListScreen(navController, sharedViewModel)
        }
        composable("Login") {
            LoginScreen(navController, sharedViewModel)
        }
        composable("Editor") {
            EditorScreen(navController, sharedViewModel)
        }
        composable("Setting") {
            SettingScreen(navController, sharedViewModel)
        }
    }
}

// EpisodeListScreen.kt
@Composable
fun EpisodeListScreen(
    navController: NavController,
    sharedViewModel: SharedViewModel = viewModel()
) {
    // ...
}

在 Jetpack Compose Navigation 中共享 viewModel

Sharing viewModel within Jetpack Compose Navigation

android-viewmodel

android-jetpack-navigation

android-jetpack-compose