Jetpack Compose 中的作用域状态

Question

在所有应用程序中，总会有以下三种状态范围：

使用 Compose，可以通过以下方式实现“每屏状态”：

NavHost(navController, startDestination = startRoute) {
    ...
    composable(route) {
       ...
       val perScreenViewModel = viewModel()  // This will be different from
    }
    composable(route) {
       ...
       val perScreenViewModel = viewModel()  // this instance
    }
    ...
}

“应用状态”可以通过以下方式实现：

val appStateViewModel = viewModel()
NavHost(navController, startDestination = startRoute) {
    ...
}

但是“作用域状态”呢？我们如何在 Compose 中实现它？

Answer 1

这正是navigation graph scoped view models 的用途。

这涉及两个步骤：

1. 找到与您希望将 ViewModel 范围限定为的图形关联的 NavBackStackEntry

2. 将其传递给viewModel()。

对于第 1 部分），您有两个选择。如果您知道导航图的路线（通常您应该知道），您可以直接使用getBackStackEntry：

// Note that you must always use remember with getBackStackEntry
// as this ensures that the graph is always available, even while
// your destination is animated out after a popBackStack()
val navigationGraphEntry = remember {
  navController.getBackStackEntry("graph_route")
}
val navigationGraphScopedViewModel = viewModel(navigationGraphEntry)

但是，如果您想要更通用的东西，您可以使用目标本身中的信息检索返回堆栈条目 - 它的 parent：

fun NavBackStackEntry.rememberParentEntry(): NavBackStackEntry {
  // First, get the parent of the current destination
  // This always exists since every destination in your graph has a parent
  val parentId = navBackStackEntry.destination.parent!!.id

  // Now get the NavBackStackEntry associated with the parent
  // making sure to remember it
  return remember {
    navController.getBackStackEntry(parentId)
  }
}

这允许您编写如下内容：

val parentEntry = it.rememberParentEntry()
val navigationGraphScopedViewModel = viewModel(parentEntry)

虽然parent 目标将等于简单导航图的根图，但当您使用nested navigation 时，父级是图的中间层之一：

NavHost(navController, startDestination = startRoute) {
    ...
  navigation(startDestination = nestedStartRoute, route = nestedRoute) {
    composable(route) {
      // This instance will be the same
      val parentViewModel: YourViewModel = viewModel(it.rememberParentEntry())
    }
    composable(route) {
      // As this instance
      val parentViewModel: YourViewModel = viewModel(it.rememberParentEntry())
    }
  }
  navigation(startDestination = nestedStartRoute, route = secondNestedRoute) {
    composable(route) {
        // But this instance is different
      val parentViewModel: YourViewModel = viewModel(it.rememberParentEntry())
    }
  }
  composable(route) {
     // This is also different (the parent is the root graph)
     // but the root graph has the same scope as the whole NavHost
     // so this isn't particularly helpful
     val parentViewModel: YourViewModel = viewModel(it.rememberParentEntry())
  }
  ...
}

请注意，您不仅限于直接父级：每个父级导航图都可用于提供更大的范围。

Answer 2

来自Compose and other libraries - Hilt 文档

要检索范围为导航路线的ViewModel 实例，请将目标根作为参数传递：

val loginBackStackEntry = remember { navController.getBackStackEntry("Parent") }
val loginViewModel: LoginViewModel = hiltViewModel(loginBackStackEntry)

没有 Hilt 也可以做到这一点

val loginBackStackEntry = remember { navController.getBackStackEntry("Parent") }
val loginViewModel: LoginViewModel = viewModel(loginBackStackEntry)

这实现了 @ianhanniballake 实现的相同目标，但代码更少

注意：导航图有自己的 route = "Parent"

完整代码示例

使用 Jetpack 组合和导航的作用域状态示例

// import androidx.hilt.navigation.compose.hiltViewModel
// import androidx.navigation.compose.getBackStackEntry

@Composable
fun MyApp() {
    NavHost(navController, startDestination = startRoute) {
        navigation(startDestination = innerStartRoute, route = "Parent") {
            // ...
            composable("exampleWithRoute") { backStackEntry ->
                val parentEntry = remember {navController.getBackStackEntry("Parent")}
                val parentViewModel = hiltViewModel<ParentViewModel>(parentEntry)
                ExampleWithRouteScreen(parentViewModel)
            }
        }
    }
}