在屏幕导航之间维护视图模型中的值

Question

我有一个视图模型，其值由Screen1更新，但我希望屏幕2获得这些值:

@HiltViewModel
class VM @Inject constructor(): ViewModel() {
var appState :Boolean by mutableStateOf(false)
}
@Destination
@Composable
fun ScreenOne(nav: DestinationsNavigator, vm: VM = hiltViewModel()){
//update the vm here.
vm.appState.value = true;
nav.navigate(ScreenTwoDestination());
}
@Destination
@Composable
fun ScreenTwo(vm: VM = hiltViewModel()){
Log.i(vm.appState); // this never changes and is also false even though the vm was correctly updated before the navigation happened.
}

screenttwo如何拥有视图模型的修改数据?

Answer 1

问题是ViewModel的新实例不能访问以前的ViewModel的状态。您必须传递ViewModel变量，而不是创建一个新变量。

我通常不建议在Navigation上这样做，但您需要将vm值从ScreenOne作为参数传递给接收@Composable,ScreenTwo。您还需要更新Navigation代码，以便它可以接受传入的参数并将其传递给ScreenTwo。

@HiltViewModel
class VM @Inject constructor(): ViewModel() {
var appState :Boolean by mutableStateOf(false)
}
@Destination
@Composable
fun ScreenOne(nav: DestinationsNavigator, vm: VM = hiltViewModel()){
vm.appState.value = true;
// notice how I added the "vm" variable as an argument
// This allows us to pass the existing instance of the ViewModel
nav.navigate(ScreenTwoDestination(vm));
}
@Destination
@Composable
fun ScreenTwo(vm: VM = hiltViewModel()){
Log.i(vm.appState); 

另一方面，您也可以将ViewModel从@Composables中一起提升:

@HiltViewModel
class VM @Inject constructor(): ViewModel() {
var appState :Boolean by mutableStateOf(false)
}
val vm = hiltViewModel()
@Destination
@Composable
fun ScreenOne(nav: DestinationsNavigator){
vm.appState.value = true;
nav.navigate(ScreenTwoDestination());
}
@Destination
@Composable
fun ScreenTwo(){
Log.i(vm.appState); 

郑重声明，这两种方法都不是理想的选择。最好的选择是在其他地方处理数据，并有两个独立的viewmodel可以访问它。