Save nested nav key in instance state

Change-Id: If1155bfbe080eb4df3c59faaec0fb4cd4da3821d

app/src/main/kotlin/com/google/samples/apps/nowinandroid/ui/interests2pane/Interests2PaneViewModel.kt

@@ -27,7 +27,8 @@ import javax.inject.Inject
 class Interests2PaneViewModel @Inject constructor(
     private val savedStateHandle: SavedStateHandle,
 ) : ViewModel() {
-    val selectedTopicId: StateFlow<String?> = savedStateHandle.getStateFlow(TOPIC_ID_ARG, null)
+    val selectedTopicId: StateFlow<String?> =
+        savedStateHandle.getStateFlow(TOPIC_ID_ARG, savedStateHandle[TOPIC_ID_ARG])
 
     fun onTopicClick(topicId: String?) {
         savedStateHandle[TOPIC_ID_ARG] = topicId

app/src/main/kotlin/com/google/samples/apps/nowinandroid/ui/interests2pane/InterestsListDetailScreen.kt

@@ -30,6 +30,8 @@ import androidx.compose.runtime.getValue
 import androidx.compose.runtime.key
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
+import androidx.compose.runtime.saveable.Saver
+import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.runtime.setValue
 import androidx.hilt.navigation.compose.hiltViewModel
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
@@ -98,7 +100,11 @@ internal fun InterestsListDetailScreen(
     var nestedNavHostStartDestination by remember {
         mutableStateOf(selectedTopicId?.let(::createTopicRoute) ?: TOPIC_ROUTE)
     }
-    var nestedNavKey by remember { mutableStateOf(UUID.randomUUID()) }
+    var nestedNavKey by rememberSaveable(
+        stateSaver = Saver({ it.toString() }, UUID::fromString),
+    ) {
+        mutableStateOf(UUID.randomUUID())
+    }
     val nestedNavController = key(nestedNavKey) {
         rememberNavController()
     }