Interests: derive selected topic from navigation state

feature/interests/impl/src/main/kotlin/com/google/samples/apps/nowinandroid/feature/interests/impl/InterestsScreen.kt

@@ -47,8 +47,6 @@ fun InterestsScreen(
         uiState = uiState,
         followTopic = viewModel::followTopic,
         onTopicClick = {
-            // TODO: this violates SSOT, events should go through the ViewModel
-            viewModel.onTopicClick(it)
             onTopicClick(it)
         },
         shouldHighlightSelectedTopic = shouldHighlightSelectedTopic,

feature/interests/impl/src/main/kotlin/com/google/samples/apps/nowinandroid/feature/interests/impl/InterestsViewModel.kt

@@ -16,14 +16,16 @@
 
 package com.google.samples.apps.nowinandroid.feature.interests.impl
 
-import androidx.lifecycle.SavedStateHandle
+import androidx.compose.runtime.snapshotFlow
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
 import com.google.samples.apps.nowinandroid.core.data.repository.UserDataRepository
 import com.google.samples.apps.nowinandroid.core.domain.GetFollowableTopicsUseCase
 import com.google.samples.apps.nowinandroid.core.domain.TopicSortField
 import com.google.samples.apps.nowinandroid.core.model.data.FollowableTopic
+import com.google.samples.apps.nowinandroid.core.navigation.NavigationState
 import com.google.samples.apps.nowinandroid.feature.interests.api.navigation.InterestsNavKey
+import com.google.samples.apps.nowinandroid.feature.topic.api.navigation.TopicNavKey
 import dagger.assisted.Assisted
 import dagger.assisted.AssistedFactory
 import dagger.assisted.AssistedInject
@@ -36,22 +38,23 @@ import kotlinx.coroutines.launch
 
 @HiltViewModel(assistedFactory = InterestsViewModel.Factory::class)
 class InterestsViewModel @AssistedInject constructor(
-    private val savedStateHandle: SavedStateHandle,
     val userDataRepository: UserDataRepository,
     getFollowableTopics: GetFollowableTopicsUseCase,
-    // TODO: see comment below
+    @Assisted val navigationState: NavigationState,
     @Assisted val key: InterestsNavKey,
 ) : ViewModel() {
 
-    // TODO: this should no longer be necessary, the currently selected topic should be
-    //  available through the navigation state
-    // Key used to save and retrieve the currently selected topic id from saved state.
-    private val selectedTopicIdKey = "selectedTopicIdKey"
-
-    private val selectedTopicId = savedStateHandle.getStateFlow(
-        key = selectedTopicIdKey,
-        initialValue = key.initialTopicId,
-    )
+    // Derive selected topic from navigation state
+    private val selectedTopicId: StateFlow<String?> =
+        snapshotFlow {
+            navigationState.currentSubStack
+                .lastOrNull { it is TopicNavKey }
+                ?.let { (it as TopicNavKey).id }
+        }.stateIn(
+            scope = viewModelScope,
+            started = SharingStarted.WhileSubscribed(5_000),
+            initialValue = key.initialTopicId,
+        )
 
     val uiState: StateFlow<InterestsUiState> = combine(
         selectedTopicId,
@@ -69,15 +72,9 @@ class InterestsViewModel @AssistedInject constructor(
         }
     }
 
-    fun onTopicClick(topicId: String?) {
-        // TODO: This should modify the navigation state directly rather than just updating the
-        //  savedStateHandle
-        savedStateHandle[selectedTopicIdKey] = topicId
-    }
-
     @AssistedFactory
     interface Factory {
-        fun create(key: InterestsNavKey): InterestsViewModel
+        fun create(navigationState: NavigationState, key: InterestsNavKey): InterestsViewModel
     }
 }
 

feature/interests/impl/src/main/kotlin/com/google/samples/apps/nowinandroid/feature/interests/impl/navigation/InterestsEntryProvider.kt

@@ -36,7 +36,7 @@ fun EntryProviderScope<NavKey>.interestsEntry(navigator: Navigator) {
         },
     ) { key ->
         val viewModel = hiltViewModel<InterestsViewModel, InterestsViewModel.Factory> {
-            it.create(key)
+            it.create(navigator.state, key)
         }
         InterestsScreen(
             // TODO: This event should either be provided by the ViewModel or by the navigator, not both

feature/interests/impl/src/test/kotlin/com/google/samples/apps/nowinandroid/interests/impl/InterestsViewModelTest.kt

@@ -16,17 +16,18 @@
 
 package com.google.samples.apps.nowinandroid.interests.impl
 
-import androidx.lifecycle.SavedStateHandle
-import androidx.navigation.testing.invoke
+import androidx.navigation3.runtime.NavBackStack
 import com.google.samples.apps.nowinandroid.core.domain.GetFollowableTopicsUseCase
 import com.google.samples.apps.nowinandroid.core.model.data.FollowableTopic
 import com.google.samples.apps.nowinandroid.core.model.data.Topic
+import com.google.samples.apps.nowinandroid.core.navigation.NavigationState
 import com.google.samples.apps.nowinandroid.core.testing.repository.TestTopicsRepository
 import com.google.samples.apps.nowinandroid.core.testing.repository.TestUserDataRepository
 import com.google.samples.apps.nowinandroid.core.testing.util.MainDispatcherRule
 import com.google.samples.apps.nowinandroid.feature.interests.api.navigation.InterestsNavKey
 import com.google.samples.apps.nowinandroid.feature.interests.impl.InterestsUiState
 import com.google.samples.apps.nowinandroid.feature.interests.impl.InterestsViewModel
+import com.google.samples.apps.nowinandroid.feature.topic.api.navigation.TopicNavKey
 import kotlinx.coroutines.flow.collect
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.test.UnconfinedTestDispatcher
@@ -66,13 +67,23 @@ class InterestsViewModelTest {
 
     @Before
     fun setup() {
+        val initialTopicId = testInputTopics[0].topic.id
+        val interestsKey = InterestsNavKey(initialTopicId = initialTopicId)
+
         viewModel = InterestsViewModel(
-            savedStateHandle = SavedStateHandle(
-                route = InterestsNavKey(initialTopicId = testInputTopics[0].topic.id),
-            ),
             userDataRepository = userDataRepository,
             getFollowableTopics = getFollowableTopicsUseCase,
-            InterestsNavKey(initialTopicId = testInputTopics[0].topic.id),
+            navigationState = NavigationState(
+                startKey = interestsKey,
+                topLevelStack = NavBackStack(interestsKey),
+                subStacks = mapOf(
+                    interestsKey to NavBackStack(
+                        interestsKey,
+                        TopicNavKey(id = initialTopicId), // Add TopicNavKey to the sub-stack
+                    ),
+                ),
+            ),
+            key = interestsKey,
         )
     }