Back button: fixed destinations

app/src/main/java/org/mydomain/myscan/MainActivity.kt

@@ -64,15 +64,15 @@ class MainActivity : ComponentActivity() {
             val cameraPermission = rememberCameraPermissionState()
             MyScanTheme {
                 val navigation = Navigation(
-                    toHomeScreen = { viewModel.navigateTo(Screen.Home) },
-                    toCameraScreen = { viewModel.navigateTo(Screen.Camera) },
-                    toDocumentScreen = { viewModel.navigateTo(Screen.Document()) },
-                    toAboutScreen = { viewModel.navigateTo(Screen.About) },
-                    toLibrariesScreen = { viewModel.navigateTo(Screen.Libraries) },
+                    toHomeScreen = { viewModel.navigateTo(Screen.Main.Home) },
+                    toCameraScreen = { viewModel.navigateTo(Screen.Main.Camera) },
+                    toDocumentScreen = { viewModel.navigateTo(Screen.Main.Document()) },
+                    toAboutScreen = { viewModel.navigateTo(Screen.Overlay.About) },
+                    toLibrariesScreen = { viewModel.navigateTo(Screen.Overlay.Libraries) },
                     back = { viewModel.navigateBack() }
                 )
                 when (val screen = currentScreen) {
-                    is Screen.Home -> {
+                    is Screen.Main.Home -> {
                         val recentDocs by viewModel.recentDocuments.collectAsStateWithLifecycle()
                         HomeScreen(
                             cameraPermission = cameraPermission,
@@ -83,17 +83,17 @@ class MainActivity : ComponentActivity() {
                             onOpenPdf = { file -> openPdf(file.toUri()) }
                         )
                     }
-                    is Screen.Camera -> {
+                    is Screen.Main.Camera -> {
                         CameraScreen(
                             viewModel,
                             navigation,
                             liveAnalysisState,
                             onImageAnalyzed = { image -> viewModel.liveAnalysis(image) },
-                            onFinalizePressed = { viewModel.navigateTo(Screen.Document()) },
+                            onFinalizePressed = navigation.toDocumentScreen,
                             cameraPermission = cameraPermission
                         )
                     }
-                    is Screen.Document -> {
+                    is Screen.Main.Document -> {
                         DocumentScreen (
                             document = document,
                             initialPage = screen.initialPage,
@@ -109,14 +109,14 @@ class MainActivity : ComponentActivity() {
                             ),
                             onStartNew = {
                                 viewModel.startNewDocument()
-                                viewModel.navigateTo(Screen.Home) },
+                                viewModel.navigateTo(Screen.Main.Home) },
                             onDeleteImage =  { id -> viewModel.deletePage(id) }
                         )
                     }
-                    is Screen.About -> {
+                    is Screen.Overlay.About -> {
                         AboutScreen(onBack = navigation.back, onViewLibraries = navigation.toLibrariesScreen)
                     }
-                    is Screen.Libraries -> {
+                    is Screen.Overlay.Libraries -> {
                         LibrariesScreen(onBack = navigation.back)
                     }
                 }

app/src/main/java/org/mydomain/myscan/MainViewModel.kt

@@ -73,9 +73,9 @@ class MainViewModel(
     val liveAnalysisState: StateFlow<LiveAnalysisState> = _liveAnalysisState.asStateFlow()
     private var lastSuccessfulLiveAnalysisState: LiveAnalysisState? = null
 
-    private val _screenStack = MutableStateFlow<List<Screen>>(listOf(Screen.Home))
-    val currentScreen: StateFlow<Screen> = _screenStack.map { it.last() }
-        .stateIn(viewModelScope, SharingStarted.Eagerly, _screenStack.value.last())
+    private val _navigationState = MutableStateFlow(NavigationState.initial())
+    val currentScreen: StateFlow<Screen> = _navigationState.map { it.current }
+        .stateIn(viewModelScope, SharingStarted.Eagerly, _navigationState.value.current)
 
     private val _pageIds = MutableStateFlow(imageRepository.imageIds())
     val documentUiModel: StateFlow<DocumentUiModel> =
@@ -159,12 +159,12 @@ class MainViewModel(
         }
     }
 
-    fun navigateTo(screen: Screen) {
-        _screenStack.update { it + screen }
+    fun navigateTo(destination: Screen) {
+        _navigationState.update { it.navigateTo(destination) }
     }
 
     fun navigateBack() {
-        _screenStack.update { stack -> if (stack.size > 1) stack.dropLast(1) else stack }
+        _navigationState.update { stack -> stack.navigateBack() }
     }
 
     fun onImageCaptured(imageProxy: ImageProxy?) {

app/src/main/java/org/mydomain/myscan/Navigation.kt

@@ -15,11 +15,15 @@
 package org.mydomain.myscan
 
 sealed class Screen {
-    object Home : Screen()
-    object Camera : Screen()
-    data class Document(val initialPage: Int = 0) : Screen()
-    object About : Screen()
-    object Libraries : Screen()
+    sealed class Main : Screen() {
+        object Home : Main()
+        object Camera : Main()
+        data class Document(val initialPage: Int = 0) : Main()
+    }
+    sealed class Overlay : Screen() {
+        object About : Overlay()
+        object Libraries : Overlay()
+    }
 }
 
 data class Navigation(
@@ -30,3 +34,30 @@ data class Navigation(
     val toLibrariesScreen: () -> Unit,
     val back: () -> Unit,
 )
+
+@ConsistentCopyVisibility
+data class NavigationState private constructor(val stack: List<Screen>) {
+
+    companion object {
+        fun initial() = NavigationState(listOf(Screen.Main.Home))
+    }
+
+    val current: Screen get() = stack.last()
+
+    fun navigateTo(destination: Screen): NavigationState {
+        return if (destination is Screen.Overlay) {
+            copy(stack = stack + destination)
+        } else {
+            copy(stack = listOf(destination))
+        }
+    }
+
+    fun navigateBack(): NavigationState {
+        return when (current) {
+            is Screen.Main.Home -> this // Back handled by system
+            is Screen.Main.Camera -> copy(stack = listOf(Screen.Main.Home))
+            is Screen.Main.Document -> copy(stack = listOf(Screen.Main.Camera))
+            is Screen.Overlay -> copy(stack = stack.dropLast(1))
+        }
+    }
+}

app/src/main/java/org/mydomain/myscan/view/CameraScreen.kt

@@ -154,7 +154,7 @@ fun CameraScreen(
         pageListState =
             CommonPageListState(
                 document = document,
-                onPageClick = { index -> viewModel.navigateTo(Screen.Document(index)) },
+                onPageClick = { index -> viewModel.navigateTo(Screen.Main.Document(index)) },
                 listState = listState,
                 onLastItemPosition = { offset -> thumbnailCoords.value = offset },
             ),

app/src/test/java/org/mydomain/myscan/NavigationTest.kt

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2025 Pierre-Yves Nicolas
+ *
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation, either version 3 of the License, or (at your option)
+ * any later version.
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ * You should have received a copy of the GNU General Public License along with
+ * this program. If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.mydomain.myscan
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+import org.mydomain.myscan.Screen.Main.Camera
+import org.mydomain.myscan.Screen.Main.Document
+import org.mydomain.myscan.Screen.Main.Home
+import org.mydomain.myscan.Screen.Overlay.About
+import org.mydomain.myscan.Screen.Overlay.Libraries
+
+class NavigationTest {
+
+    @Test
+    fun empty_ScreenStack() {
+        val empty = NavigationState.initial()
+        assertThat(empty.current).isEqualTo(Home)
+        assertThat(empty.navigateBack()).isEqualTo(empty)
+    }
+
+    @Test
+    fun navigate_between_fixed_screens() {
+        val atHome = NavigationState.initial()
+        val atCamera = atHome.navigateTo(Camera)
+        val atDocument = atHome.navigateTo(Document())
+
+        assertThat(atHome.current).isEqualTo(Home)
+        assertThat(atCamera.current).isEqualTo(Camera)
+        assertThat(atDocument.current).isEqualTo(Document())
+
+        assertThat(atCamera.navigateTo(Document())).isEqualTo(atDocument)
+        assertThat(atDocument.navigateTo(Home)).isEqualTo(atHome)
+        assertThat(atDocument.navigateTo(Camera)).isEqualTo(atCamera)
+
+        assertThat(atHome.navigateBack()).isEqualTo(atHome)
+        assertThat(atCamera.navigateBack()).isEqualTo(atHome)
+        assertThat(atDocument.navigateBack()).isEqualTo(atCamera)
+    }
+
+    @Test
+    fun navigate_to_secondary_screens() {
+        val atHome = NavigationState.initial()
+        val atCamera = atHome.navigateTo(Camera)
+
+        val atAboutAfterHome = atHome.navigateTo(About)
+        assertThat(atAboutAfterHome.current).isEqualTo(About)
+        assertThat(atAboutAfterHome.navigateBack()).isEqualTo(atHome)
+
+        val atAboutAfterCamera = atCamera.navigateTo(About)
+        assertThat(atAboutAfterCamera.current).isEqualTo(About)
+        assertThat(atAboutAfterCamera.navigateBack()).isEqualTo(atCamera)
+
+        val atLibrariesAfterCameraAbout = atAboutAfterCamera.navigateTo(Libraries)
+        assertThat(atLibrariesAfterCameraAbout.current).isEqualTo(Libraries)
+        assertThat(atLibrariesAfterCameraAbout.navigateBack()).isEqualTo(atAboutAfterCamera)
+    }
+}