Change the back stack system.

app/src/main/java/com/pixelized/biblib/ui/MainActivity.kt

@@ -13,7 +13,7 @@ import com.pixelized.biblib.ui.composable.screen.LoginScreenComposable
 import com.pixelized.biblib.ui.composable.screen.MainScreenComposable
 import com.pixelized.biblib.ui.composable.screen.SplashScreenComposable
 import com.pixelized.biblib.ui.theme.BibLibTheme
-import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel.Screen
+import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel.Navigable.Screen
 import com.pixelized.biblib.ui.viewmodel.navigation.NavigationViewModel
 
 class MainActivity : ComponentActivity() {

app/src/main/java/com/pixelized/biblib/ui/composable/screen/LoginScreenComposable.kt

@@ -51,6 +51,7 @@ import com.pixelized.biblib.ui.viewmodel.book.IBooksViewModel
 import com.pixelized.biblib.ui.viewmodel.credential.CredentialViewModel
 import com.pixelized.biblib.ui.viewmodel.credential.ICredentialViewModel
 import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel
+import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel.Navigable.Screen
 import com.pixelized.biblib.ui.viewmodel.navigation.NavigationViewModel
 import kotlinx.coroutines.delay
 
@@ -215,7 +216,7 @@ private fun LoginScreenNavigationComposable(
 
     val bookLoadingState by bookViewModel.state.observeAsState()
     if (bookLoadingState is IBooksViewModel.State.Finished) {
-        navigationViewModel.navigateTo(INavigationViewModel.Screen.MainScreen)
+        navigationViewModel.navigateTo(Screen.MainScreen)
     }
 }
 

app/src/main/java/com/pixelized/biblib/ui/composable/screen/MainScreenComposable.kt

@@ -17,7 +17,7 @@ import com.pixelized.biblib.ui.theme.BibLibTheme
 import com.pixelized.biblib.ui.viewmodel.book.BooksViewModel
 import com.pixelized.biblib.ui.viewmodel.book.IBooksViewModel
 import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel
-import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel.Page
+import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel.Navigable.Page
 import com.pixelized.biblib.ui.viewmodel.navigation.NavigationViewModel
 
 @Preview

app/src/main/java/com/pixelized/biblib/ui/composable/screen/SplashScreenComposable.kt

@@ -24,6 +24,7 @@ import com.pixelized.biblib.ui.viewmodel.authentication.IAuthenticationViewModel
 import com.pixelized.biblib.ui.viewmodel.book.BooksViewModel
 import com.pixelized.biblib.ui.viewmodel.book.IBooksViewModel
 import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel
+import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel.Navigable.Screen
 import com.pixelized.biblib.ui.viewmodel.navigation.NavigationViewModel
 import kotlinx.coroutines.delay
 import java.util.*
@@ -85,14 +86,14 @@ fun SplashScreenComposable(
                 LaunchedEffect(LAUNCH_EFFECT_NAVIGATION) {
                     contentVisible.value = false
                     delay(1000)
-                    navigationViewModel.navigateTo(INavigationViewModel.Screen.MainScreen)
+                    navigationViewModel.navigateTo(Screen.MainScreen)
                 }
             }
         } else {
             LaunchedEffect(LAUNCH_EFFECT_NAVIGATION) {
                 contentVisible.value = false
                 delay(1000)
-                navigationViewModel.navigateTo(INavigationViewModel.Screen.LoginScreen)
+                navigationViewModel.navigateTo(Screen.LoginScreen)
             }
         }
     }

app/src/main/java/com/pixelized/biblib/ui/viewmodel/navigation/INavigationViewModel.kt

@@ -5,31 +5,33 @@ import androidx.lifecycle.MutableLiveData
 import com.pixelized.biblib.ui.data.BookUio
 
 interface INavigationViewModel {
-    val screen: LiveData<Screen>
+    val screen: LiveData<Navigable.Screen>
-    val page: LiveData<Page>
+    val page: LiveData<Navigable.Page>
-
-    fun navigateTo(screen: Screen): Boolean
-    fun navigateTo(page: Page): Boolean
 
+    fun navigateTo(navigable: Navigable, addToBackStack: Boolean = false): Boolean
     fun navigateBack(): Boolean
 
-    sealed class Screen {
+    sealed class Navigable {
-        object SplashScreen : Screen()
+        sealed class Screen : Navigable() {
-        object MainScreen : Screen()
+            object SplashScreen : Screen()
-        object LoginScreen : Screen()
+            object MainScreen : Screen()
+            object LoginScreen : Screen()
+        }
+
+        sealed class Page : Navigable() {
+            object HomePage : Page()
+            data class Detail(val book: BookUio) : Page()
+        }
     }
 
-    sealed class Page {
+    class Mock(
-        object HomePage : Page()
+        screen: Navigable.Screen = Navigable.Screen.SplashScreen,
-        data class Detail(val book: BookUio) : Page()
+        page: Navigable.Page = Navigable.Page.HomePage
-    }
+    ) : INavigationViewModel {
+        override val screen: LiveData<Navigable.Screen> = MutableLiveData(screen)
+        override val page: LiveData<Navigable.Page> = MutableLiveData(page)
 
-    class Mock(screen: Screen = Screen.SplashScreen, page: Page = Page.HomePage) : INavigationViewModel {
+        override fun navigateTo(navigable: Navigable, addToBackStack: Boolean): Boolean = false
-        override val screen: LiveData<Screen> = MutableLiveData(screen)
-        override val page: LiveData<Page> = MutableLiveData(page)
-
-        override fun navigateTo(screen: Screen): Boolean = false
-        override fun navigateTo(page: Page): Boolean = false
         override fun navigateBack(): Boolean = false
     }
 }

app/src/main/java/com/pixelized/biblib/ui/viewmodel/navigation/NavigationViewModel.kt

@@ -3,38 +3,53 @@ package com.pixelized.biblib.ui.viewmodel.navigation
 import androidx.lifecycle.LiveData
 import androidx.lifecycle.MutableLiveData
 import androidx.lifecycle.ViewModel
-import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel.Page
+import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel.Navigable
-import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel.Screen
+import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel.Navigable.Page
+import com.pixelized.biblib.ui.viewmodel.navigation.INavigationViewModel.Navigable.Screen
 import java.util.*
 
 class NavigationViewModel : ViewModel(), INavigationViewModel {
-
+    private val stack = Stack<Navigable?>()
-    private val stack = Stack<Page>()
-
     private val _screen = MutableLiveData<Screen>(Screen.SplashScreen)
-    override val screen: LiveData<Screen> get() = _screen
-
     private val _page = MutableLiveData<Page>(Page.HomePage)
+
+    override val screen: LiveData<Screen> get() = _screen
     override val page: LiveData<Page> get() = _page
 
-    override fun navigateTo(screen: Screen): Boolean {
+    override fun navigateTo(navigable: Navigable, addToBackStack: Boolean): Boolean {
-        _screen.postValue(screen)
+        when (navigable) {
-        return true
+            is Page -> _page.postValue(navigable)
-    }
+            is Screen -> _screen.postValue(navigable)
-
+        }
-    override fun navigateTo(page: Page): Boolean {
+        if (addToBackStack) {
-        _page.postValue(page)
+            stack.push(navigable)
-        stack.push(page)
+        } else {
+            stack.push(null)
+        }
         return true
     }
 
     override fun navigateBack(): Boolean {
-        stack.pop()
+        return when (val navigable: Navigable? = popBackStack()) {
-        return if (stack.empty()) {
+            is Page -> {
-            false
+                _page.postValue(navigable)
+                true
+            }
+            is Screen -> {
+                _screen.postValue(navigable)
+                true
+            }
+            else -> false
+        }
+    }
+
+    private fun popBackStack(): Navigable? {
+        return if (stack.isEmpty()) {
+            null
         } else {
-            _page.postValue(stack.peek())
+            stack.pop()
-            true
+            while (stack.isNotEmpty() && stack.peek() == null) stack.pop()
+            return if (stack.isNotEmpty()) stack.peek() else null
         }
     }
 }