ReaderViewModel: Fix saved state (#9807)

Also save page index

app/src/main/java/eu/kanade/tachiyomi/ui/reader/ReaderViewModel.kt

@@ -79,8 +79,8 @@ import java.util.Date
 /**
  * Presenter used by the activity to perform background operations.
  */
-class ReaderViewModel(
-    private val savedState: SavedStateHandle = SavedStateHandle(),
+class ReaderViewModel @JvmOverloads constructor(
+    private val savedState: SavedStateHandle,
     private val sourceManager: SourceManager = Injekt.get(),
     private val downloadManager: DownloadManager = Injekt.get(),
     private val downloadProvider: DownloadProvider = Injekt.get(),
@@ -120,6 +120,15 @@ class ReaderViewModel(
             field = value
         }
 
+    /**
+     * The visible page index of the currently loaded chapter. Used to restore from process kill.
+     */
+    private var chapterPageIndex = savedState.get<Int>("page_index") ?: -1
+        set(value) {
+            savedState["page_index"] = value
+            field = value
+        }
+
     /**
      * The chapter loader for the loaded manga. It'll be null until [manga] is set.
      */
@@ -198,7 +207,10 @@ class ReaderViewModel(
             .distinctUntilChanged()
             .filterNotNull()
             .onEach { currentChapter ->
-                if (!currentChapter.chapter.read) {
+                if (chapterPageIndex >= 0) {
+                    // Restore from SavedState
+                    currentChapter.requestedPage = chapterPageIndex
+                } else if (!currentChapter.chapter.read) {
                     currentChapter.requestedPage = currentChapter.chapter.last_page_read
                 }
                 chapterId = currentChapter.chapter.id!!
@@ -490,6 +502,7 @@ class ReaderViewModel(
             it.copy(currentPage = pageIndex + 1)
         }
         readerChapter.requestedPage = pageIndex
+        chapterPageIndex = pageIndex
 
         if (!incognitoMode && page.status != Page.State.ERROR) {
             readerChapter.chapter.last_page_read = pageIndex