Fix loading indicator showing randomly

lib/widget/horizontal_page_viewer.dart

@@ -4,6 +4,7 @@ import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 import 'package:nc_photos/k.dart' as k;
 import 'package:nc_photos/platform/k.dart' as platform_k;
+import 'package:nc_photos/widget/page_changed_listener.dart';
 
 class HorizontalPageViewer extends StatefulWidget {
   HorizontalPageViewer({
@@ -14,14 +15,9 @@ class HorizontalPageViewer extends StatefulWidget {
     HorizontalPageViewerController? controller,
     this.viewportFraction = 1,
     this.canSwitchPage = true,
+    this.onPageChanged,
   })  : controller = controller ?? HorizontalPageViewerController(),
-        super(key: key) {
-    this.controller._pageController = PageController(
-      initialPage: initialPage,
-      viewportFraction: viewportFraction,
-      keepPage: false,
-    );
-  }
+        super(key: key);
 
   @override
   createState() => _HorizontalPageViewerState();
@@ -32,13 +28,26 @@ class HorizontalPageViewer extends StatefulWidget {
   final HorizontalPageViewerController controller;
   final double viewportFraction;
   final bool canSwitchPage;
+  final ValueChanged<int>? onPageChanged;
 }
 
 class _HorizontalPageViewerState extends State<HorizontalPageViewer> {
   @override
-  void initState() {
+  initState() {
     super.initState();
     _pageFocus.requestFocus();
+
+    widget.controller._pageController = PageController(
+      initialPage: widget.initialPage,
+      viewportFraction: widget.viewportFraction,
+      keepPage: false,
+    );
+    if (widget.onPageChanged != null) {
+      widget.controller._pageController.addListener(PageChangedListener(
+        widget.controller._pageController,
+        onPageChanged: widget.onPageChanged,
+      ).call);
+    }
   }
 
   @override
@@ -52,6 +61,12 @@ class _HorizontalPageViewerState extends State<HorizontalPageViewer> {
         : _buildContent(context);
   }
 
+  @override
+  dispose() {
+    widget.controller._pageController.dispose();
+    super.dispose();
+  }
+
   Widget _buildWebContent(BuildContext context) {
     assert(platform_k.isWeb);
     // support switching pages with keyboard on web

lib/widget/page_changed_listener.dart

@@ -0,0 +1,22 @@
+import 'package:flutter/widgets.dart';
+
+class PageChangedListener {
+  PageChangedListener(
+    this.pageController, {
+    this.onPageChanged,
+  }) : _prevPage = pageController.initialPage;
+
+  void call() {
+    if (pageController.hasClients) {
+      final page = pageController.page!.round();
+      if (page != _prevPage) {
+        onPageChanged?.call(page);
+        _prevPage = page;
+      }
+    }
+  }
+
+  final PageController pageController;
+  final ValueChanged<int>? onPageChanged;
+  int _prevPage;
+}

lib/widget/viewer.dart

@@ -108,6 +108,9 @@ class _ViewerState extends State<Viewer> {
             controller: _viewerController,
             viewportFraction: _viewportFraction,
             canSwitchPage: _canSwitchPage(),
+            onPageChanged: (_) {
+              setState(() {});
+            },
           ),
           _buildBottomAppBar(context),
           _buildAppBar(context),
@@ -340,21 +343,18 @@ class _ViewerState extends State<Viewer> {
           ImageViewer.preloadImage(widget.account, nextFile);
         }
       }
-      setState(() {
-        _pageStates[index]!.hasLoaded = true;
-        _isViewerLoaded = true;
-      });
     }
+    setState(() {
+      _pageStates[index]!.hasLoaded = true;
+      _isViewerLoaded = true;
+    });
   }
 
   void _onVideoLoaded(int index) {
-    if (_viewerController.currentPage == index &&
-        !_pageStates[index]!.hasLoaded) {
-      setState(() {
-        _pageStates[index]!.hasLoaded = true;
-        _isViewerLoaded = true;
-      });
-    }
+    setState(() {
+      _pageStates[index]!.hasLoaded = true;
+      _isViewerLoaded = true;
+    });
   }
 
   void _onVideoPlay() {