feat: Swipe to next or previous image in image viewer

lib/pages/image_viewer/image_viewer.dart

@@ -10,28 +10,61 @@ import '../../utils/matrix_sdk_extensions/event_extension.dart';
 
 class ImageViewer extends StatefulWidget {
   final Event event;
+  final Timeline? timeline;
   final BuildContext outerContext;
 
-  const ImageViewer(this.event, {required this.outerContext, super.key});
+  const ImageViewer(
+    this.event, {
+    required this.outerContext,
+    this.timeline,
+    super.key,
+  });
 
   @override
   ImageViewerController createState() => ImageViewerController();
 }
 
 class ImageViewerController extends State<ImageViewer> {
+  @override
+  void initState() {
+    super.initState();
+    allEvents = widget.timeline?.events
+            .where((event) => event.messageType == MessageTypes.Image)
+            .toList()
+            .reversed
+            .toList() ??
+        [widget.event];
+    var index =
+        allEvents.indexWhere((event) => event.eventId == widget.event.eventId);
+    if (index < 0) index = 0;
+    pageController = PageController(initialPage: index);
+  }
+
+  late final PageController pageController;
+
+  late final List<Event> allEvents;
+
+  int get _index => pageController.page?.toInt() ?? 0;
+
+  Event get currentEvent => allEvents[_index];
+
+  bool get canGoNext => _index < allEvents.length - 1;
+
+  bool get canGoBack => _index > 0;
+
   /// Forward this image to another room.
   void forwardAction() => showScaffoldDialog(
         context: context,
         builder: (context) => ShareScaffoldDialog(
-          items: [ContentShareItem(widget.event.content)],
+          items: [ContentShareItem(currentEvent.content)],
         ),
       );
 
   /// Save this file with a system call.
-  void saveFileAction(BuildContext context) => widget.event.saveFile(context);
+  void saveFileAction(BuildContext context) => currentEvent.saveFile(context);
 
   /// Save this file with a system call.
-  void shareFileAction(BuildContext context) => widget.event.shareFile(context);
+  void shareFileAction(BuildContext context) => currentEvent.shareFile(context);
 
   static const maxScaleFactor = 1.5;