info: keep alive info page (by metadata sliver) but only fetch metadata when necessary

lib/widgets/fullscreen/fullscreen_page.dart

@@ -299,6 +299,7 @@ class FullscreenVerticalPageView extends StatefulWidget {
 class _FullscreenVerticalPageViewState extends State<FullscreenVerticalPageView> {
   bool _isInitialScale = true;
   ValueNotifier<Color> _backgroundColorNotifier = ValueNotifier(Colors.black);
+  ValueNotifier<bool> _infoPageVisibleNotifier = ValueNotifier(false);
 
   @override
   void initState() {
@@ -343,8 +344,12 @@ class _FullscreenVerticalPageViewState extends State<FullscreenVerticalPageView>
         scrollDirection: Axis.vertical,
         controller: widget.verticalPager,
         physics: _isInitialScale ? const PageScrollPhysics() : const NeverScrollableScrollPhysics(),
-        onPageChanged: widget.onVerticalPageChanged,
+        onPageChanged: (page) {
+          widget.onVerticalPageChanged(page);
+          _infoPageVisibleNotifier.value = page == FullscreenBodyState.infoPage;
+        },
         children: [
+          // fake page for opacity transition between collection and fullscreen views
           const SizedBox(),
           ImagePage(
             collection: widget.collection,
@@ -359,7 +364,11 @@ class _FullscreenVerticalPageViewState extends State<FullscreenVerticalPageView>
               if (notification is BackUpNotification) widget.onImagePageRequested();
               return false;
             },
-            child: InfoPage(collection: widget.collection, entry: widget.entry),
+            child: InfoPage(
+              collection: widget.collection,
+              entry: widget.entry,
+              visibleNotifier: _infoPageVisibleNotifier,
+            ),
           ),
         ],
       ),

lib/widgets/fullscreen/info/info_page.dart

@@ -14,11 +14,13 @@ import 'package:tuple/tuple.dart';
 class InfoPage extends StatefulWidget {
   final ImageCollection collection;
   final ImageEntry entry;
+  final ValueNotifier<bool> visibleNotifier;
 
   const InfoPage({
     Key key,
     @required this.collection,
     @required this.entry,
+    this.visibleNotifier,
   }) : super(key: key);
 
   @override
@@ -97,7 +99,11 @@ class InfoPageState extends State<InfoPage> {
                     ),
                     SliverPadding(
                       padding: horizontalPadding,
-                      sliver: MetadataSectionSliver(entry: entry, columnCount: split ? 2 : 1),
+                      sliver: MetadataSectionSliver(
+                        entry: entry,
+                        columnCount: split ? 2 : 1,
+                        visibleNotifier: widget.visibleNotifier,
+                      ),
                     ),
                     SliverPadding(
                       padding: EdgeInsets.only(bottom: 8 + mqViewInsetsBottom),

lib/widgets/fullscreen/info/metadata_section.dart

@@ -10,34 +10,56 @@ import 'package:flutter_staggered_grid_view/flutter_staggered_grid_view.dart';
 class MetadataSectionSliver extends StatefulWidget {
   final ImageEntry entry;
   final int columnCount;
+  final ValueNotifier<bool> visibleNotifier;
 
   const MetadataSectionSliver({
     @required this.entry,
     @required this.columnCount,
+    this.visibleNotifier,
   });
 
   @override
   State<StatefulWidget> createState() => _MetadataSectionSliverState();
 }
 
-class _MetadataSectionSliverState extends State<MetadataSectionSliver> {
+class _MetadataSectionSliverState extends State<MetadataSectionSliver> with AutomaticKeepAliveClientMixin {
   Map _metadata;
+  String _loadedMetadataUri;
+
+  bool get isVisible => widget.visibleNotifier.value;
 
   @override
   void initState() {
     super.initState();
+    _registerWidget(widget);
     _getMetadata();
   }
 
   @override
   void didUpdateWidget(MetadataSectionSliver oldWidget) {
     super.didUpdateWidget(oldWidget);
+    _unregisterWidget(oldWidget);
+    _registerWidget(widget);
     _getMetadata();
   }
 
+  @override
+  void dispose() {
+    _unregisterWidget(widget);
+    super.dispose();
+  }
+
+  void _registerWidget(MetadataSectionSliver widget) {
+    widget.visibleNotifier.addListener(_getMetadata);
+  }
+
+  void _unregisterWidget(MetadataSectionSliver widget) {
+    widget.visibleNotifier.removeListener(_getMetadata);
+  }
+
   @override
   Widget build(BuildContext context) {
-    debugPrint('$runtimeType build');
+    super.build(context);
     final directoryNames = (_metadata?.keys?.toList() ?? [])..sort();
     return SliverStaggeredGrid.countBuilder(
       crossAxisCount: widget.columnCount,
@@ -57,10 +79,19 @@ class _MetadataSectionSliverState extends State<MetadataSectionSliver> {
   }
 
   Future<void> _getMetadata() async {
-    debugPrint('$runtimeType _getMetadata');
+    if (_loadedMetadataUri == widget.entry.uri) return;
-    _metadata = await MetadataService.getAllMetadata(widget.entry);
+    if (isVisible) {
+      _metadata = await MetadataService.getAllMetadata(widget.entry);
+      _loadedMetadataUri = widget.entry.uri;
+    } else {
+      _metadata = null;
+      _loadedMetadataUri = null;
+    }
     if (mounted) setState(() {});
   }
+
+  @override
+  bool get wantKeepAlive => true;
 }
 
 class _Directory extends StatelessWidget {