🐛 Fix keep offset jumping recovery issues (#174)

Author: AlexV525

lib/src/delegates/asset_picker_builder_delegate.dart

@@ -188,9 +188,7 @@ abstract class AssetPickerBuilderDelegate<Asset, Path> {
   /// Be aware that the method will do nothing when [keepScrollOffset] is true.
   /// 注意当 [keepScrollOffset] 为 true 时方法不会进行释放。
   void dispose() {
-    if (keepScrollOffset) {
-      return;
-    }
+    Constants.scrollPosition = null;
     gridScrollController.dispose();
     permission.dispose();
     permissionOverlayHidden.dispose();
@@ -586,11 +584,12 @@ abstract class AssetPickerBuilderDelegate<Asset, Path> {
   Widget build(BuildContext context) {
     // Schedule the scroll position's restoration callback if this feature
     // is enabled and offsets are different.
-    if (keepScrollOffset &&
-        Constants.scrollPosition != null &&
-        !gridScrollController.hasClients) {
+    if (keepScrollOffset && Constants.scrollPosition != null) {
       SchedulerBinding.instance!.addPostFrameCallback((_) {
-        gridScrollController.jumpTo(Constants.scrollPosition!.pixels);
+        // Update only if the controller has clients.
+        if (gridScrollController.hasClients) {
+          gridScrollController.jumpTo(Constants.scrollPosition!.pixels);
+        }
       });
     }
     return AnnotatedRegion<SystemUiOverlayStyle>(

lib/src/widget/asset_picker.dart

@@ -262,7 +262,10 @@ class AssetPickerState<Asset, Path> extends State<AssetPicker<Asset, Path>>
   void dispose() {
     WidgetsBinding.instance!.removeObserver(this);
     AssetPicker.unregisterObserve(_onLimitedAssetsUpdated);
-    widget.builder.dispose();
+    // Skip delegate's dispose when it's keeping scroll offset.
+    if (!widget.builder.keepScrollOffset) {
+      widget.builder.dispose();
+    }
     super.dispose();
   }