🐛 Fix generic type when building indicators

Author: AlexV525

example/lib/customs/pickers/directory_file_asset_picker.dart

@@ -535,6 +535,29 @@ class FileAssetPickerBuilder
     );
   }
 
+  @override
+  Widget loadingIndicator(BuildContext context) {
+    return Selector<FileAssetPickerProvider, bool>(
+      selector: (_, FileAssetPickerProvider p) => p.isAssetsEmpty,
+      builder: (BuildContext context, bool isAssetsEmpty, Widget? w) {
+        if (loadingIndicatorBuilder != null) {
+          return loadingIndicatorBuilder!(context, isAssetsEmpty);
+        }
+        return Center(child: isAssetsEmpty ? emptyIndicator(context) : w);
+      },
+      child: Center(
+        child: SizedBox.fromSize(
+          size: Size.square(Screens.width / gridCount / 3),
+          child: CircularProgressIndicator(
+            valueColor: AlwaysStoppedAnimation<Color>(
+              theme.iconTheme.color ?? Colors.grey,
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+
   @override
   Widget assetsGridBuilder(BuildContext context) {
     int totalCount = provider.currentAssets.length;

lib/src/delegates/asset_picker_builder_delegate.dart

@@ -359,6 +359,16 @@ abstract class AssetPickerBuilderDelegate<Asset, Path> {
   /// Android设备的选择器布局
   Widget androidLayout(BuildContext context);
 
+  /// Loading indicator.
+  /// 加载指示器
+  ///
+  /// Subclasses need to implement this due to the generic type limitation, and
+  /// not all delegates use [AssetPickerProvider].
+  ///
+  /// See also:
+  /// - [DefaultAssetPickerBuilderDelegate.loadingIndicator] as an example.
+  Widget loadingIndicator(BuildContext context);
+
   /// GIF image type indicator.
   /// GIF 类型图片指示
   Widget gifIndicator(BuildContext context, Asset asset) {
@@ -424,24 +434,6 @@ abstract class AssetPickerBuilderDelegate<Asset, Path> {
     );
   }
 
-  /// Loading indicator.
-  /// 加载指示器
-  Widget loadingIndicator(BuildContext context) {
-    return Selector<AssetPickerProvider<Asset, Path>, bool>(
-      selector: (_, AssetPickerProvider<Asset, Path> p) => p.isAssetsEmpty,
-      builder: (BuildContext context, bool isAssetsEmpty, Widget? w) {
-        if (loadingIndicatorBuilder != null) {
-          return loadingIndicatorBuilder!(context, isAssetsEmpty);
-        }
-        return Center(child: isAssetsEmpty ? emptyIndicator(context) : w);
-      },
-      child: PlatformProgressIndicator(
-        color: theme.iconTheme.color,
-        size: context.mediaQuery.size.width / gridCount / 3,
-      ),
-    );
-  }
-
   /// Item widgets when the thumb data load failed.
   /// 资源缩略数据加载失败时使用的部件
   Widget failedItemBuilder(BuildContext context) {
@@ -1018,6 +1010,23 @@ class DefaultAssetPickerBuilderDelegate
     );
   }
 
+  @override
+  Widget loadingIndicator(BuildContext context) {
+    return Selector<DefaultAssetPickerProvider, bool>(
+      selector: (_, DefaultAssetPickerProvider p) => p.isAssetsEmpty,
+      builder: (BuildContext context, bool isAssetsEmpty, Widget? w) {
+        if (loadingIndicatorBuilder != null) {
+          return loadingIndicatorBuilder!(context, isAssetsEmpty);
+        }
+        return Center(child: isAssetsEmpty ? emptyIndicator(context) : w);
+      },
+      child: PlatformProgressIndicator(
+        color: theme.iconTheme.color,
+        size: context.mediaQuery.size.width / gridCount / 3,
+      ),
+    );
+  }
+
   @override
   Widget assetsGridBuilder(BuildContext context) {
     return Selector<DefaultAssetPickerProvider, AssetPathEntity?>(