✨ Introduce AssetSelectPredicate

Author: AlexV525

lib/src/delegates/asset_picker_builder_delegate.dart

@@ -2,6 +2,7 @@
 /// [Author] Alex (https://github.com/AlexV525)
 /// [Date] 2020-10-29 21:50
 ///
+import 'dart:async';
 import 'dart:io';
 import 'dart:math' as math;
 import 'dart:typed_data';
@@ -22,6 +23,12 @@ typedef IndicatorBuilder = Widget Function(
   bool isAssetsEmpty,
 );
 
+typedef AssetSelectPredicate<Asset> = FutureOr<bool> Function(
+  BuildContext context,
+  Asset asset,
+  bool isSelected,
+);
+
 /// The delegate to build the whole picker's components.
 ///
 /// By extending the delegate, you can customize every components on you own.
@@ -41,6 +48,7 @@ abstract class AssetPickerBuilderDelegate<Asset, Path> {
     this.loadingIndicatorBuilder,
     this.allowSpecialItemWhenEmpty = false,
     this.keepScrollOffset = false,
+    this.selectPredicate,
   })  : assert(
           pickerTheme == null || themeColor == null,
           'Theme and theme color cannot be set at the same time.',
@@ -100,6 +108,10 @@ abstract class AssetPickerBuilderDelegate<Asset, Path> {
   /// 选择器是否可以从同样的位置开始选择
   final bool keepScrollOffset;
 
+  /// Predicate whether an asset can be selected or unselected.
+  /// 判断资源可否被选择
+  final AssetSelectPredicate<Asset>? selectPredicate;
+
   /// The [ScrollController] for the preview grid.
   final ScrollController gridScrollController = ScrollController();
 
@@ -629,6 +641,7 @@ class DefaultAssetPickerBuilderDelegate
     IndicatorBuilder? loadingIndicatorBuilder,
     bool allowSpecialItemWhenEmpty = false,
     bool keepScrollOffset = false,
+    AssetSelectPredicate<AssetEntity>? selectPredicate,
     this.gridThumbSize = Constants.defaultGridThumbSize,
     this.previewThumbSize,
     this.specialPickerType,
@@ -648,6 +661,7 @@ class DefaultAssetPickerBuilderDelegate
           loadingIndicatorBuilder: loadingIndicatorBuilder,
           allowSpecialItemWhenEmpty: allowSpecialItemWhenEmpty,
           keepScrollOffset: keepScrollOffset,
+          selectPredicate: selectPredicate,
         );
 
   /// Thumbnail size in the grid.
@@ -1642,7 +1656,15 @@ class DefaultAssetPickerBuilderDelegate
         );
         final GestureDetector selectorWidget = GestureDetector(
           behavior: HitTestBehavior.opaque,
-          onTap: () {
+          onTap: () async {
+            final bool? selectPredicateResult = await selectPredicate?.call(
+              context,
+              asset,
+              selected,
+            );
+            if (selectPredicateResult == false) {
+              return;
+            }
             if (selected) {
               provider.unSelectAsset(asset);
               return;

lib/src/widget/asset_picker.dart

@@ -46,6 +46,7 @@ class AssetPicker<Asset, Path> extends StatefulWidget {
     IndicatorBuilder? loadingIndicatorBuilder,
     SpecialItemPosition specialItemPosition = SpecialItemPosition.none,
     bool allowSpecialItemWhenEmpty = false,
+    AssetSelectPredicate<AssetEntity>? selectPredicate,
     bool useRootNavigator = true,
     Curve routeCurve = Curves.easeIn,
     Duration routeDuration = const Duration(milliseconds: 300),
@@ -111,6 +112,7 @@ class AssetPicker<Asset, Path> extends StatefulWidget {
           specialItemBuilder: specialItemBuilder,
           loadingIndicatorBuilder: loadingIndicatorBuilder,
           allowSpecialItemWhenEmpty: allowSpecialItemWhenEmpty,
+          selectPredicate: selectPredicate,
         ),
       ),
     );