support custom loading widget delegate

CHANGELOG.md

 # CHANGELOG
 
 ## [0.1.0] support video
+
 API incompatibility
 
 ImageXXX rename AssetXXX
 
 ## [0.0.8] fix bug
+
 DefaultCheckBoxBuilderDelegate params checkColor not valid bug
 
 ## [0.0.7] fix bug
+
 fix dividerColor not valid bug
 
 ## [0.0.6] add checkbox delegate
+
 users can use CheckBoxDelegate to custom preview right bottom widget
 
 ## [0.0.5] add a params

README.md

 [![pub package](https://img.shields.io/pub/v/photo.svg)](https://pub.dartlang.org/packages/photo)
 ![Hex.pm](https://img.shields.io/hexpm/l/plug.svg)
 
-
 image picker, multi picker
 use flutter as ui
 
 ![image](https://github.com/CaiJingLong/some_asset/blob/master/image_picker1.gif)
 
 ## API incompatibility
+
 API incompatibility
 
 because support video, so the ImagePathEntity and ImageEntity rename to AssetPathEntity and AssetEntity.
     }
     setState(() {});
   }
-
 ```
 
 ## whole example
 
-you can see [github](https://github.com/caijinglong/flutter_photo/blob/master/example/)  [main.dart](https://github.com/caijinglong/flutter_photo/blob/master/example/lib/main.dart)
+you can see [github](https://github.com/caijinglong/flutter_photo/blob/master/example/) [main.dart](https://github.com/caijinglong/flutter_photo/blob/master/example/lib/main.dart)
 
 ## about android
 
 
 see the [github](https://github.com/bumptech/glide#proguard)
 
-
 ## about ios
 
 Because the album is a privacy privilege, you need user permission to access it. You must to modify the `Info.plist` file in Runner project.
 
-
 like next
 
 ```plist
 ```
 
 xcode like image
-![in xcode](https://github.com/CaiJingLong/some_asset/blob/master/flutter_photo2.png)
+![in xcode](https://github.com/CaiJingLong/some_asset/blob/master/flutter_photo2.png)

lib/photo.dart

 import 'dart:async';
 
 import 'package:flutter/material.dart';
-import 'package:photo_manager/photo_manager.dart';
-
+import 'package:photo/src/delegate/checkbox_builder_delegate.dart';
+import 'package:photo/src/delegate/loading_delegate.dart';
+import 'package:photo/src/delegate/sort_delegate.dart';
 import 'package:photo/src/entity/options.dart';
 import 'package:photo/src/provider/i18n_provider.dart';
 import 'package:photo/src/ui/dialog/not_permission_dialog.dart';
 import 'package:photo/src/ui/photo_app.dart';
-import 'package:photo/src/delegate/sort_delegate.dart';
-import 'package:photo/src/delegate/checkbox_builder_delegate.dart';
+import 'package:photo_manager/photo_manager.dart';
 
+export 'package:photo/src/delegate/checkbox_builder_delegate.dart';
+export 'package:photo/src/delegate/loading_delegate.dart';
+export 'package:photo/src/delegate/sort_delegate.dart';
 export 'package:photo/src/provider/i18n_provider.dart'
     show I18NCustomProvider, I18nProvider, CNProvider, ENProvider;
-export 'package:photo/src/delegate/sort_delegate.dart';
-export 'package:photo/src/delegate/checkbox_builder_delegate.dart';
 
 class PhotoPicker {
   static PhotoPicker _instance;
     I18nProvider provider = I18nProvider.chinese,
     SortDelegate sortDelegate,
     CheckBoxBuilderDelegate checkBoxBuilderDelegate,
+    LoadingDelegate loadingDelegate,
   }) {
     assert(provider != null, "provider must be not null");
     assert(context != null, "context must be not null");
     sortDelegate ??= SortDelegate.common;
     checkBoxBuilderDelegate ??= DefaultCheckBoxBuilderDelegate();
 
+    loadingDelegate ??= DefaultLoadingDelegate();
+
     var options = Options(
       rowCount: rowCount,
       dividerColor: dividerColor,
       thumbSize: thumbSize,
       sortDelegate: sortDelegate,
       checkBoxBuilderDelegate: checkBoxBuilderDelegate,
+      loadingDelegate: loadingDelegate,
     );
 
     return PhotoPicker()._pickImage(

lib/src/delegate/loading_delegate.dart

+import 'package:flutter/material.dart';
+import 'package:photo_manager/photo_manager.dart';
+
+abstract class LoadingDelegate {
+  Widget buildBigImageLoading(
+      BuildContext context, AssetEntity entity, Color themeColor);
+
+  Widget buildPreviewLoading(
+      BuildContext context, AssetEntity entity, Color themeColor);
+}
+
+class DefaultLoadingDelegate extends LoadingDelegate {
+  @override
+  Widget buildBigImageLoading(
+      BuildContext context, AssetEntity entity, Color themeColor) {
+    return Center(
+      child: Container(
+        width: 30.0,
+        height: 30.0,
+        child: CircularProgressIndicator(
+          valueColor: AlwaysStoppedAnimation(themeColor),
+        ),
+      ),
+    );
+  }
+
+  @override
+  Widget buildPreviewLoading(
+      BuildContext context, AssetEntity entity, Color themeColor) {
+    return Center(
+      child: Container(
+        width: 30.0,
+        height: 30.0,
+        child: CircularProgressIndicator(
+          valueColor: AlwaysStoppedAnimation(themeColor),
+        ),
+      ),
+    );
+  }
+}

lib/src/entity/options.dart

 import 'package:flutter/material.dart';
 import 'package:photo/src/delegate/checkbox_builder_delegate.dart';
+import 'package:photo/src/delegate/loading_delegate.dart';
 import 'package:photo/src/delegate/sort_delegate.dart';
 
 class Options {
 
   final CheckBoxBuilderDelegate checkBoxBuilderDelegate;
 
+  final LoadingDelegate loadingDelegate;
+
   const Options({
     this.rowCount,
     this.maxSelected,
     this.thumbSize,
     this.sortDelegate,
     this.checkBoxBuilderDelegate,
+    this.loadingDelegate,
   });
 }

lib/src/ui/page/photo_main_page.dart

 import 'dart:typed_data';
 
 import 'package:flutter/material.dart';
+import 'package:photo/src/delegate/loading_delegate.dart';
 import 'package:photo/src/engine/lru_cache.dart';
 import 'package:photo/src/entity/options.dart';
 import 'package:photo/src/provider/config_provider.dart';
             entity: data,
             themeColor: themeColor,
             size: options.thumbSize,
+            loadingDelegate: options.loadingDelegate,
           ),
           _buildMask(containsEntity(data)),
           _buildSelected(data),
 
   final int size;
 
+  final LoadingDelegate loadingDelegate;
+
   const ImageItem({
     Key key,
     this.entity,
     this.themeColor,
     this.size = 64,
+    this.loadingDelegate,
   }) : super(key: key);
 
   @override
           return _buildImageItem(futureData);
         }
         return Center(
-          child: Container(
-            width: 20.0,
-            height: 20.0,
-            child: CircularProgressIndicator(
-              valueColor: AlwaysStoppedAnimation(themeColor),
-            ),
-          ),
+          child:
+              loadingDelegate.buildPreviewLoading(context, entity, themeColor),
         );
       },
     );

lib/src/ui/page/photo_preview_page.dart

     var data = list[index];
     return BigPhotoImage(
       assetEntity: data,
+      loadingWidget: _buildLoadingWidget(data),
     );
   }
 
+  Widget _buildLoadingWidget(AssetEntity entity) {
+    return options.loadingDelegate.buildBigImageLoading(context, entity, themeColor);
+  }
+
   void _onPageChanged(int value) {
     pageChangeController.add(value);
   }
               themeColor: themeColor,
               entity: item,
               size: options.thumbSize,
+              loadingDelegate: options.loadingDelegate,
             ),
             IgnorePointer(
               child: StreamBuilder(
 
 class _BigPhotoImageState extends State<BigPhotoImage>
     with AutomaticKeepAliveClientMixin {
-
-  Widget get loadingWidget{
+  Widget get loadingWidget {
     return widget.loadingWidget ?? Container();
   }