-(void)addAssetURL:(NSURL*)assetURL toAlbum:(NSString*)albumName withCompletionBlock:(SaveImageCompletion)completionBlock
{
    //相簿存在標示
    __block BOOL albumWasFound = NO;
    
    ALAssetsLibrary *assetsLibrary = [[ALAssetsLibrary alloc] init];
    //search all photo albums in the library
    [assetsLibrary enumerateGroupsWithTypes:ALAssetsGroupAlbum usingBlock:^(ALAssetsGroup *group, BOOL *stop)
     {
         
         //判斷相簿是否存在
         if ([albumName compare: [group valueForProperty:ALAssetsGroupPropertyName]]==NSOrderedSame) {
             
             //存在
             albumWasFound = YES;
             
             //get a hold of the photo's asset instance
             [assetsLibrary assetForURL: assetURL
                            resultBlock:^(ALAsset *asset) {
                                
                                //add photo to the target album
                                [group addAsset: asset];
                                
                                //run the completion block
                                completionBlock(nil);
                                
                            } failureBlock: completionBlock];
             return;
         }
         
         //如果不存在該相簿建立
         if (group==nil && albumWasFound==NO)
         {
             __weak ALAssetsLibrary* weakSelf = assetsLibrary;
             
             //建立相簿
             [assetsLibrary addAssetsGroupAlbumWithName:albumName resultBlock:^(ALAssetsGroup *group)
              {
                  
                  //get the photo's instance
                  [weakSelf assetForURL: assetURL
                            resultBlock:^(ALAsset *asset)
                   {
                       
                       //add photo to the newly created album
                       [group addAsset: asset];
                       
                       //call the completion block
                       completionBlock(nil);
                       
                   } failureBlock: completionBlock];
                  
              } failureBlock: completionBlock];
             return;
         }
         
     }failureBlock:completionBlock];
}