ALAssetsLibrary* library = [[ALAssetsLibrary alloc] init];
    [library enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos
                           usingBlock:libraryGroupsEnumeration
                         failureBlock:failureblock];
ALAssetsGroupEnumerationResultsBlock groupEnumerAtion = ^(ALAsset *result, NSUInteger index, BOOL *stop){
        if (result!=NULL) {

            if ([[result valueForProperty:ALAssetPropertyType] isEqualToString:ALAssetTypePhoto]) {

                [self._dataArray addObject:result];
            }

        }
    };

ALAssetsLibraryGroupsEnumerationResultsBlock
    libraryGroupsEnumeration = ^(ALAssetsGroup* group, BOOL* stop){
        //within the group enumeration block.filter to enumerate just photos.
        [group setAssetsFilter:[ALAssetsFilter allPhotos]];
        if (group!=nil) {
            NSString *g=[NSString stringWithFormat:@"%@",group];
            NSLog(@"gg:%@",g);//gg:ALAssetsGroup - Name:Camera Roll, Type:Saved Photos, Assets count:71
            [group enumerateAssetsUsingBlock:groupEnumerAtion];
        }
        else {
            dispatch_async(dispatch_get_global_queue(0, 0), ^{
                [self saveToDB:self._dataArray];

            });
        }

    };

Assume that my Camera Roll have 100 photos,and I want get first 30 to save to my database.but in above code ,I must wait for 100 results fisishing.After 30 write to database , continue to get another 30 until end. because get 100 or even more photos will delay my UI refresh.It looks not comfortable. Thanks a lot!

what should I write.?

Try this one

if (result!=NULL) {

    if ([[result valueForProperty:ALAssetPropertyType] isEqualToString:ALAssetTypePhoto])         {

          [self._dataArray addObject:result];

            if([self._dataArray count] == 30){

               dispatch_async(dispatch_get_global_queue(0, 0), ^{

                 NSArray *array = [[NSArray alloc] initWithArray:self._dataArray]; //please change the array declaration to top of this method. Because the Block will not allow to do it here.
                 [self._dataArray removeAllObjects];
                 [self saveToDB:array];
                 //array release,if not using ARC

           });
       }
    }

}

and

if (group!=nil) {

            [group enumerateAssetsUsingBlock:groupEnumerAtion];
        }
        else if([self._dataArray count] > 0) {

              dispatch_async(dispatch_get_global_queue(0, 0), ^{

                    [self saveToDB:self._dataArray];

            });
        }

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