objective-c - Objective-C / ALAssetslibrary - メソッドを同期的に動作するように変更するにはどうすればよいですか

Question

Alassetslibrary を使用して、画像に関する情報を正しく取得するメソッドを実装しました。ご存知のように、非同期で動作しますが、私の場合は同期で動作する必要があります。画像に関する情報を読み込んでメソッドに戻るだけです。更新を必要としない唯一のビューとしての私のアプリケーション。メソッドを変更するにはどうすればよいですか? 現時点では、「return xmlList;」という行です。結果は常に null ですが、NSlog を使用したため、アセットがブロックで正しく読み取られていることがわかります。

-(NSString *) getPhotos{

NSMutableArray *idList = [[NSMutableArray alloc] init];
NSMutableArray *widthList = [[NSMutableArray alloc] init];
NSMutableArray *heightList = [[NSMutableArray alloc] init];
NSMutableArray *orientationList = [[NSMutableArray alloc] init];
NSMutableArray *dateList = [[NSMutableArray alloc] init];

__block XMLWriter* xmlWriter = [[XMLWriter alloc]init];
__block NSString *xmlList;
__block NSString *test;
__block ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
__block NSString *description = [[NSString alloc] init];
[library enumerateGroupsWithTypes:ALAssetsGroupAll usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
    if (group) {
        [group setAssetsFilter:[ALAssetsFilter allPhotos]];
        [group enumerateAssetsUsingBlock:^(ALAsset *asset, NSUInteger index, BOOL *stop){
            if (asset){
                NSString *description = [asset description];
                NSRange first = [description rangeOfString:@"URLs:"];
                NSRange second = [description rangeOfString:@"?id="];
                NSString *path = [description substringWithRange: NSMakeRange(first.location + first.length, second.location - (first.location + first.length))];
                [idList addObject:path];

                NSDictionary *data = [[asset defaultRepresentation] metadata];

                NSNumber *width = [[[asset defaultRepresentation] metadata] objectForKey:@"PixelWidth"];
                NSString *widthString = [NSString stringWithFormat:@"%@", width];
                [widthList addObject:widthString];

                NSNumber *height = [[[asset defaultRepresentation] metadata] objectForKey:@"PixelHeight"];
                NSString *heightString = [NSString stringWithFormat:@"%@", height];
                [heightList addObject:heightString];

                NSNumber *orientation = [[[asset defaultRepresentation] metadata] objectForKey:@"Orientation"];
                NSString *orientationString = [NSString stringWithFormat:@"%@", orientation];
                if(orientationString != NULL){
                    [orientationList addObject:orientationString];
                } else {
                    NSString *noOrientation = [[NSString alloc] init];
                    noOrientation = @"No orientation avaiable";
                    [dateList addObject:noOrientation];
                }

                NSString *dateString = [[[asset defaultRepresentation] metadata] objectForKey:@"DateTime"];
                if(dateString != NULL){
                    [dateList addObject:dateString];
                } else {
                    NSString *noDate = [[NSString alloc] init];
                    noDate = @"No date avaiable";
                    [dateList addObject:noDate];
                }
            }
        }];
    }
    if (group == nil){
        [xmlWriter writeStartDocumentWithEncodingAndVersion:@"UTF-8" version:@"1.0"];
        [xmlWriter writeStartElement:@"Data"];
        int i = 0;
        for(i = 0; i<=[idList count]-1; i++){
            [xmlWriter writeStartElement:@"Photo"];
            [xmlWriter writeAttribute:@"Id" value:[idList objectAtIndex:i]];
            [xmlWriter writeAttribute:@"Width" value:[widthList objectAtIndex:i]];
            [xmlWriter writeAttribute:@"Height" value:[heightList objectAtIndex:i]];
            [xmlWriter writeAttribute:@"Orientation" value:[orientationList objectAtIndex:i]];
            [xmlWriter writeAttribute:@"Date" value:[dateList objectAtIndex:i]];
            [xmlWriter writeEndElement:@"Photo"];
        }
        [xmlWriter writeEndElement:@"Data"];
        [xmlWriter writeEndDocument];
        xmlList = [xmlWriter toString];
    }

} failureBlock:^(NSError *error) {
    NSLog(@"error enumerating AssetLibrary groups %@\n", error);
}];

return xmlList;

}