Can't read PHAsset content asynchronous

I want to read specified file (photo from camera roll) asynchronous, but it does not work for me.

Variable tempData gets nil untill I change config requestOptionForPhotos.synchronous to YES, then everything is ok, but I don't want to perform this code synchronous.

Is it possible that I'm blocking access to photo by requesting to the same file in other thread? I'm newbie in objective-c and iOS programming and I don't know how does it works.

  NSURL *assetUrl = [[NSURL alloc] initWithString:filepath];
  PHFetchResult *collection = [PHAsset fetchAssetsWithALAssetURLs:[NSArray arrayWithObject:assetUrl] options:nil];

  PHImageRequestOptions *requestOptionForPhotos = [[PHImageRequestOptions alloc] init];
  requestOptionForPhotos.networkAccessAllowed = YES;
  requestOptionForPhotos.synchronous = NO;

  __block BOOL isFinished = NO;
  __block NSData * tempData = nil;

  for(PHAsset *asset in collection) {
    [[PHImageManager defaultManager]
     requestImageForAsset:asset
     targetSize:CGSizeMake(80, 80)
     contentMode:PHImageContentModeAspectFill
     options:requestOptionForPhotos
     resultHandler:^(UIImage *result, NSDictionary *info) {
       tempData = UIImagePNGRepresentation(result);
       isFinished = YES;
     }];
  }