不知道该怎么做

我弄得一团糟，试图找到一种熟练的方法来解决这个问题，但是却陷入困境。我现在的问题是，我要在执行代码之前先执行代码，这与我的代码块有关。

以下代码的问题在于，在添加postToFormmy之前，该方法已被调用并执行convertedImages。我该如何解决这个问题，以等待它发生？还是我会以完全错误的方式来解决这个问题？（我也需要了解内存使用情况，需要在发布信息后释放内存。）

这是包含问题的代码：

// create serial queue
dispatch_queue_t queue = dispatch_queue_create("myQueue", 0);

// create dispatch group
dispatch_group_t group = dispatch_group_create();

for(id photos in photoUrls) {
    NSString *urlString = photos;

    // enqueue operation in queue
    dispatch_async(queue, ^{
        // enter the group
        dispatch_group_enter(group);

        // do something async, I do use another dispatch_queue for example
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
            // wrap in autoreleasepool to release memory upon completion
            @autoreleasepool {
                // here for example the nested operation sleeps for two seconds
                NSURL *url = [NSURL URLWithString:urlString];
                ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];

                [library assetForURL:url resultBlock:^(ALAsset *asset) {

                    //TODO: Deal with JPG or PNG
                    NSData *imageData = UIImageJPEGRepresentation([self thumbnailForAsset:asset maxPixelSize:1000], 0);
                    NSString *base64 = [imageData base64EncodedString];
                    NSLog(@"base64::%@",base64);
                    [convertedImages addObject:base64];

                } failureBlock:^(NSError *error) {
                    NSLog(@"that didn't work %@", error);
                }];


                dispatch_group_leave(group);
            }
        });

        // wait until the nested async operation leaves the group (e.g finishes its job)
        dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

        NSLog(@"Finished single operation.");
    });
}

// will be called once all operations complete
dispatch_async(queue, ^{
    NSLog(@"Finished all jobs. ");
    NSLog(@"how many::%lu",(unsigned long)convertedImages.count);
    [jsonWithPhotos setObject:convertedImages forKey:@"photo64"];
    [self postToForm];
});