ios - How do I return a variable from a block inside a method?

Question

Say I have this method that given a URL returns a UIImage:

- (void)getUIImageFromURL:(NSURL *)URL {
    NSURLRequest *request = [NSURLRequest requestWithURL:URL];
    AFHTTPRequestOperation *imageOperation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
    imageOperation.responseSerializer = [AFImageResponseSerializer serializer];

    [imageOperation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
        return (UIImage *)responseObject;
    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
    }];

    [imageOperation start];
}

But it keeps giving me this error:

Incompatible block pointer types sending 'UIImage *(^)(AFHTTPRequestOperation *__strong, _strong id)' to parameter of type 'void (^)(AFHTTPRequestOperation *_strong, __strong id)'

I'm somewhat new to blocks, so perhaps I'm approaching this completely backwards. How best would I implement a method like this?

Answer 1

You cannot return an image from inside a block. This is an asynchronous API and cannot be used in the manner you are attempting to. Either use a blocking API, where the method is blocking until the image is downloaded (a bad solution), or implement support for the asynchronous API. For instance, pass a completion block to your getImage method and call it with in the completion block of the download operation. In this block, do what you need with the image.