Question

Hi I’m working through the Stanford iOS development class. I have a question regarding threading. I understand UIKit calls should be handled by the main thread. I was wondering if something like this is legal?

- (UIImage *)mapViewController:(MapViewController *)sender imageForAnnotation:(id<MKAnnotation>)annotation {
    FlickrPhotoAnnotation *fpa = (FlickrPhotoAnnotation *) annotation;
    NSURL *url = [FlickrFetcher urlForPhoto:fpa.photo format:FlickrPhotoFormatSquare];

    __block UIImage *image;
    dispatch_queue_t downloadQ = dispatch_queue_create("download queue", NULL);
    dispatch_async(downloadQ, ^{
        NSData *data = [NSData dataWithContentsOfURL:url];
        if (data) {
            dispatch_async(dispatch_get_main_queue(), ^{
                image = [UIImage imageWithData:data];
            });
        }
    });
    dispatch_release(downloadQ);
    return image;
}

or I should just return NSData and handle all the threading in the calling method?

Thanks

Answer 1

Your code won’t do what you want.

You are putting an asynchronous block into a queue, and then immediately returning from the method. You are not guaranteed that the block will actually run before you return — in fact, the odds are that it won’t.

So you’ll return the current value of image. Since you didn’t initialize it, it’s probably garbage. Whoever calls this method will try to use a garbage pointer to an image, and (if you’re lucky) crash. If you had initialized it to nil:

__block UIImage *image = nil;

that would be a little more polite.

The problem here is: your method must return a UIImage, so you must wait for the time it takes to make a fully constructed UIImage before you return. There is zero benefit to doing this on some other thread — you’re still waiting the same amount of time, and switching threads just adds overhead.

In order to load the image in a usefully asynchronous way, you need some way to asynchronously tell the caller when the image is done loading, via a callback to a delegate method or block. For example, look at NSURLConnection in Foundation. It has an older method that calls back via a delegate, and a newer method (+sendAsynchronousRequest:queue:completionHandler:) that calls back via a block.