Objective C: Download Thread not affecting UI, even possible? - objective-c

Question

I'm working through the Standford course on ItunesU and there they say a thread to download something should not do anything to the UI, this should only happen on the Main Thread.
Well, in my example I'm downloading a picture from Flicker and I want to setup this picture (via a segue) in a UIScrollView. So while I'm downloading this picture in the "side" thread I'm setting the image property of the UIScrollview to image etc. However this doesn't work obviously, because I don't know the imagesize yet and I also don't have a reference to that image object yet to set right?
So how do you handle that? I hope I'm clear..here is my example:
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(NSIndexPath *)sender{
NSDictionary *selectedPhoto = [self.photos objectAtIndex:sender.row];
[self.defaults addPhotoToRecentlyViewed:selectedPhoto];
[self.defaults saveDefaults];
PhotoViewer *photoViewer = segue.destinationViewController;
UIActivityIndicatorView *spinner = [[UIActivityIndicatorView alloc]initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
photoViewer.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc]initWithCustomView:spinner];
[spinner startAnimating];
dispatch_queue_t photoDownload = dispatch_queue_create("photoviewever", nil);
dispatch_async(photoDownload, ^{
NSData *data = [NSData dataWithContentsOfURL:[FlickrFetcher urlForPhoto:selectedPhoto format:FlickrPhotoFormatLarge]];
UIImage *image = [UIImage imageWithData:data];
photoViewer.image = image;
dispatch_async(dispatch_get_main_queue(), ^{
photoViewer.title = [selectedPhoto objectForKey:FLICKR_PHOTO_TITLE];
photoViewer.navigationItem.rightBarButtonItem = nil;
});
});
}
and my PhotoViewer:
#import "PhotoViewer.h"
#interface PhotoViewer ()
#property (nonatomic, strong) IBOutlet UIScrollView *scrollView;
#property (nonatomic, strong) IBOutlet UIImageView *imageView;
#end
#implementation PhotoViewer
#synthesize scrollView = _scrollView;
#synthesize imageView = _imageView;
#synthesize image = _image;
- (void)viewDidLoad
{
[super viewDidLoad];
self.scrollView.delegate = self;
[self setupImage];
}
- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView{
return self.imageView;
}
- (void)setupImage{
self.imageView.image = self.image;
self.imageView.frame = CGRectMake(0, 0, self.image.size.width, self.image.size.height);
self.scrollView.contentSize = self.image.size;
[self.imageView setNeedsDisplay];
}
#end

Answer 1

You should move this line:
photoViewer.image = image;
into the "completion" block you dispatch back to the main thread. PhotoViewer appears to be a view controller, and as such it's subject to the same "main thread only" rules as other UI. Also, it looks like you'll need to call -setupImage again from that main thread "completion" block. (Otherwise the image will never get pushed into the image view, as you noted.)
Also in -setupImage, you want to check that self.image returns a non-nil value before addressing into it. Depending on which compiler you're using, the behavior of calling a struct-returning Objective C method is 'undefined' (i.e. self.image.size returns a CGSize struct). (Although in recent compilers, it returns a zero filled struct.)