gistfile1.m

#import "JTAsynchronousImageView.h"

@interface JTAsynchronousImageView ()
@property (nonatomic, copy) void (^loadingBlock)(JTAsynchronousImageViewLoadCompletionHandler completionHandler);
@property (nonatomic, strong) void (^cancellationBlock)();
@end

@implementation JTAsynchronousImageView {
    // Queue for the image that is currently loading.
	dispatch_queue_t _currentQueue;
}

- (void)awakeFromNib
{
	self.placeholderImage = self.image;
}

- (void)loadWithBlock:(void (^)(JTAsynchronousImageViewLoadCompletionHandler completionHandler))loadingBlock
{
	if (!_loadingQueue) {
		_loadingQueue = dispatch_queue_create("net.bandes-storch.Draw.JTAsynchronousImageView", DISPATCH_QUEUE_SERIAL);
	}
	
	// Once the cancellation block has been submitted, the image has either set cancelled=YES,
	// before execution of the completion block, or it has successfully loaded.
	if (_cancellationBlock) NSLog(@"Cancelling load %@", self);
	if (_cancellationBlock) dispatch_async(_currentQueue, _cancellationBlock);
	
	self.image = _placeholderImage;
	
	_currentQueue = _loadingQueue;
	_loadingBlock = [loadingBlock copy];
	
	JTAsynchronousImageView *__weak bself = self;
	BOOL __block cancelled = NO;
	
	dispatch_async(_loadingQueue, ^{
		typeof(bself) __strong self = bself;
		if (!self) return;
		
		self.loadingBlock(^(UIImage *image) {
			
			// Continue asynchronously, allowing the cancellation block to run if it has already been submitted
			dispatch_async(_loadingQueue, ^{
				typeof(bself) __strong self = bself;
				if (!self) return;
				self.cancellationBlock = nil;
				self.loadingBlock = nil;
				
				if (cancelled) return;
				
				dispatch_async(dispatch_get_main_queue(), ^{
					typeof(bself) __strong self = bself;
					if (!self) return;
					
					self.image = image;
					[self invalidateIntrinsicContentSize];
					[self.superview setNeedsLayout];
				});
			});
		});
	});
	
	_cancellationBlock = [^{
		cancelled = YES;
	} copy];
}

@end