Loading an image into UIImage asynchronously

I wrote a custom class to do just this, using blocks and GCD:

WebImageOperations.h

#import <Foundation/Foundation.h>@interface WebImageOperations : NSObject {}// This takes in a string and imagedata object and returns imagedata processed on a background thread+ (void)processImageDataWithURLString:(NSString *)urlString andBlock:(void (^)(NSData *imageData))processImage;@end

WebImageOperations.m

#import "WebImageOperations.h"#import <QuartzCore/QuartzCore.h>@implementation WebImageOperations+ (void)processImageDataWithURLString:(NSString *)urlString andBlock:(void (^)(NSData *imageData))processImage{    NSURL *url = [NSURL URLWithString:urlString];    dispatch_queue_t callerQueue = dispatch_get_current_queue();    dispatch_queue_t downloadQueue = dispatch_queue_create("com.myapp.processsmagequeue", NULL);    dispatch_async(downloadQueue, ^{        NSData * imageData = [NSData dataWithContentsOfURL:url];        dispatch_async(callerQueue, ^{            processImage(imageData);        });    });    dispatch_release(downloadQueue);}@end

And in your ViewController

    - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{    // Pass along the URL to the image (or change it if you are loading there locally)    [WebImageOperations processImageDataWithURLString:entry.photo andBlock:^(NSData *imageData) {    if (self.view.window) {        UIImage *image = [UIImage imageWithData:imageData];        cell.photo.image = image;    }    }];}

It is very fast and will load the images without affecting the UI or scrolling speed of the TableView.

*** Note - This example assumes ARC is being used. If not, you will need to manage your own releases on objects)

In iOS 6 and later dispatch_get_current_queue gives deprecation warnings.

Here is an alternative that is a synthesis of the @ElJay answer above and the article by @khanlou here.

Create a category on UIImage:

UIImage+Helpers.h

@interface UIImage (Helpers)+ (void) loadFromURL: (NSURL*) url callback:(void (^)(UIImage *image))callback;@end

UIImage+Helpers.m

#import "UIImage+Helpers.h"@implementation UIImage (Helpers)+ (void) loadFromURL: (NSURL*) url callback:(void (^)(UIImage *image))callback {    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);    dispatch_async(queue, ^{        NSData * imageData = [NSData dataWithContentsOfURL:url];        dispatch_async(dispatch_get_main_queue(), ^{            UIImage *image = [UIImage imageWithData:imageData];            callback(image);        });    });}@end

Take a look at SDWebImage:

https://github.com/rs/SDWebImage

It's a fantastic set of classes that handle everything for you.

Tim