GCD in iOS

GCD (Grand Central Dispatch) is one very powerful way of multi-thread to handle many tasks at the same time. I read many articles which say that GCD is much better than other ways, such as NSThread, NSOperationQueue, and NSInvocationOperation, but I'm not sure about this. Maybe GCD is mainly focus on the CPU which has multi core after iOS 4.0 and beyond. The following are some code I wrote to get familiar with GCD.

- (void)viewDidLoad

{

    [super viewDidLoad];

    self.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithImage:[UIImageimageNamed:@"AddStack.png"] style:UIBarButtonSystemItemAdd target:selfaction:@selector(performMethod)];

    

    self.indicator = [[UIActivityIndicatorView alloc] initWithFrame:CGRectMake(160, 200, 20,20)];

    self.indicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyleGray;

    self.indicator.hidesWhenStopped = YES;

    

    [self.view addSubview:self.indicator];

    [self.indicator release];

    

}

- (void)performMethod {

    

    self.indicator.hidden = NO;

    [self.indicator startAnimating];

    

    //Open another new async operation in the global queue

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        NSURL * url = [NSURL URLWithString:@"http://www.apple.com"];

        NSError * error;

        NSString * data = [NSString stringWithContentsOfURL:url encoding:NSUTF8StringEncodingerror:&error];

        

        if (data != nil) {

    //Inform main thread to refresh UI.

            dispatch_async(dispatch_get_main_queue(), ^{

                [self.indicator stopAnimating];

                self.indicator.hidden = YES;

                

                NSLog(@"Result: %@", data);

            });

            

        } else {

            

            dispatch_async(dispatch_get_main_queue(), ^{

                [self.indicator stopAnimating];

                self.indicator.hidden = YES;

                

                NSLog(@"error when download:%@", error);

            });

            

        }

        

    });

}

Another similiar example is as below.

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath*)indexPath

{

    static NSString *cellIdentity = @"ImageViewCell";

    

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentity];

    

    if (cell ==  nil) {

        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefaultreuseIdentifier:cellIdentity] autorelease];

    }

    

    UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(5, 5, 90, 90)];

    [cell.contentView addSubview:imageView];

    [imageView release];

    

    dispatch_async(dispatch_get_global_queue(0, 0), ^{

        UIImage *img = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:[imagesUrls objectAtIndex:indexPath.row]]]];

        dispatch_async(dispatch_get_main_queue(), ^{

            [imageView setImage:img];

        });

    });

    

    return cell;

}