IOS GCD (事例下载图片)

@interface HMViewController ()
@property (weak, nonatomic) IBOutlet UIImageView *imageView;

@end

@implementation HMViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, );
    dispatch_async(queue, ^{
        NSLog(@"--download--%@", [NSThread currentThread]);
        // 下载图片
        NSURL *url = [NSURL URLWithString:@"http://news.baidu.com/z/resource/r/image/2014-06-22/2a1009253cf9fc7c97893a4f0fe3a7b1.jpg"];
        NSData *data = [NSData dataWithContentsOfURL:url]; // 这行会比较耗时
        UIImage *image = [UIImage imageWithData:data];

        // 回到主线程显示图片
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"--imageView--%@", [NSThread currentThread]);
            self.imageView.image = image;
        });
    });
}

@end