@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