- (IBAction)getImage:(id)sender {

    //1,准备URL

    NSString *str = @"http://e.hiphotos.baidu.com/image/h%3D1200%3Bcrop%3D0%2C0%2C1920%2C1200/sign=e13d66699825bc31345d059a6eefb6d2/6159252dd42a2834f85385ac5ab5c9ea14cebfa4.jpg";

    NSURL *url = [NSURL URLWithString:str];

    //2，创建请求对象

    NSURLRequest *request = [NSURLRequest requestWithURL:url];

    //链接对象，block 异步

    __block MXTViewController *weakSelf = self;

    NSOperationQueue *queue = [[NSOperationQueue new] autorelease];

    [NSURLConnection sendAsynchronousRequest:request queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {

        //获取data

        UIImage *image = [UIImage imageWithData:data];

        //回到主线程更新页面

        dispatch_sync( dispatch_get_main_queue(), ^{

            //block 不可以使用_imageView 的方式访问属性，会造成self 引用计数+1 ,不能使用强引用

            //要使用weak

            weakSelf.imageView.image = image;

        });

    }];

GET异步 请求图片步骤