iOS 之 后台下载，前台显示模式，双 block

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    //耗时的操作
    NSURL *url = [NSURL URLWithString:@""];
    NSData *data = [[[NSData alloc] initWithContentsOfURL:url autorelease];
    UIImage *image = [[[UIImage alloc] initWithData:data] autorelease];

    dispatch_async(dispatch_get_main_queue(),^{
        //更新界面
        _imageView.image = image;
    });
});