GCD队列组

// 1.队列组

    dispatch_group_t group = dispatch_group_create();

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    

    // 2.下载图片

    __block UIImage * image1 = nil;

    dispatch_group_async(group, queue, ^{

        NSURL * url1 = [NSURL URLWithString:@"https://www.baidu.com/img/bd_logo1.png"];

        NSData * data1 = [NSData dataWithContentsOfURL:url1];

        image1 = [UIImage imageWithData:data1];

    });

    

    __block UIImage * image2 = nil;

    dispatch_group_async(group, queue, ^{

        NSURL * url2 = [NSURL URLWithString:@"https://www.baidu.com/img/bd_logo1.png"];

        NSData * data2 = [NSData dataWithContentsOfURL:url2];

        image2 = [UIImage imageWithData:data2];

    });

    

    // 3.合并图片(保证组里面所有的任务执行完之后，再执行notify函数里面的block)

    dispatch_group_notify(group, queue, ^{

        //开启一个位图上下文

        UIGraphicsBeginImageContextWithOptions(image1.size, NO, 0.0);

        //绘制第1张图片

        CGFloat image1W = image1.size.width;

        CGFloat image1H = image1.size.height;

        [image1 drawInRect:CGRectMake(0, 0, image1W, image1H)];

        

        //绘制第2张图片

        CGFloat image2W = image2.size.width * 0.5;

        CGFloat image2H = image2.size.height * 0.5;

        CGFloat image2Y = image1H - image2H;

        [image2 drawInRect:CGRectMake(0, image2Y, image2W, image2H)];

        

        //得到上下文中的图片

        UIImage * fullImage = UIGraphicsGetImageFromCurrentImageContext();

        

        //结束上下文

        UIGraphicsEndImageContext();

        

        // 4.回到主线程显示图片

        dispatch_async(dispatch_get_main_queue(), ^{

            self.imageView.image = fullImage;

        });

    });