iOS-GCD 队列组的使用

    // 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1
    __block UIImage *image1 = nil;
    dispatch_group_async(group, queue, ^{
        NSURL *url1 = [NSURL URLWithString:@"http://111.jpg"];
        NSData *data1 = [NSData dataWithContentsOfURL:url1];
        image1 = [UIImage imageWithData:data1];
    });
    
    // 3.下载图片2
    __block UIImage *image2 = nil;
    dispatch_group_async(group, queue, ^{
        NSURL *url2 = [NSURL URLWithString:@"http://222.png"];
        NSData *data2 = [NSData dataWithContentsOfURL:url2];
        image2 = [UIImage imageWithData:data2];
    });
    
    // 4.合并图片 (保证执行完组里面的所有任务之后，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();
        
        // 5.回到主线程显示图片
        dispatch_async(dispatch_get_main_queue(), ^{
            self.imageView.image = fullImage;
        });
    });