OC

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    [self downloadImage];
}

-(void)downloadImage{
    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_group_async(group, queue, ^{
        NSURL *url = [NSURL URLWithString:@"https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1530167525247&di=f09b0f4412a2825e4ae641079691cd14&imgtype=0&src=http%3A%2F%2Fpic35.photophoto.cn%2F20150617%2F0013026499624253_b.png"];
        NSData *data = [NSData dataWithContentsOfURL:url];
        self.image1 = [UIImage imageWithData:data];
    });
    
    dispatch_group_async(group, queue, ^{
        NSURL *url = [NSURL URLWithString:@"https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1530167525247&di=184be5b28b34bf0670e9cb065fef673c&imgtype=0&src=http%3A%2F%2Fpic22.nipic.com%2F20120724%2F16351_204302573152_2.png"];
        NSData *data = [NSData dataWithContentsOfURL:url];
        self.image2 = [UIImage imageWithData:data];
    });
    
    
    dispatch_group_notify(group,queue , ^{
        
        //1创建图形上下文
        UIGraphicsBeginImageContext(CGSizeMake(200, 200));
        
        //2.画图
        [self.image1 drawInRect:CGRectMake(0, 0, 200, 100)];
        self.image1 = nil;//此时不需要图片了
        
        //3.画图
        [self.image2 drawInRect:CGRectMake(0, 100, 200, 100)];
        self.image2 = nil;//此时不需要图片了
        
        //4.从图形上下文获取图片
        UIImage *tempImage = UIGraphicsGetImageFromCurrentImageContext();
        
        //5.关闭上下文
        UIGraphicsEndImageContext();
        
        //6更新UI
        dispatch_async(dispatch_get_main_queue(), ^{
            self.imageView.image = tempImage;
        });
    });
}

swift 版 下载图片

    //保证 下载完图片在分享
    private func getDownloadImage(urlStrArr:[String], block:@escaping (_ imageArr:[UIImage])->()){
        var downloadImageArr : [UIImage] = []
        var tempBlock : ((_ imageArr:[UIImage])->())? = nil
        tempBlock = block
        let group = DispatchGroup()
        let diyQueue = DispatchQueue(label: "diyQueue")
        
        for (_, urlStr) in urlStrArr.enumerated(){
            group.enter()
            diyQueue.async {
                if let tempUrl = URL.init(string: urlStr){
                    if  let tempData = try? Data.init(contentsOf: tempUrl){
                        if let tempImage = UIImage.init(data: tempData){
                            downloadImageArr.append(tempImage)
                            group.leave()
                        }
                    }
                }
            }
        }
        
        //走完队列
        group.notify(queue: diyQueue) {
            
            //这个地方可以合并数据, 图片合并操作等等
            //主线程刷新
            DispatchQueue.main.async {
                tempBlock?(downloadImageArr)
            }
        }
    }

调用

        getDownloadImage(urlStrArr: [dataStruct.relationQrCodeUrl]) { [weak self](imageArr:[UIImage]) in
            self?.downloadView.updateImage(downImage: imageArr[0])
        }