GCD队列组

@interface
ViewController ()

@property (nonatomic,strong)UIImage
*image1;

@property (nonatomic,strong)UIImage
*image2;

@end

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent
*)event{

dispatch_queue_t queue =
dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,
0);

/*

//1.下载图片1

dispatch_async(queue, ^{

NSURL *url = [NSURL URLWithString:@"图片"];

NSData *data = [NSData dataWithContentsOfURL:url];

UIImage *image = [UIImage imageWithData:data];

});

//2.下载图片2

dispatch_async(queue, ^{

NSURL *url = [NSURL URLWithString:@"图片"];

NSData *data = [NSData dataWithContentsOfURL:url];

UIImage *image = [UIImage imageWithData:data];

});

// 方法一：拦截方法，保证前两个任务完成，再做3.图片合成

//并发队列不知道前面谁先请求下来，所以加dispatch_barrier_async

//dispatch_barrier_async(, <#^(void)block#>)

//3.将图片1，图片2合成一张新的图片

//4.将新图片显示出来

*/

// 方法二：

// 创建一个队列组

dispatch_group_t group =
dispatch_group_create();

dispatch_group_async(group, queue, ^{

NSURL *url = [NSURL
URLWithString:@"图片"];

NSData *data = [NSData
dataWithContentsOfURL:url];

_image1 = [UIImage
imageWithData:data];

});

dispatch_group_async(group, queue, ^{

NSURL *url = [NSURL
URLWithString:@"图片"];

NSData *data = [NSData
dataWithContentsOfURL:url];

_image2 = [UIImage
imageWithData:data];

});

//3.将图片1，图片2合成一张新的图片

// 任务一和任务二都执行完，才执行dispatch_group_notify

dispatch_group_notify(group, queue, ^{

// 开启新的图像上下文

UIGraphicsBeginImageContext(CGSizeMake(100,
100));

//绘制图片

[self.image1
drawInRect:CGRectMake(0,
0, 50,
100)];

[self.image2
drawInRect:CGRectMake(50,
0, 50,
100)];

//取得上下文图片

UIImage *image =
UIGraphicsGetImageFromCurrentImageContext();

//结束上下文

UIGraphicsEndImageContext();

//回到主线程显示图片

dispatch_async(dispatch_get_main_queue(), ^{

//4将图片显示过来，到storyboard

self.imageView.image = image;

});

});

}