- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {//开一个子线程dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{//下载图片NSURL *url = [NSURL URLWithString:@"http://e.hiphotos.baidu.com/image/pic/item/14ce36d3d539b600be63e95eed50352ac75cb7ae.jpg"];NSData *data = [NSData dataWithContentsOfURL:url];UIImage *img = [UIImage imageWithData:data];dispatch_async(dispatch_get_main_queue(), ^{//回到主线程self.imgVIew.image = img;});//在这里使用同步还是异步，区别是前者是按顺序依次执行，后者是先执行到最后再回到主线程NSLog(@"________");});
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {//创建子线程[self performSelectorInBackground:@selector(downLoad) withObject:nil];
}//在子线程中下载图片
- (void)downLoad {NSURL *url = [NSURL URLWithString:@"http://e.hiphotos.baidu.com/image/pic/item/e7cd7b899e510fb34395d1c3de33c895d0430cd1.jpg"];NSData *data = [NSData dataWithContentsOfURL:url];UIImage *img = [UIImage imageWithData:data];//下载完毕，返回给主线程图片    [self.imgVIew performSelector:@selector(setImage:) onThread:[NSThread mainThread] withObject:img waitUntilDone:YES];}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {[[[NSOperationQueue alloc] init] addOperationWithBlock:^{//下载图片NSURL *url = [NSURL URLWithString:@"http://e.hiphotos.baidu.com/image/pic/item/e7cd7b899e510fb34395d1c3de33c895d0430cd1.jpg"];NSData *data = [NSData dataWithContentsOfURL:url];UIImage *img = [UIImage imageWithData:data];//回到主线程[[NSOperationQueue mainQueue] addOperationWithBlock:^{self.imgView.image = img;}];}];}