GCD异步下载

#pragma mark 下载
- (void)download:(id)sender {
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    // 异步执行下载任务
    dispatch_async(queue, ^{
        NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:@"http://localhost:8080/Server/images/image01.jpg"]];
       
        UIImage *image = [UIImage imageWithData:data];
        
        // 获取跟主线程相关的队列
        dispatch_queue_t  main = dispatch_get_main_queue();
        dispatch_async(main, ^{
            self.imageView.image = image;
        });
    });
   
    dispatch_async(queue, ^{
        NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:@"http://localhost:8080/Server/images/image02.jpg"]];
       
        UIImage *image = [UIImage imageWithData:data];
        
        // 获取跟主线程相关的队列
        dispatch_queue_t  main = dispatch_get_main_queue();
        dispatch_async(main, ^{
            self.imageView2.image = image;
        });
    });
}

#pragma mark 同步执行
- (void)sync {
    // 获取一个全局队列
    dispatch_queue_t  queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_sync(queue, ^{
        NSLog(@"执行了一个异步任务-%@", [NSThread currentThread]);
    });
}

#pragma mark 迭代
- (void)diedai {    

    // 手动创建队列

    // dispatch_queue_t queue = dispatch_queue_create("cn.com.queue", NULL);

   // 手动管理内存

   // dispatch_release(queue)

    // 获取一个全局队列
    dispatch_queue_t  queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_apply(10, queue,
      ^(size_t i) {
          NSLog(@"%i", i);
    });
}

// 等group里面的所有任务都执行完后，再回到主线程调用block

- (void)count {
    int a = 2;
    int b = 3;
   
    __block int c;
    __block int d;
    
    // 创建一个组
    dispatch_group_t  group = dispatch_group_create();
    dispatch_queue_t  queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    // 并发、异步执行任务
    dispatch_group_async(group, queue, ^{
        c = a + b; // 5秒;
    });
   
    dispatch_group_async(group, queue, ^{
        d = a * b; // 3秒
    });
     
    dispatch_queue_t  main = dispatch_get_main_queue();
    
    // 等group里面的所有任务都执行完后，再回到主线程调用block
    dispatch_group_notify(group, main, ^{
        int e = c + d;
        NSLog(@"e=%i %@", e, [NSThread currentThread]);
       
        dispatch_release(group);
    });   
}