iOS并发执行

平时运用总会遇到几个网络请求需要同时进行：

    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0); //创建信号量
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); //创建全局并行
    dispatch_group_t group = dispatch_group_create();
    
    dispatch_group_async(group, queue, ^{
        
        NSLog(@"-->> 这是并发任务A");
        for (int i = 0; i<100; i++) {
            NSLog(@"正在处理A %d",i);
        }
        dispatch_semaphore_signal(semaphore);
    });
    
    dispatch_group_async(group, queue, ^{
        
        NSLog(@"-->> 这是并发任务B");
        for (int i = 0; i<100; i++) {
            NSLog(@"正在处理B %d",i);
        }
        dispatch_semaphore_signal(semaphore);
    });
    
    dispatch_group_async(group, queue, ^{
        NSLog(@"-->> 这是并发任务C");
        for (int i = 0; i<100; i++) {
            NSLog(@"正在处理C %d",i);
        }
        dispatch_semaphore_signal(semaphore);
    });
    
    dispatch_group_async(group, queue, ^{
        NSLog(@"-->> 这是并发任务D");
        for (int i = 0; i<100; i++) {
            NSLog(@"正在处理D %d",i);
        }
        dispatch_semaphore_signal(semaphore);
    });
    
    dispatch_group_notify(group, queue, ^{ //四个请求对应四次信号等待
        
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        NSLog(@"----->>>> 渲染UI");
    });