信号量的详细用法，可以用此方法进行队列管理

-(void)dispatchSignal{

    //crate的value表示，最多几个资源可访问
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(3);
    dispatch_queue_t quene = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    //任务1
    dispatch_async(quene, ^{
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        NSLog(@"run task 1");
        sleep(1);
        NSLog(@"complete task 1");
        dispatch_semaphore_signal(semaphore);
    });
    //任务2
    dispatch_async(quene, ^{
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        NSLog(@"run task 2");
        sleep(1);
        NSLog(@"complete task 2");
        dispatch_semaphore_signal(semaphore);
    });
    //任务3
    dispatch_async(quene, ^{
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        NSLog(@"run task 3");
        sleep(1);
        NSLog(@"complete task 3");
        dispatch_semaphore_signal(semaphore);
    });

}

队列组：

适用场景：分别异步执行2个耗时任务，然后当2个耗时任务都执行完毕后再回到主线程执行任务。这时候我们可以用到 GCD 的队列组。

- (void)groupNotify {

    NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    NSLog(@"group---begin");
    
    dispatch_group_t group =  dispatch_group_create();
    
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 追加任务1
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"1---%@",[NSThread currentThread]);      // 打印当前线程
        }
    });
    
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 追加任务2
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"2---%@",[NSThread currentThread]);      // 打印当前线程
        }
    });
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        // 等前面的异步任务1、任务2都执行完毕后，回到主线程执行下边任务
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"3---%@",[NSThread currentThread]);      // 打印当前线程
        }
        NSLog(@"group---end");
    });
}
//    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

// dispatch_async(queue, ^{
//
//
// });

//group的用法
APIHTTPService *httpService = [APIHTTPService sharedService];
__block NSError *blockError;
dispatch_group_t group = dispatch_group_create();

/* 先获取Token */
dispatch_group_enter(group);
[httpService fetchTokenWithCompletionHandler:^(NSString *token, NSError *error) {
    if (error) {
        NSLog(@"[MBStitchAPIService] Fetch token failed");
        blockError = error;
    }
    dispatch_group_leave(group);
}];
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);