方式一

dispatch_group_t group = dispatch_group_create();
dispatch_group_enter(group);
开启异步任务
  successBlock{
     dispatch_group_leave(group);
}
failedBlock{
    dispatch_group_leave(group);
}
dispatch_group_notify(group, dispatch_get_main_queue(), ^{

异步任务完成后 需要做的事情....

});

方式二

self.lock = [[NSConditionLock alloc]initWithCondition:1];
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        [self.lock lockWhenCondition:1];
        for (int i = 0; i < 10; i ++) {
            NSLog(@"---%d---",i);
        }
        [self.lock unlockWithCondition:2];
    });
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        [self.lock lockWhenCondition:2];
        for (int i = 0; i < 10; i ++) {
            NSLog(@"+++%d+++",i);
        }
        [self.lock unlockWithCondition:3];
    });
    
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        [self.lock lockWhenCondition:3];
        for (int i = 0; i < 10; i ++) {
            NSLog(@"***%d***",i);
        }
        [self.lock lock];
    });

方式三

dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_SERIAL);
    dispatch_async(queue, ^{
        sleep(3);
        NSLog(@"111111111111111111");
    });
    dispatch_async(queue, ^{
        sleep(2);
        NSLog(@"2222222222222222222");
        
    });
    dispatch_async(queue, ^{
        sleep(1);
        NSLog(@"333333333333333333");
     
    });

GCD信号量详解

- (void)viewDidLoad {
    [super viewDidLoad];
    //表示最多5个异步任务会进来访问
    self.semaphore = dispatch_semaphore_create(5);
    for (int i = 0; i < 20; i ++) {
        [[[NSThread alloc]initWithTarget:self selector:@selector(test) object:nil] start];
    }
}
- (void)test
{
//dispatch_semaphore_wait 判断信号量的值 如果信号量的值 > 0 就让信号量的值减1 然后继续往下执行
//如果信号量的值<= 0就会休眠等待 直到信号量的值>0 然后让信号量的值减1 然后继续往下执行
//DISPATCH_TIME_FOREVER表示一直等 
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    sleep(2);
    NSLog(@"%@",[NSThread currentThread]);
//让信号量的值+1
    dispatch_semaphore_signal(self.semaphore);
}

 self.semaphore = dispatch_semaphore_create(1);
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
        for (int i = 0; i < 10; i ++) {
            NSLog(@"--------111---------");
        }
        dispatch_semaphore_signal(self.semaphore);
    });
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
        for (int i = 0; i < 10; i ++) {
            NSLog(@"++++++++222+++++++++");
        }
        dispatch_semaphore_signal(self.semaphore);
    });
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
        for (int i = 0; i < 10; i ++) {
            NSLog(@"*********333***********");
        }
        dispatch_semaphore_signal(self.semaphore);
    });

方式四

 dispatch_async(dispatch_get_global_queue(0, 0), ^{
        @synchronized (self) {
            for (int i = 0; i < 10; i ++) {
                NSLog(@"------1111------");
            }
        }
        
    });
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        @synchronized (self) {
            for (int i = 0; i < 10; i ++) {
                NSLog(@"------2222------");
            }
        }
        
    });
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        @synchronized (self) {
            for (int i = 0; i < 10; i ++) {
                NSLog(@"*****3333******");
            }
        }
    });

栅栏函数

//dispatch_barrier_async 必须传入自己创建的并发队列 不能是全局并发队列 也不能是串行队列
    dispatch_queue_t queue = dispatch_queue_create("rw_queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        NSLog(@"多读");
    });
    dispatch_barrier_async(queue, ^{
        NSLog(@"单写");
    });