异步线程GCD

-(void)a_loadGCD
{
    //创建队列组
    dispatch_group_t group = dispatch_group_create();
    //进入这个组
    dispatch_group_enter(group);
    dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)( 5* NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            //这个组的任务完成时离开
            dispatch_group_leave(group);
            NSLog(@"111-----1111");
        });
        NSLog(@"1111111");
        
    });
    dispatch_group_enter(group);
    dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)( 3* NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            //这个组的任务完成时离开
            dispatch_group_leave(group);
            NSLog(@"222-----222");
        });
        NSLog(@"22222222");
    });
    dispatch_group_enter(group);
    dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)( 1* NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            //这个组的任务完成时离开
            dispatch_group_leave(group);
            NSLog(@"3333-----33333");
        });
        NSLog(@"3333333");
    });
    
    //当所有的任务都完成后会发送这个通知
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"GCD_2 三个任务全部完成");
    });
}

//信号量

-(void)b_loadGCD
{
    //crate的value表示，最多几个资源可访问
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
    dispatch_queue_t quene = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    for (int i = 0; i<5; i++) {
        dispatch_async(quene, ^{
            //等待信号，当信号总量小于0的时候就会一直等待，否则就继续执行，并让信号总量-1。
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
            NSLog(@"run 任务 %d",i);
NSInteger arc = arc4random()%4;
            NSLog(@"-------%ld",(long)arc);
            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(arc * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
                NSLog(@"complete 任务 %d",i);
                //完成一个任务，发送一个信号，会让信号总量加1
                dispatch_semaphore_signal(semaphore);
            });
            
        });
    }
    
}