5、dispatch_queue

将子线程里的任务放入串行队列，也可以保证不同子线程里的任务不会同时执行，也可以解决线程同步问题。
卖票和存钱/取钱问题：

- (instancetype)init {
    if (self = [super init]) {
        self.ticketQueue = dispatch_queue_create("ticketQueue", DISPATCH_QUEUE_SERIAL);
        self.moneyQueue = dispatch_queue_create("moneyQueue", DISPATCH_QUEUE_SERIAL);
    }
    return self;
}
-  (void)__saleTicket {
    dispatch_sync(self.ticketQueue, ^{
        [super __saleTicket];
    });
}
- (void)__saveMoney {
    dispatch_sync(self.moneyQueue, ^{
        [super __saveMoney];
    });
}
- (void)__drawMoney {
    dispatch_sync(self.moneyQueue, ^{
        [super __drawMoney];
    });
}