//獲得隊列
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    //創(chuàng)建一個定時器
    self.time = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    //設(shè)置開始時間
    dispatch_time_t start = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.0 * NSEC_PER_SEC));
    //設(shè)置時間間隔
    uint64_t interval = (uint64_t)(15.0* NSEC_PER_SEC);
    //設(shè)置定時器
    dispatch_source_set_timer(self.time, start, interval, 0);
    //設(shè)置回調(diào)
    WEAKSELF
    dispatch_source_set_event_handler(self.time, ^{
        [weakSelf requestCurrentLoacation];
        if (isAgain == NO) { //取消定時器
            dispatch_cancel(self.time);
        }
    });
    //啟動定時器 (默認是關(guān)閉的)
    dispatch_resume(self.time);

@property (nonatomic, strong)dispatch_source_t time;