多个网络请求的并发情况

可在最后执行的一个操作里进行界面刷新

    dispatch_group_t group = dispatch_group_create();
    
    /*
     
     dispatch_group_enter：通知group，下面的任务马上要放到group中执行了。
     dispatch_group_leave：通知group，任务完成了，该任务要从group中移除了。
     
     */
    
    //轮播图
    dispatch_group_enter(group);
    [self sendADCycleImageRequestFinished:^(BOOL isFinished) {
        dispatch_group_leave(group);
    }];
    
    //热门样板间
    dispatch_group_enter(group);
    [self sendHotHouseTypeRequestFinished:^(BOOL isFinished) {
        dispatch_group_leave(group);
    }];
    
    //热卖商品
    dispatch_group_enter(group);
    [self sendHotGoodsRequestFinished:^(BOOL isFinished) {
        dispatch_group_leave(group);
    }];
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        
        [self.refresh mk_endRefreshing];

        [self.collectionView reloadData];
    });