GCD之group正确用法

等待多个异步操作结束，再做汇总操作：

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_group_t group = dispatch_group_create();

    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        [[HttpRequest sharedRequest] getWithURLString:api_applyListQuery_queryApplySchedule parameters:@{} success:^(id responseObject) {
            
            dispatch_group_leave(group);
            
        } failure:^(NSError *error) {
            
            dispatch_group_leave(group);
            
        }];
    });

    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        [[HttpRequest sharedRequest] getWithURLString:api_applyListQuery_queryApplySchedule parameters:@{} success:^(id responseObject) {
            
            dispatch_group_leave(group);
            
        } failure:^(NSError *error) {
            
            dispatch_group_leave(group);
            
        }];
    });

    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        // 汇总操作
    });

注意：
dispatch_group_enter(group);和 dispatch_group_leave(group); 要一一对应，否则会崩溃