数组的懒加载

//懒加载方式初始化数组(要用到数组时才初始化)
-(NSMutableArray *)nearbyArray
{
    if (_nearbyArray == nil) {
        _nearbyArray = [NSMutableArray array];
    }
    return _nearbyArray;
}

//调用

LimitModel *model = self.nearbyArray[i];