保存自定义对象数组的nsDictionary

我有一本字典(如果有区别的话它是可变的),它包含nsarray,而nsarray又包含(nsobject的子类)s 我已经实现了initwithcoder和encodewithcoder,如下所示:

-(id)initWithCoder:(NSCoder *)aDecoder {
    self = [super init];
    self.Title = [aDecoder decodeObjectForKey:@"Title"];
    self.SiteAPIURL = [aDecoder decodeObjectForKey:@"SiteAPIURL"];
    self.ID = [aDecoder decodeObjectForKey:@"ID"];
    return self;
}

-(void) encodeWithCoder:(NSCoder *)aCoder {
    [aCoder encodeObject:self.Title forKey:@"Title"];
    [aCoder encodeObject:self.SiteAPIURL forKey:@"SiteAPIURL"];
    [aCoder encodeObject:self.ID forKey:@"ID"];
}

但是当我这样做的时候,它不能正常保存 [dictionary writeToFile:savepath atomically:YES]; 那么我做错了什么,序列化到底是什么,我需要使用nskeyedarchiver还是其他什么?