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我正在尝试缓存 Web 请求。基本上我有一个使用 facebook 用户的朋友列表的应用程序，但我不想在他们每次登录时都抓住它。也许每月刷新一次。在文档目录的 plist 中缓存好友列表似乎对此功能有意义。我这样做如下：

- (void)writeToDisk {
    NSLog(@"writing cache to disk, where cache = %@", cache);
    BOOL res = [cache writeToFile:[FriendCache persistentPath] atomically:YES];
    NSLog(@"reading cache from disk immediately after writing, res = %d", res);
    NSMutableArray *temp = [[NSMutableArray alloc] initWithContentsOfFile:[FriendCache persistentPath]];
    NSLog(@"cache read in = %@", temp);
}

+ (NSString *)persistentPath {
    NSString *documentsDirectory = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) objectAtIndex:0];
    return [documentsDirectory stringByAppendingPathComponent:@"FriendCache.plist"];
}

这些是我正在使用的 FriendCache 单例的成员，它基本上包装了 NSMutableArray。我已经验证 peristentPath 方法正在返回有效路径。正如您在 writeToDisk 方法中看到的那样，我验证缓存中有数据，然后打印写入结果并检查是否可以读回任何数据。永远不会读回数据，因为结果文件写入为0。

缓存打印的输出很长，但这里是缩写版本：

2010-12-28 13:35:23.006 AppName[51607:207] writing cache to disk, where cache = (
        {
        birthday = "<null>";
        name = "Some Name1";
        pic = "http://profile.ak.fbcdn.net/hprofile-ak-snc4/hs1324.snc4/7846385648654.jpg";
        "pic_big" = "http://profile.ak.fbcdn.net/hprofile-ak-snc4/hs442.snc4/784365789465746.jpg";
        "pic_square" = "http://profile.ak.fbcdn.net/hprofile-ak-snc4/hs1324.snc4/7846357896547.jpg";
        sex = female;
        status = "<null>";
        uid = 892374897165;
    },
        {
        birthday = "<null>";
        name = "Some Name2";
        pic = "http://profile.ak.fbcdn.net/hprofile-ak-snc4/hs625.ash1/54636536547_s.jpg";
        "pic_big" = "http://profile.ak.fbcdn.net/hprofile-ak-snc4/hs170.ash2/65465656365666_n.jpg";
        "pic_square" = "http://profile.ak.fbcdn.net/hprofile-ak-snc4/hs625.ash1/654635656547_q.jpg";
        sex = female;
        status = "<null>";
        uid = 7658436;
    },

...

我检查的一件事是在使用 writeToFile 时，我必须确保我正在编写的对象具有有效的 plist 对象。我确实检查了这一点，这是我构造缓存对象的方式：

- (void)request:(FBRequest*)request didLoad:(id)result{
    NSMutableArray *friendsInfo = [[[NSMutableArray alloc] init] autorelease];

    for (NSDictionary *info in result) {
        NSString *friend_id = [NSString stringWithString:[[info objectForKey:@"uid"] stringValue]];
        NSString *friend_name = nil;
        NSString *friend_sex = nil;
        NSString *friend_relationship_status = nil;
        NSString *friend_current_location = nil;
        if ([info objectForKey:@"name"] != [NSNull null]) {
            friend_name = [NSString stringWithString:[info objectForKey:@"name"]];
        }
        if ([info objectForKey:@"relationship_status"] != [NSNull null]) {
            friend_relationship_status = [NSString stringWithString:[info objectForKey:@"relationship_status"]];
        }
        if ([info objectForKey:@"sex"] != [NSNull null]) {
            friend_sex = [NSString stringWithString:[info objectForKey:@"sex"]];
        }
        if ([info objectForKey:@"current_location"] != [NSNull null]) {
            friend_current_location = [[info objectForKey:@"current_location"] objectForKey:@"name"];
        }
        NSString *friend_pic_square = [info objectForKey:@"pic_square"];
        NSString *friend_status = [info objectForKey:@"status"];
        NSString *friend_pic = [info objectForKey:@"pic"];
        NSString *friend_pic_big = [info objectForKey:@"pic_big"];
        NSString *friend_birthday = [info objectForKey:@"birthday"];
        NSDictionary *friend_info = [NSDictionary dictionaryWithObjectsAndKeys:
                                            friend_id,@"uid",
                                            friend_name, @"name", 
                                            friend_pic_square, @"pic_square", 
                                            friend_status, @"status",
                                            friend_sex, @"sex",
                                            friend_pic, @"pic",
                                            friend_pic_big, @"pic_big",
                                            friend_birthday, @"birthday",
                                            friend_relationship_status, @"relationship_status",
                                            friend_current_location, @"current_location",
                                            nil];

        // If the friend qualifies as a single of your gender, add to the friend cache
        if ( [AppHelpers friendQualifies:friend_info] == YES) {
            [[FriendCache sharedInstance] push:friend_info];
        }
    }
    [[FriendCache sharedInstance] writeToDisk];

}

我的推送方法只是包装了 NSMutableArray 推送：

- (void)push:(id)o {
    [cache addObject:o];
}

你能想到写入失败的任何原因吗？

谢谢！

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2 回答 2

3

正如我们已经指出的，这是因为NSNull对象的使用。

避免这种情况的最好方法是创建一个Friend具有所有必需属性的对象。然后您可以轻松设置nil值，这是对象无法做到的NSDictionary（好吧，您必须删除密钥，这不是很好的做法）。

然后，通过实施该NSCoding协议，您可以轻松地归档（序列化）您的自定义对象。

这是处理数据的一种更好的方法，并且将来会变得更加容易。您将能够在Friend对象上调用消息，这是NSDictionary.

于 2010-12-28T19:26:42.783 回答
1

使用 NSError-aware API for NSPropertyListSerialization 来获取数据和 NSData NSError 感知写入 API，以便您获得有意义的错误，帮助您了解您的问题可能是什么。

于 2010-12-28T19:16:57.097 回答