ios writetofile IOS开发中使用writeToFile时的注意事项

总会有一些坑在前面等着你

我们先来看一下后台返回的部分json数据,稍后再来分析问题,仔细看一下userId和userCode两个字段,其他不用看

"list": [{
   "classId": 5000285,
   "className": "考勤（A）班",
   "schoolId": 50011,
   "schoolName": "星星局测中学25",
   "classLeaderUserId": 2000163,
   "parentList": [{
    "userId": 2000790,
    "userName": "zhaomin",
    "gender": "0",
    "mobile": "15071362222",
    "email": "",
    "areaCode": "440105",
    "avatarUrl": "",
    "userCode": "2000790",
    "id": 1542,
    "roleType": 2,
    "nickName": "zhaomin"
   }, {
    "userId": 2000846,
    "userName": "刘玄德",
    "gender": "1",
    "mobile": "18825113388",
    "email": "",
    "areaCode": "440105",
    "avatarUrl": "",
    "userCode": "2000846",
    "id": 1631,
    "roleType": 2,
    "nickName": "刘玄德"
   }],

问题背景

这个问题是在我集成环信IM的时候,由于需要处理用户头像和昵称问题,所以会将联系人的头像url和用户昵称做一个本地缓存,缓存的方式就是采用简单的写入plist文件来处理.之所以使用plist,是因为简单方便,而且可以满足开发,所以就没有采用其他的缓存方式.

问题就是出现在写入plist文件上面.

遇到问题

在获取到后台返回的联系人数据以后,我就将返回的list进行筛选,只是筛选出所需的用户姓名和头像地址.返回字段中,userId和userCode看似一样,其实解析出来,前者是NSNuber类型,后者是NSString类型,当时只记得后台直接使用Sqlite语句,将userCode=userId,根本没有考虑到类型问题.心想,既然这样,不如直接使用userId得了,于是将' [userNameDict setObject:dict[@"userName"] forKey:dict[@"userCode"]];'换成了'[userNameDict setObject:dict[@"userName"] forKey:dict[@"userId"]];'.问题就是出现在换了一个字段上.

刚开始没有发现问题,因为之前一直使用userCode字段取值作为字典的key,所以在本地已经有了缓存.直到有一天,重新安装App测试时才发现,聊天界面的头像和昵称都不在显示,才最终想到当初换了了一个字段取值.

但是,更换为userId后,打印出来的字典一模一样,就是writeToFile写入plist时总是失败.后来使用isEqualToDictionary方法比较两个字典又是不一样的.问题实在难找,当然解决办法就是切换为原来的userCode,但是遇到问题一向不想通过回避的方式去解决,所以就排查原因,甚至去比较过所有的key和value值,发现还是一样.最后,感觉实在找不出问题所在,于是去查看返回数据,于是便发现了,字段userId和userCode所对应的Value值的类型是不一样的.这才得出一下结论

如果是可变字典,那么在使用'setObject: forKey:'方法时,如果key使用的是NSNumber类型的key,会导致writeToFile失败.

至于为什么是这样,有待进一步研究,当然,如果有人遇到过并找出原因,也可以回复一下,相互学习,共同进步.

附上当时代码

- (void)saveContactListDict:(id)list {
 NSMutableArray *contactListArray = [NSMutableArray array];
 for (NSDictionary *dict in list) {
  for (NSString *key in dict) {
   if ([dict[key] isKindOfClass:[NSArray class]]) {
    [contactListArray addObjectsFromArray:dict[key]];
   }
  }
 }
 NSMutableDictionary *userNameDict = [NSMutableDictionary dictionary];
 NSMutableDictionary *avatarurlDict = [NSMutableDictionary dictionary];
 NSMutableDictionary *avatarurlAndNameDict = [NSMutableDictionary dictionary];
 for (NSDictionary *dict in contactListArray) {
  if (dict[@"userId"] == nil) {
   return;
  }
  [userNameDict setObject:dict[@"userName"] forKey:dict[@"userId"]];
  NSString *url =dict[@"avatarUrl"];
  NSString *avatarUrl = [CPUtil getThumUrl:url size:CGSizeMake(200, 200)];
  [avatarurlDict setObject:avatarUrl forKey:dict[@"userId"]];
  if (dict[@"userName"] == nil) {
   return;
  }
  [avatarurlAndNameDict setObject:avatarUrl forKey:dict[@"userName"]];
 }
 NSString *path = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).firstObject;
 NSString *userNameDictPath = [path stringByAppendingPathComponent:@"userNameDict.plist"];
 NSString *avatarurlDictPath = [path stringByAppendingPathComponent:@"avatarurlDict.plist"];
 NSString *avatarurlAndNameDictPath = [path stringByAppendingPathComponent:@"avatarurlAndNameDict.plist"];
 [userNameDict writeToFile:userNameDictPath atomically:YES];
 [avatarurlDict writeToFile:avatarurlDictPath atomically:YES];
 [avatarurlAndNameDict writeToFile:avatarurlAndNameDictPath atomically:YES];
}

分析问题

实际开发当中,总是有细节的东西,虽然有时候觉得,这些东西太基础,但是就在这些基础的知识上,我们却忽略了一些本应该注意的点.好比说我们明明知道向数组中添加元素的时候,元素不能为空,记得考虑为nil,null的情况.这谁都知道,但是却最容易被忽略,因为你无法确定后台的数据返回什么,包括那些规范文档明确要求不能为nil的字段,都有可能返回一个nil or Null .这个时候开始想静静了.明白这个世界其实没有必然的东西.另外,数组越界问题也一直都在,当然为了防止App直接闪退,你可以选择去覆盖系统的方法......好了,言归正传.我们看一下苹果官方文档,回顾一下基础的东西,文档中关于NSDictionary和writeToFile有下面两段内容

NSDictionary

*A key-value pair within a dictionary is called an entry. Each entry consists of one object that represents the key and a second object that is that key's value. Within a dictionary, the keys are unique. That is, no two keys in a single dictionary are equal (as determined by isEqual(_:)). In general, a key can be any object (provided that it conforms to the NSCopying protocol—see below), but note that when using key-value coding the key must be a string (see Accessing Object Properties). Neither a key nor a value can be nil; if you need to represent a null value in a dictionary, you should use NSNull.*

这里说,字典中的key可以是遵守NSCopying协议的任何对象类型,但是 key-value coding中的key必须是一个string.

'- (BOOL)writeToFile:(NSString *)path atomically:(BOOL)useAuxiliaryFile;'

This method recursively validates that all the contained objects are property list objects (instances of NSData, NSDate, NSNumber, NSString, NSArray, or NSDictionary) before writing out the file, and returns NO if all the objects are not property list objects, since the resultant file would not be a valid property list.

这里描述了写入文件的对象要求,也就是平时常用的 NSData, NSDate, NSNumber, NSString, NSArray, or NSDictionary这些类型,当然自定义类型不可以.

解决问题

当然最后的处理就是将NSNumber格式化为NSString,看下代码

 NSString *dictKey = [NSString stringWithFormat:@"%@",dict[@"userId"]];
 [userNameDict setObject:dict[@"userName"] forKey:dictKey];

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