iPhone 上的 JSON 和核心数据 [英] JSON and Core Data on the iPhone

问题描述

我有一个核心数据对象图(由一对多关系链接的两个实体组成).

I have a core data object graph (consisting of two entities linked by a to-many relationship).

我很好奇，作为一个相对缺乏经验的 iPhone 开发人员，是否有人可以推荐一种方法，以及适用于 iPhone 的合适的 JSON 实现，这将使我能够:

I was curious, as a relatively inexperienced iPhone developer, whether anyone could recommend an approach, and a suitable JSON implementation for the iPhone, which would allow me to:

1. 将核心数据记录转换为JSON字符串(同时保持实体之间的关系)；和

1. convert the core data records into a JSON string (whilst maintaining the relationship between the entities); and

将 JSON 字符串转换回核心数据对象(再次保留实体之间的关系).

convert the JSON string back into core data objects (again preserving the relationship between entities).

我已经搜索了有关这一点的教程/代码示例，但未成功，因此我们将不胜感激.

I have searched, unsuccessfully, for a tutorial/code sample on this point so any assistance would be gratefully received.

推荐答案

首先，选择一个 JSON 库来使用，我个人喜欢 TouchJSON，但其他几个也很不错.复杂的部分虽然不是很困难，但将您的托管对象转换为适合转换的结构.我写得很快，所以它可能有一两个错误:)

First, pick a JSON library to use, I personally like TouchJSON but several others out there are quite nice as well. The complicated part, although not very hard, is to convert your managed objects into suitable structures for the conversion. I wrote this real quick so it may have an error or two :)

您调用的方法是:

- (NSString*)jsonStructureFromManagedObjects:(NSArray*)managedObjects;
- (NSArray*)managedObjectsFromJSONStructure:(NSString*)json withManagedObjectContext:(NSManagedObjectContext*)moc;

实现如下:

- (NSDictionary*)dataStructureFromManagedObject:(NSManagedObject*)managedObject
{
  NSDictionary *attributesByName = [[managedObject entity] attributesByName];
  NSDictionary *relationshipsByName = [[managedObject entity] relationshipsByName];
  NSMutableDictionary *valuesDictionary = [[managedObject dictionaryWithValuesForKeys:[attributesByName allKeys]] mutableCopy];
  [valuesDictionary setObject:[[managedObject entity] name] forKey:@"ManagedObjectName"];
  for (NSString *relationshipName in [relationshipsByName allKeys]) {
    NSRelationshipDescription *description = [[[managedObject entity] relationshipsByName] objectForKey:relationshipName];
    if (![description isToMany]) {
      NSManagedObject *relationshipObject = [managedObject valueForKey:relationshipName];
      [valuesDictionary setObject:[self dataStructureForManagedObject:relationshipObject] forKey:relationshipName];
      continue;
    }
    NSSet *relationshipObjects = [managedObject objectForKey:relationshipName];
    NSMutableArray *relationshipArray = [[NSMutableArray alloc] init];
    for (NSManagedObject *relationshipObject in relationshipObjects) {
      [relationshipArray addObject:[self dataStructureForManagedObject:relationshipObject]];
    }
    [valuesDictionary setObject:relationshipArray forKey:relationshipName];
  }
  return [valuesDictionary autorelease];
}

- (NSArray*)dataStructuresFromManagedObjects:(NSArray*)managedObjects
{
  NSMutableArray *dataArray = [[NSMutableArray alloc] init];
  for (NSManagedObject *managedObject in managedObjects) {
    [dataArray addObject:[self dataStructureForManagedObject:managedObject]];
  }
  return [dataArray autorelease];
}

- (NSString*)jsonStructureFromManagedObjects:(NSArray*)managedObjects
{
  NSArray *objectsArray = [self dataStructuresFromManagedObjects:managedObjects];
  NSString *jsonString = [[CJSONSerializer serializer] serializeArray:objectsArray];
  return jsonString;
}

- (NSManagedObject*)managedObjectFromStructure:(NSDictionary*)structureDictionary withManagedObjectContext:(NSManagedObjectContext*)moc
{
  NSString *objectName = [structureDictionary objectForKey:@"ManagedObjectName"];
  NSManagedObject *managedObject = [NSEntityDescription insertNewObjectForEntityForName:objectName inManagedObjectContext:moc];
  [managedObject setValuesForKeysWithDictionary:structureDictionary];

  for (NSString *relationshipName in [[[managedObject entity] relationshipsByName] allKeys]) {
    NSRelationshipDescription *description = [relationshipsByName objectForKey:relationshipName];
    if (![description isToMany]) {
      NSDictionary *childStructureDictionary = [structureDictionary objectForKey:relationshipName];
      NSManagedObject *childObject = [self managedObjectFromStructure:childStructureDictionary withManagedObjectContext:moc];
      [managedObject setObject:childObject forKey:relationshipName];
      continue;
    }
    NSMutableSet *relationshipSet = [managedObject mutableSetForKey:relationshipName];
    NSArray *relationshipArray = [structureDictionary objectForKey:relationshipName];
    for (NSDictionary *childStructureDictionary in relationshipArray) {
      NSManagedObject *childObject = [self managedObjectFromStructure:childStructureDictionary withManagedObjectContext:moc];
      [relationshipSet addObject:childObject];
    }
  }
  return managedObject;
}

- (NSArray*)managedObjectsFromJSONStructure:(NSString*)json withManagedObjectContext:(NSManagedObjectContext*)moc
{
  NSError *error = nil;
  NSArray *structureArray = [[CJSONDeserializer deserializer] deserializeAsArray:json error:&error];
  NSAssert2(error == nil, @"Failed to deserialize
%@
%@", [error localizedDescription], json);
  NSMutableArray *objectArray = [[NSMutableArray alloc] init];
  for (NSDictionary *structureDictionary in structureArray) {
    [objectArray addObject:[self managedObjectFromStructure:structureDictionary withManagedObjectContext:moc]];
  }
  return [objectArray autorelease];
}

现在这是递归的，因此如果您不小心，您很容易最终翻译整个持久存储.注意您的关系并确保它们只在对象树中向下"，以便您只得到您想要翻译的对象.

Now this is recursive so you can easily end up translating your entire persistent store if you are not careful. Watch your relationships and make sure that they only go "down" the object tree so that you only get the objects you want translated.

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