iPhone 上的 JSON 和核心数据

Question

我有一个核心数据对象图（由通过一对多关系链接的两个实体组成）。

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

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

2. 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）。

我已经搜索了关于这一点的教程/代码示例，但未成功，因此我们将不胜感激。

Answer 1

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，但是将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:)

你调用的方法是：

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

而实现如下：

- (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\n%@\n%@", [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，所以如果你不小心的话，你可以很容易地翻译整个持久存储。注意您的关系并确保它们只“向下”移动对象树，以便您只获得要翻译的对象。

Answer 2

我只是想指出一个导致代码崩溃的小错字，希望这可以为您节省几分钟。

- (NSArray*)dataStructuresFromManagedObjects:(NSArray*)managedObjects {

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

NSMutableArray *dataArray = [[NSArray alloc] init]; // This should be NSMutableArray

真的应该是NSMutableArray *dataArray = [[NSMutableArray alloc] init];

仅此而已。

谢谢

Answer 3

Synchronizing Core Data with Rails 是一个详细的演示文稿，其中包括将 Core Data 对象序列化/反序列化到 JSON 的示例代码（跳到幻灯片 55 了解 Core Data 部分）。他的示例代码假设了一个没有关系的相当简单的模型，尽管我认为它很容易扩展。

该演示文稿还详细介绍了如何使您的 Core Data 模型与基于 REST 的 Web 应用程序保持同步，并提供了一些有用的库的指针，包括 ObjectiveResource 和 ASIHTTPRequest。不确定这是否是您想要做的，但即使是核心数据代码也值得一看。

Answer 4

如果您的托管对象中有NSDate，如上面在其中一个cmets 中所述，那么序列化包含NSDate 的对象时会遇到问题。一个简单的解决方法是使用objective-c 类别将JSONDataRepresentation 方法添加到NSDate。

将这两个文件添加到您的项目中：

NSdate.h：

#import <Foundation/Foundation.h>

@interface NSDate (jsondatarepresentation) 

- (NSData*) JSONDataRepresentation;

@end

NSDate.m：

#import "NSDate.h"

@implementation NSDate (jsondatarepresentation)

- (NSData*) JSONDataRepresentation {
    return [[[NSNumber numberWithDouble:[self timeIntervalSince1970]] stringValue] dataUsingEncoding:NSUTF8StringEncoding];
}

@end

Answer 5

只是想 id 发布一个快速更新这个问题。我按照 Marcus 和 Brandon 的回答提出了这个用于 JSON 导出的方法（它仍然使用 TouchJSON）：

- (NSData*)jsonStructureFromManagedObjects:(NSArray*)managedObjects
{
    NSArray *objectsArray = [self dataStructuresFromManagedObjects:managedObjects];
    NSData *jsonData      = [[CJSONSerializer serializer] serializeArray:objectsArray error:nil];
    return jsonData;
}

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

- (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 userInfo] objectForKey:@"isExportable"] boolValue] == YES) {

            if (![description isToMany]) {
                NSManagedObject *relationshipObject = [managedObject valueForKey:relationshipName];
                if (relationshipObject) {
                    [valuesDictionary setObject:[self dataStructureFromManagedObject:relationshipObject] forKey:relationshipName];
                }

                continue;
            }

            NSSet *relationshipObjects        = [managedObject valueForKey:relationshipName];
            NSMutableArray *relationshipArray = [[NSMutableArray alloc] init];

            for (NSManagedObject *relationshipObject in relationshipObjects) {
                [relationshipArray addObject:[self dataStructureFromManagedObject:relationshipObject]];
            }

            [valuesDictionary setObject:relationshipArray forKey:relationshipName];

        }

    }
    return valuesDictionary;
}

我无法让导入工作，也许这与我不确定我正在使用 Magical Record 的事实有关，所以我只是循环通过传入的 JSON 流并手动创建对象......

Answer 6

有一个库可以为您进行 JSON 同步：https://github.com/sixdegrees/lidenbrock

Answer 7

我看到了这篇文章，效果很好。

http://touchalicious.com/blog/2009/10/25/turn-core-data-models-into-json.html

由于这是递归的，多对多关系将不断循环。为了避免这种情况，我在我的核心数据模型中的关系的用户信息字典中添加了一个“isExportable”键。然后，您可以检查此键并选择在没有它的情况下不循环关系。

if ([property isKindOfClass:[NSRelationshipDescription class]])
    {
        NSRelationshipDescription *relationshipDescription = (NSRelationshipDescription *)property;

        if ([[[relationshipDescription userInfo] objectForKey:@"isExportable"] boolValue] == YES)
        {
            NSString *name = [relationshipDescription name];

            if ([relationshipDescription isToMany])
            {
                NSMutableArray *arr = [properties valueForKey:name];
                if (!arr)
                {
                    arr = [[NSMutableArray alloc] init];
                    [properties setValue:arr forKey:name];
                }

                for (NSManagedObject *o in [self mutableSetValueForKey:name])
                {
                    [arr addObject:[o propertiesDictionary]];
                }
            }
            else
            {
                NSManagedObject *o = [self valueForKey:name];
                [properties setValue:[o propertiesDictionary] forKey:name];
            }
        }
    }
}

Answer 8

Marcus S. Zarra 启发了我将递归的想法带入工作版本。在这个版本中，您不需要在 CoreData 中设置密钥，您可以将其剪切并粘贴到您的项目中:-)

// MARK: - encoding and decoding CoreData entity to dictionary

func dataStructureFromManagedObject( managedObject:NSManagedObject?, parentEntity: NSEntityDescription? = nil) -> NSMutableDictionary {
    if (managedObject != nil) {
        var attributesByName: NSDictionary = managedObject!.entity.attributesByName
        var relationshipsByName: NSDictionary  = managedObject!.entity.relationshipsByName
        var valuesImmutableDictionary: NSDictionary = managedObject!.dictionaryWithValuesForKeys( attributesByName.allKeys)
        var valuesDictionary: NSMutableDictionary = valuesImmutableDictionary.mutableCopy() as NSMutableDictionary
        valuesDictionary.setObject( managedObject!.entity.name!, forKey: "ManagedObjectName")
        for relationshipNameObject in relationshipsByName.allKeys {
            var relationshipName: NSString = relationshipNameObject as  NSString
            var relationshipDescription: NSRelationshipDescription? = relationshipsByName.objectForKey( relationshipName) as? NSRelationshipDescription
            if !relationshipDescription!.toMany {
                // ono to one
                if parentEntity == nil || (relationshipDescription! as NSRelationshipDescription).destinationEntity != parentEntity! {
                    // no parent or relationship is "downward" -> object for relationship must be added
                    var relationshipObject: NSManagedObject? = managedObject!.valueForKey( relationshipName) as? NSManagedObject
                    var relationshipObjectDictionary: NSMutableDictionary = self.dataStructureFromManagedObject( relationshipObject, parentEntity: managedObject?.entity)
                    valuesDictionary.setObject( relationshipObjectDictionary, forKey: relationshipName)
                } else {
                    // relationship is "upward" -> nothing to do
                }
            } else {
                // one to many -> all objects must be added
                var relationshipObjects: NSSet = managedObject!.mutableSetValueForKey( relationshipName)
                var relationshipArray:NSMutableArray = []
                for relationshipObjectRaw in relationshipObjects {
                    var relationshipObject:NSManagedObject? = relationshipObjectRaw as? NSManagedObject
                    if relationshipObject != nil && !relationshipObject!.entity.isKindOfEntity( managedObject!.entity) {
                        relationshipArray.addObject(self.dataStructureFromManagedObject( relationshipObject, parentEntity: managedObject?.entity))
                    }
                }
                valuesDictionary.setObject( relationshipArray, forKey: relationshipName)
            }
        }
        return valuesDictionary
    } else {
        return NSMutableDictionary()
    }
}

func managedObjectFromStructure( structureDictionary: NSDictionary, moc: NSManagedObjectContext, parentObject: NSManagedObject? = nil) -> NSManagedObject {
    if structureDictionary.count > 0 {
        var objectName:NSString = structureDictionary.objectForKey( "ManagedObjectName") as NSString
        var managedObject:NSManagedObject = NSEntityDescription.insertNewObjectForEntityForName( objectName, inManagedObjectContext: moc) as NSManagedObject
        var relationshipsByName: NSDictionary  = managedObject.entity.relationshipsByName
        var realObjectStructure:NSMutableDictionary = structureDictionary.mutableCopy() as NSMutableDictionary
        realObjectStructure.removeObjectForKey( "ManagedObjectName")
        for key in realObjectStructure.allKeys {
            // search for "ManagedObjectName" relationship entrys and delete them before filling the managedObject from this structure
            for relationshipName in relationshipsByName.allKeys {
                if relationshipName as NSString == key as NSString {
                    realObjectStructure.removeObjectForKey( key)
                }
            }
        }
        managedObject.setValuesForKeysWithDictionary( realObjectStructure)
        // the main object with attributes is created. Now care about the relationships
        for relationshipName in managedObject.entity.relationshipsByName.keys {
            var description:NSRelationshipDescription = relationshipsByName.objectForKey( relationshipName) as NSRelationshipDescription
            if !description.toMany {
                // to one relationship
                if parentObject == nil || description.destinationEntity != parentObject!.entity {
                    // no parent or relationship is "downward" -> recurse structure to add
                    var childStructureDictionary:NSDictionary = structureDictionary.objectForKey( relationshipName) as NSDictionary
                    if childStructureDictionary.count > 0 {
                        // dictionary not empty -> object must be created and added
                        var childObject:NSManagedObject? = self.managedObjectFromStructure( childStructureDictionary, moc: moc, parentObject: managedObject)
                        // validateForUpdate
                        var error:NSError?
                        if !managedObject.validateForUpdate( &error) {
                            println("Error: Object not in valid state for update!!! -> \(error)")
                        } else {
                            managedObject.setValue( childObject, forKey: relationshipName as NSString)
                        }
                    } else {
                        // relationship is "upward" -> nothing to do
                    }
                }
            } else {
                // to many relationship
                var relationshipSet:NSMutableSet = managedObject.mutableSetValueForKey( relationshipName as NSString)
                var relationshipArray:NSArray = structureDictionary.objectForKey( relationshipName as NSString) as NSArray
                for childStructureDictionary in relationshipArray {
                    if childStructureDictionary.count > 0 {
                        // dictionary not empty -> object must be created and added
                        var childObject:NSManagedObject = self.managedObjectFromStructure( childStructureDictionary as NSDictionary, moc: moc, parentObject: managedObject)
                        // validateForUpdate
                        var error:NSError?
                        if !managedObject.validateForUpdate( &error) {
                            println( "Error: Object not in valid state for update!!! -> \(error)")
                        } else {
                            relationshipSet.addObject( childObject)
                        }
                    } else {
                        // no object was behind the relationship -> nothing to do
                    }
                }
                // save set
                managedObject.setValue( relationshipSet, forKey: relationshipName as NSString)
            }
        }
        // final check validateForUpdate
        var error:NSError?
        if !managedObject.validateForUpdate( &error) {
            println( "Error: Object not in valid state for update although all previous check are passed!!! -> \(error)")
        }
        return managedObject
    } else {
        println( "Error: structure for object was empty. this should not happen at this point")
        var objectName:NSString = structureDictionary.objectForKey( "ManagedObjectName") as NSString
        var managedObject:NSManagedObject = NSEntityDescription.insertNewObjectForEntityForName( objectName, inManagedObjectContext: moc) as NSManagedObject
        return managedObject
    }
}

func dataStructuresFromManagedObjects( managedObjects: NSArray) -> NSArray {
    var dataArray:NSMutableArray = []
    for managedObject in managedObjects {
        dataArray.addObject( self.dataStructureFromManagedObject(managedObject as? NSManagedObject))
    }
    return dataArray
}

这里的关键是将父实体作为参数传递给递归，这样我们就可以决定我们必须用数据填充哪个关系。所以这两个函数：dataStructureFromManagedObject 和 managedObjectFromStructure 可以将 CoreData 中的任何实体对象编码和解码到字典中，然后再返回到对象中。