IPhone 上的 JSON 和核心数据

我有一个核心数据对象图(由两个通过对多关系链接的实体组成)。

我很好奇，作为一个相对缺乏经验的 iPhone 开发人员，是否有人能够推荐一种方法，以及一个适合 iPhone 的 JSON 实现，这将允许我:

  1. 将核心数据记录转换为 JSON 字符串(同时维护实体之间的关系) ; 以及

  2. 将 JSON 字符串转换回核心数据对象(同样保留实体之间的关系)。

在这一点上，我已经搜索了一个教程/代码示例，但是没有找到，所以如果有任何帮助，我会非常感激。

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小开
最佳答案

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 :)

The methods you call are:

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

And the implementation is as follows:

- (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];
}

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.

小开

Synchronizing Core Data with Rails is a detailed presentation that includes sample code for serializing/deserializing your Core Data objects to/from JSON (skip to slide 55 for the Core Data part). His sample code assumes a fairly simple model without relationships, though I think it would be pretty easy to extend.

The presentation also goes into some detail about keeping your Core Data model in sync with a REST-based web application, with pointers to some useful libraries, including ObjectiveResource and ASIHTTPRequest. Not sure if that's what you're trying to do, but it's worth a look even for the Core Data code.

小开

I just wanted to point out a small typo, that caused the code to crash, and hopefully this will save you a few min.

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


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

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

really should be NSMutableArray *dataArray = [[NSMutableArray alloc] init];

that is all.

thank you

小开

If you have an NSDate in your managed object, as mentioned above in one of the comments, you'll have problems serializing the object containing the NSDate. A simple fix is to add a JSONDataRepresentation method to NSDate using objective-c categories.

Add these two files to your project:

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
小开

There is a lib that does the JSON synchronization for you : https://github.com/sixdegrees/lidenbrock

小开

I came across this post which works very well.

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

Since this is recursive, many-to-many relationships are going to keep looping through themselves. To avoid this, I added an "isExportable" key to the user info dictionary of the relationships in my Core Data model. You can then check for this key and choose to not loop through relationships without it.

enter image description here

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];
}
}
}
}
小开

Just thought id post a quick update to this question. I followed the Answers by Marcus and Brandon and came up with this for JSON exporting (it uses TouchJSON still):

- (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;
}

I couldn't get the import working, maybe that has something to do with the fact that I'm using Magical Record I'm not sure, so Im just looping through the incoming JSON stream and creating objects manually...

小开

Marcus S. Zarra has inspired me to bring the recursive idea to a working version. In this version you don't need to set a key in CoreData and you can cut and paste it in your project :-)

// 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
}

The key here is to pass the parent entity as argument to the recursion, so we can decide which relationship we have to fill with data. So the both functions: dataStructureFromManagedObject and managedObjectFromStructure can encode and decode any entity object from CoreData into a dictionary and back into an object.


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