cocoa - 将 CoreData 实体保存在 NSUserDefaults 中

Question

想象一个 CoreData 实体(例如名为 searchEngine )。
NSManagedObjectContext管理该实体的一些“实例”。
最终用户将能够选择他的 "standard searchEngine"与 NSPopupButton .
selected object NSPopupButton 的应该绑定(bind)到 NSUserDefaults。
问题:

1) @try{save}

a) If you try to save the selected "instance" directly to NSUserDefaults there comes something like this:

-[NSUserDefaults setObject:forKey:]: Attempt to insert non-property value ' (entity: searchEngine; id: 0x156f60  ; data: {    url = "http://google.de/";    someAttribute = 1;    name = "google";})' of class 'searchEngine'.

b) If you try to convert the "instance" to NSData comes this:

-[searchEngine encodeWithCoder:]: unrecognized selector sent to instance 0x1a25b0

So any idea how to get this entities in a plist-compatible data?

2) @尝试{注册默认}

Usually the registerDefaults: method is implemented in + (void)initialize. The problem here is that this method is called before CoreData loads the saved entities from his database. So I can't set a default to a no-existing object, right?

我知道，问题很长...但是:尝试{[我提供:详细信息]} ;D

Answer 1

如果需要存储对特定托管对象的引用，请使用其托管对象 ID 的 URI 表示形式:

NSURL *moIDURL = [[myManagedObject objectID] URIRepresentation];

然后您可以将该 URL 保存为用户默认值。

要检索托管对象，您可以使用:

NSManagedObjectID *moID = [myPersistentStoreCoordinator managedObjectIDForURIRepresentation:moIDURL];
NSManagedObject *myManagedObject = [myContext objectWithID:moID];

唯一需要注意的是，您必须确保原始托管对象 ID 是永久的 - 如果您已经保存了该对象，这不是问题，或者您可以使用 obtainPermanentIDsForObjects:error: .