NSManagedObject作为NSDictionary键？

问题描述

在我的应用程序中，我有一个 NSDictionary ，其键应该是 NSManagedObject 子类的实例。

In my app, I have a NSDictionary whose keys should be instances of a subclass of NSManagedObject.

但是，问题是 NSManagedObject 没有实现 NSCopying 协议，这意味着 NSManagedObject 的核心数据对象/实例可以用作字典键，即使 - [hash] code>方法对他们很好。

The problem, however, is that NSManagedObject does not implement the NSCopying protocol which means that no Core Data objects / instances of NSManagedObject can be used as dictionary keys even though the -[hash] method works fine for them.

我应该怎么办？

推荐答案

有四个选项：

1. 使用不同的对象作为字典键，然后从中查找。 [object objectID] 或 + [NSValue valueWithNonretainedObject：] 似乎最明显


2. 使用 CFDictionaryCreateMutable（）创建一个具有保留键的字典，而不是复制，然后调用 CFDictionarySetValue （）以存储对象


3. 在OS X或iOS6 +上， [NSMapTable mapTableWithStrongToStrongObjects] 纯Objective-C相当于 CFMutableDictionary


4. 为您的托管对象实现 NSCopying 子类，以便它返回self（如果您不使用ARC，则带有引用计数）

1. Use a different object as the dictionary key instead, and lookup from that. [object objectID] or +[NSValue valueWithNonretainedObject:] seem the most obvious
2. Use CFDictionaryCreateMutable() to create a dictionary with retained keys, rather than copied, instead, and then call CFDictionarySetValue() to store the objects
3. On OS X or iOS6+, [NSMapTable mapTableWithStrongToStrongObjects] gives you a purely Objective-C equivalent to CFMutableDictionary
4. Implement NSCopying for your managed object subclass, such that it returns self (with a bumped reference count if you're not using ARC)





注意

+ valueWithNonretainedObject：是非常危险的，因为它可能留下悬空指针;

Notes

+valueWithNonretainedObject: is pretty dangerous, since it's possible to be left with a dangling pointer; likely best to avoid.

存储对象ID很好，除了新对象开始使用临时 / em> ID。当上下文保存到磁盘时（或 -obtainPermanentIDsForObjects：... 被调用），该ID将更改为永久ID。

Storing object IDs is fine, apart from the fact that new objects start out life with a temporary ID. That ID then changes to a permanent one when the context is saved to disk (or -obtainPermanentIDsForObjects:… is called). Your mapping code needs to be smart enough to handle this unless it can guarantee that all incoming objects already have a permanent ID.

实施 NSCopying

Implementing NSCopying like this feels a bit icky, but should work just fine. As it happens, this is exactly the approach NSURLSessionTask takes, I presume for dictionary friendliness.

在OS X之前10.8 Mountain Lion，以前可以创建一个常规的 NSMutableDictionary ，然后调用 CFDictionarySetValue（）。这不是这样的;新的字典现在具有在CF级指定的适当的复制回调，而不是纯粹是 NSMutableDictionary 的特征。

Prior to OS X 10.8 Mountain Lion, it used to be possible to create a regular NSMutableDictionary and then call CFDictionarySetValue() for it. That's no longer the case though; new dictionaries now have proper copy callbacks specified down at the CF level, rather than purely being a feature of NSMutableDictionary.

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