Swift: 为什么从 NSManagedObject 继承会破坏我的属性？

Question

我是一个完全 Swift/IOS 的新手，关于 CoreData 的一些东西我根本无法理解。

我有一个 class，其中包含一些由指定初始化程序初始化的非可选属性。然后，如果我将 class 设置为从 NSManagedObject 继承，然后突然出现错误

Stored property X requires an initial value or should be @NSManaged.

为什么 Swift 突然认为我的属性没有初始化，即使它们显然是？

此外，我读到@NSManaged "tells the compiler that the storage and implementation of the properties will be handled by CoreData"，但这到底是什么意思？

任何答案将不胜感激..

Answer 1

实际上我昨天刚读到这篇文章。

Yes, it kinda really acts like @dynamic -- technically it might be identical even. Semantically there is a slight difference:

@dynamic says 'compiler, don't check if my properties are also implemented. There might be no code you can see but I guarantee it will work at runtime'

@NSManaged now says 'compiler, don't check those properties as I have Core Data to take care of the implementation - it will be there at runtime'

so you could even say: @NSManaged is syntactic sugar that is a more narrow version of dynamic :)

取自此

swift 的一大推动是使语言极其安全，在这种情况下，检查属性是否在编译时实现。如果我理解正确，CoreData 并不完全符合这些编译时检查，因此添加 @NSManaged 让编译器知道变量将被处理。

来自 Apple：

You use the @NSManaged attribute to inform the Swift compiler that Core Data provides the storage and implementation of a declaration at runtime.