NSManagedObject init/dealloc 等效项
我在 NSManagedObject 子类 bar 中有一个对象 ivar foo ，只要该对象存在，我就需要始终在那里。
为了确保正确创建 foo，我对 awakeFromInsert 进行了子类化，以便在创建 bar 时创建 foo。我在 awakeFromFetch 中做了同样的事情，以确保从存储中获取 bar 时 foo 存在。
为了解决这个问题，我在 willTurnIntoFault 和prepareForDeletion 中释放了foo。
然而，事实证明，当我删除 bar 时，prepareForDeletion 和 willTurnIntoFault 都会被调用，释放 foo 两次。
我意识到我可能不能在prepareForDeletion中释放它，但我想知道这里的最佳实践是什么，所以我理解什么时候会变成错误，等等。对于一个普通的对象，我只需创建init 中的 foo 并在 dealloc 中销毁它。
谢谢！
I have an object ivar foo inside a NSManagedObject subclass bar that I need to be there at all times, as long as the object exists.
To make sure foo is created properly, I've subclassed awakeFromInsert to create foo when bar is created. I've done the same in awakeFromFetch, to make sure foo is there when bar is fetched from the store.
To counteract that, I release foo inside willTurnIntoFault and in prepareForDeletion.
However, it turns out that when I do delete bar, both prepareForDeletion and then willTurnIntoFault are called, releasing foo twice.
I realize I can probably just not release it in prepareForDeletion then, but I'd like to know what the best practice is here, so I understand when something gets turned into a fault, etc. For a normal object, I'd just create foo in init and destroy it in dealloc.
Thanks!
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不要仅仅释放 ivar，而是释放它并将其设置为 nil。释放
nil没有任何效果，因此即使发生两次也没关系。更好的是，使
foo成为具有retain语义的属性，并始终通过-setFoo:设置它。Instead of just releasing the ivar, release it and set it to
nil. Releasingnilhas no effect, so you'll be OK if it happens twice.Better yet, make
fooa property withretainsemantics and always set it via-setFoo:.