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ios - Observing an NSNotification inside a NSManagedObject

问题描述:

I want my NSManagedObject to listen for a notification from a timer class that will post an NSnotification every second. This is needed to update a value in my NSManagedObject.

Problem is as the CD lifecycle is out of my control it appears that i'm getting duplicate NSNotifications which I have found out is due to the multiple contexts that a NSManagedObject could be in.

How canI listen for this notification reliably inside my NSManagedObject?

网友答案:

This is a normal side effect of the way Core Data works. You're creating multiple objects that represent the same underlying data. All of them are registering for the same notification, so all of them get it. Listening for a notification like this is not a very good idea, because this duplication is a fundamental part of how the system works.

If the objects that should respond to the notification all come from the same managed object context, there are workarounds. For example, to listen for the notification only if the object was fetched from the root context in a parent/child context setup, do something like

if ([[self managedObjectContext] parentContext] == nil) {
    ...register for notification
}

If you don't use parent/child context relationships, you could decide that one specific context is "the one" whose managed objects get the notification, and compare [self managedObjectContext] to that.

A better solution would be to sidestep the problem and listen for the notification somewhere else-- or else just update the value from the timer callback, without using a notification. Whenever the timer fires, update the value on one specific instance of the object. This way you'll know that you're making the change in exactly one place, on one object. Other instances from other contexts would need to merge the change to get the new value.

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