Interqueue Communication
Since objects created with or retrieved from a context can only be accessed on the queue associated with that context, the challenge becomes passing references to those objects between queues. This is arguably the biggest area where multiple threads with Core Data cause people the most issues.
If a reference to an object must be passed between queues, the best way to handle that hand-off is via the object’s -objectID property. This property is designed to be safe to access from multiple queues and is a unique identifier to the object.
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