What are the nuances of Core Data with threads?
The main rule
An NSManagedObjectContext and all of its NSManagedObjects belong to one queue: the main context to the main queue, a private context to its own internal queue. Every access (reading a property, fetch, save) must go through that context's perform { } or performAndWait { }. Violations give non-deterministic crashes and data corruption, often far from the actual mistake.
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