Friday, February 21, 2014

NSManagedObjectContext’s Parent Context

Benedict Cohen:

parentContext was introduced alongside a new concurrency model. To use parentContext both the parent and child contexts must adopt the new concurrency model. But the problem addressed by parentContext is not concurrency. Concurrency is just a problem, albeit a significant one, that needed to be solved to for parentContext to be implemented. The intent of parentContext is to improve the [atomicity] of changes. parentContext allows changes to be batch up and committed en masse. This has always been possible by using multiple NSManagedObjectContext, but parentContext allows for improved granularity of the batching.


parentContext does provides features that simplify a handful of use cases. Unfortunately the short comings of parentContext mean that it can not be adopted piecemeal. The top of the Core Data stack are managed objects. A good model will provide an interface that works at a high level of abstraction. Creating such an interface requires encapsulating implementation detail. The way that Core Data is designed means that the natural place for this code is in managed object subclasses. Because parentContext affects the behaviour of managed objects adopting it makes it difficult to write managed object subclasses without knowing the context hierarchy in which they’ll be used. Proceed with extreme caution!

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