Here is a good article on multi-context core data systems. Here is some real data from a computationally intensive function that reads from an XML file (with buffering) and performs data interpolation: Hence no data is available in the main thread. Data races can be the root cause behind flaky tests and weird crashes. Use NSTimer instead. What happened? Calls that saveContext() method from the scene delegate’s sceneDidEnterBackground() method, ensuring that Core Data updates are flushed when the scene moves to the background. Let’s say you want to insert or update a lot of users you got in a network response. You use the privateManagedObjectContext to insert in the background and save your data. The privateManagedObjectContext operates on a background thread, as noted by its concurrency type. That subclass is initialized using a custom initWithData: method and then added to a NSOperationQueue.. Core Data in Swift Part 1 - The Stack While updating a new Core Data app to Swift 2 I've found a few situations where either there doesn't seem to be any official guidance on best practices, or worse, what I'd consider bad recommendations. I don't recommend running tasks with the .background thread priority especially on the iPhone X where the task seems to be allocated on the low power cores. EDIT I don't suggest using dispatch_after if you want to cancel while moving away. Let me add one more item to that list. You could be reading values from an array from the main thread while a background thread is adding new values to that same array. Apple suggests the following way: Using thread confinement, you should not pass managed objects or managed object contexts between threads. When doing CoreData operations on a background thread it is important that the background thread uses it’s own instance of NSManagedObjectContext. In this article, we take a look at the dangers when using Core Data on multiple threads and we explore several solutions to tackle the problem. That provides for us the complete ability to use Core Data fetch requests from within SwiftUI, although we still need to add some example data to work with. In Apple’s example code the CoreData operations happen inside the main method of a custom subclass of NSOperation. When working with Core Data, it's important to always remember that Core Data isn't thread safe. Thread Safety. 1. Synchronizing Main and Background Core Data Threads (Swift 3) Posted by mmv November 27, 2016 Let’s say we have two different managedObjectContext (with one persistentStoreCoordinator). This approach is thread safe and will not block your UI. A data race can occur when multiple threads access the same memory without synchronization and at least one access is a write. 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