What happened? 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). Let me add one more item to that list. Solution: We needed to re-fetch data from background thread to main thread before background thread is destroyed. So far we learned how to make your Core Data stack not suck, and you got some code you can copy and paste. Use NSTimer instead. This approach is thread safe and will not block your UI. Here is a good article on multi-context core data systems. 1. When doing CoreData operations on a background thread it is important that the background thread uses it’s own instance of NSManagedObjectContext. 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. 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. A data race can occur when multiple threads access the same memory without synchronization and at least one access is a write. You could be reading values from an array from the main thread while a background thread is adding new values to that same array. 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. Easy to read data on the main thread; Easy to use background thread data saving; Easy to setup in-memory database (e.g. The privateManagedObjectContext operates on a background thread, as noted by its concurrency type. Query Correctly. 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. Sometimes when we have a bunch of contacts in our contact book and try to store it in Core Data we face many issues like Threading issue, UI stuck issue, Store data in background issue, etc. When working with Core Data, it's important to always remember that Core Data isn't thread safe. 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. Core Data expects to be run on a single thread. Thread Safety. Apple suggests the following way: Using thread confinement, you should not pass managed objects or managed object contexts between threads. EDIT I don't suggest using dispatch_after if you want to cancel while moving away. for caching, or unit testing) Out of the box support for automatic migrations of the database between app releases; Easy to setup database modeling tool (with Interface Builder) Core Data Example Here you can find the easiest way to sync contacts with Core Data, you can easily get the array of deleted, updated and newly added contact list. Hence no data is available in the main thread. Data races can be the root cause behind flaky tests and weird crashes. Here is some real data from a computationally intensive function that reads from an XML file (with buffering) and performs data interpolation: In Apple’s example code the CoreData operations happen inside the main method of a custom subclass of NSOperation. That subclass is initialized using a custom initWithData: method and then added to a NSOperationQueue.. You use the privateManagedObjectContext to insert in the background and save your data. 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