# defaultIsolation MainActor and Core Data background tasks

**URL:** <https://forums.swift.org/t/defaultisolation-mainactor-and-core-data-background-tasks/80569>\
**Category:** Using Swift\
**Tags:** concurrency\
**Created:** [June 20, 2025, 9:53am UTC](https://forums.swift.org/t/defaultisolation-mainactor-and-core-data-background-tasks/80569 "2025-06-20T09:53:35Z")\
**Posts on this page:** 1\
**Showing post:** 22

<div class="post-metadata">

**Author:** ![nicoreese](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/nicoreese/32/37277_2.png) [@nicoreese](https://forums.swift.org/u/nicoreese)\
**Post date:** [August 5, 2025, 9:13pm UTC](https://forums.swift.org/t/defaultisolation-mainactor-and-core-data-background-tasks/80569/22 "2025-08-05T21:13:52Z")

</div>

Looks like iOS 26 & companions address this with beta 5:

> In beta 5 SDK, CoreData changed several `Sendable` annotations to resolve compatibility issues with Swift 6’s new `MainActor` default isolation feature. These changes include marking `NSManagedObject` as `NS_SWIFT_NONISOLATED NS_SWIFT_NONSENDABLE`, marking `NSManagedObjectContext` as `NS_SWIFT_NONISOLATED NS_SWIFT_SENDABLE`, and requiring `NS_SWIFT_SENDABLE` closures for the family of `perform`, `performBlock`, `performBlockAndWait` and similar methods. These changes are ABI compatible with past releases but might introduce new warnings while building source code that violates the longstanding CoreData concurrency guidelines.
> 
> `NSManagedObject` are mutable reference types inextricably related to others in a graph and cannot be made `Sendable`. They are expected to be isolated to the scope of the `NSManagedObjectContext` that creates or fetches them. `NSManagedObjectContext` is a style of actor which encapsulates its own dispatch queue. While it’s impermissible to use many methods on `NSManagedObjectContext` from other threads, it is permissible to pass references around to invoke the `performBlock` family of methods, for the purpose of routing a `Sendable` closure to its managed dispatch queue. CoreData supports a user default `-com.apple.CoreData.ConcurrencyDebug 1` which can be used to enable additional assertions. (153848710) (FB18216198)
> 
> **Workaround:** Most new warnings detected by Swift from these changes are real data races, although some errors are not properly downgraded to warnings as the `@preconcurrency` attribute would indicate. To work around this, apply a `nonisolated(unsafe)` closure local to errors involving `MainActor` isolated state captured by a `performBlock` closure:

```swift
nonisolated(unsafe) let perform = {
                 // work
             }
             
             try context.performAndWait {
               try perform()
             }

```

> **[iOS & iPadOS 26 Beta 5 Release Notes | Apple Developer Documentation](https://developer.apple.com/documentation/ios-ipados-release-notes/ios-ipados-26-release-notes)**
>
> Update your apps to use new features, and test your apps against API changes.

---

_[View the full topic](https://forums.swift.org/t/defaultisolation-mainactor-and-core-data-background-tasks/80569)._