CoreData and Swift's Actor and Sendable

Question

I am trying to understand Swift's Actors, but failed. I am playing around with the following example.
I want to setup a LocalStore that uses an NSPersistentContainer.
The struct LocalStore should be used by a StoreManager declared as

actor StoreManager {
  private let localStore = LocalStore()
  init() {
    localStore.test()
  }
}

LocalStore is declared as

import CoreData
import Foundation

struct LocalStore: Sendable {
  private let localPersistentContainer = NSPersistentContainer()

  func test() {
    print("test")
  }
}

Here I get 2 compiler errors, shown here as comments:

import CoreData // Add '@_predatesConcurrency' to suppress 'Sendable'-related warnings from module 'CoreData'
private let localPersistentContainer = NSPersistentContainer() // Stored property 'localPersistentContainer' of 'Sendable'-conforming struct 'LocalStore' has non-sendable type 'NSPersistentContainer'

I neither know if it is save or advisable to suppress 'Sendable'-related warnings from module 'CoreData', nor what to do with the non-sendable type 'NSPersistentContainer'.
Any help is welcome.

score 4 · Accepted Answer · answered Feb 23 '22 at 10:05

By now I understand the situation a little better.
LocalStore is declared as Sendable, i.e. it should be possible to be passed around thread-safe. Now assume its property localPersistentContainer had been declared as follows:

let localPersistentContainer = NSPersistentContainer()

NSPersistentContainer has some properties that can be set, like persistentStoreDescriptions. It is thus possible that such a property can be changed within LocalStore, as well as by anything that has been passed LocalStore to, i.e. this would not be thread-safe. In this case the warning

Stored property 'localPersistentContainer' of 'Sendable'-conforming struct 'LocalStore' has non-sendable type 'NSPersistentContainer'

is justified.
However, in my case localPersistentContainer has been declared as follows:

private let localPersistentContainer = NSPersistentContainer()

This means that anything that receives a LocalStore cannot access the private property localPersistentContainer, and thus the compiler warning is not justified, since the compiler is of course aware of that.

As the other compiler warning suggested,

@_predatesConcurrency import CoreData

suppresses all Sendable warnings created by the CoreData module. While this works, it is to my mind dangerous because it suppresses also justified warnings.

CoreData and Swift's Actor and Sendable

1 Answers1