Replace Threadsafe var with Actor

I have a long established app that uses a threadsafe var to protect an array of URLs from concurrency races, etc…

class ThreadsafeVar<T>
{
  private var value = Optional<T>.none
  
  func callAsFunction() -> T?
  {
    queue.sync(flags: .barrier) { [unowned self] in value }
  }
  
  func set(_ value: T?)
  {
    queue.async(flags: .barrier) { [unowned self] in self.value = value }
  }
}

What I want to do is substitute an Actor, which I understand will be intrinsically threadsafe. So, I wrote one like this…

actor UrlManager
{
  private var value = [URL]()

  func callAsFunction() -> [URL]
  {
    value
  }

  func set(_ value: [URL])
  {
    self.value = value
  }
}

The calling code is in one of four classes, all of which implement a protocol, which contains a read-write var…

protocol FolderProtocol
{
  …
  
  var fileURLs: [URL] { get set }
}

In this particular class, the var is implemented as read-only…

  private var urls = ThreadsafeVar<[URL]>()

  var fileURLs: [URL]
  {
    get
    {
      urls()?.sorted(using: .finder, pathComponent: .fullPath) ?? []
    }
    set { }
  }

… the private var being updated from a metadata updating notification handler.

Calling the set(_:[URL]) method on the actor poses no problem…

      Task { await urls.set(foundURLs) }

But, no matter what I try - continuation, try, await, etc, I don't seem to be able to read back the array of URLs from the actor's callAsFunction() method.

The fileURLs var is called on the main thread in response to NSCollectionView delegate methods.

Other implementing classes don't need the concurrency management as they are fixed once and never changed.

Can anyone help me with this one?