CODEWITHSUNDEEP
swiftasync-awaitconcurrencygrand-central-dispatch
Karina
Karina

Reputation: 45

async await @MainActor crash "Modifications to the layout engine must not be performed from a background thread"

I have a crash when trying to reload collectionView inside the method update() which marked as @MainActor. "Modifications to the layout engine must not be performed from a background thread after it has been accessed from the main thread"

I know, that you should update the UI from main thread only. Using GCD it works

private func update() {
      DispatchQueue.main.async {
          collectionView.reloadData()
      }
}

I thought that if you mark you method with @MainActor in async/await world, then this function will be executed in the main thread automatically. But it crashes.

extension PhotosCollectionViewController: PHPhotoLibraryChangeObserver {
    func photoLibraryDidChange(_ changeInstance: PHChange) {
        update()
    }
    
    @MainActor private func update() {
        collectionView.reloadData()
    }
}

What am I missing?

Upvotes: 1

Views: 783

Answers (1)

fruitcoder
fruitcoder

Reputation: 1218

@MainActor only works when using Swift concurrency and ensures that when you are in an async context calling a @MainActor annotated function is executed on the main thread (actually the actor's executor which in this case is always the main queue).

In your example you call the function in a synchronous context which uses the same thread it was called from (as always).

Upvotes: 1

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