AsyncPublisher causes crash in rather simple situation

Using Swift
, , , ,
1

I finally managed to reduce the crash I was experiencing down to the following:

let a = ObservableObjectPublisher()
Task {
    try await Task.sleep(nanoseconds: 1_000_000)
    a.send()
    a.send()
}
await AsyncPublisher(a).first(where: { _ in true })

which produces the error message: Received an output without requesting demand:

Screen Shot 2022-04-06 at 4.48.33 PM
Screen Shot 2022-04-06 at 4.48.33 PM2628×536 75.2 KB

I suppose this is a bug in Combine, so I'll also file a bug report to Apple, but if anyone can help me understand this better so that I can work around it that would be great, because it's problematic for me that after awaiting the publisher in this way I can no longer publish to it without provoking a crash.

Thanks!

4 Likes
2

That is definitely a bug; it seems like ObservableObjectPublisher is not properly respecting demand. Do you happen to have a feedback # on it?

3

I think this is the feedback number? FB9975196

4

Got it routed to the right spot; it looks like ObservableObjectPublisher is expecting everything subscribed to it to be unlimited, I would suggest instead to use a PassthroughSubject instead if possible - that will respect demand correctly.

1 Like
5

I've been playing with a sample from a WWDC lecture and they seem to do it like this:

  var objectWillChangeSequence:
    AsyncPublisher<Publishers.Buffer<ObservableObjectPublisher>>
  {
    objectWillChange
      .buffer(size: 1, prefetch: .byRequest, whenFull: .dropOldest)
      .values
  }
2 Likes
6

Hello, I recently experienced the same crash in my app. @Philippe_Hausler is it going to be fixed anytime soon or is it actually expected behaviour?

Screen Shot 2022-11-08 at 10.11.32 PM
Screen Shot 2022-11-08 at 10.11.32 PM1916×324 44.7 KB

7

That screenshot does not tell me very much, but the assertion that it is hitting means that the publisher is breaking the rules for Combine; basically it means that it got a value without any demand applied to the subscription. If you are seeing this with composition I would suggest highly to file a feedback and include a detailed reproduction of the failure.

8

@Philippe_Hausler this is a minimum reproducible example.

import UIKit

class ViewController: UIViewController {
    let label = UILabel()

    let viewModel = ViewModel()

    override func viewDidLoad() {
        super.viewDidLoad()

        view.addSubview(label)

        NSLayoutConstraint.activate([
            label.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            label.centerYAnchor.constraint(equalTo: view.centerYAnchor),
        ])

        Task {
            for await _ in viewModel.objectWillChange.values {
                label.text = viewModel.someString
            }
        }

        viewModel.getCachedString()
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        viewModel.getUpdatedString()
    }
}

@MainActor
class ViewModel: ObservableObject {
    @Published var someString = "someValue"

    func getCachedString() {
        Task {
            try? await Task.sleep(nanoseconds: 1_000_000_000)
            someString = "cache"
        }
    }

    func getUpdatedString() {
        Task {
            try? await Task.sleep(nanoseconds: 1_000_000_000)
            someString = "server"
        }
    }
}

Just create a new Xcode project with Storyboard and paste this code in the ViewController.swift file. Run the app and it will crash :smiling_face_with_tear:

9

That is the same as the case given in this thread; the workaround is to add a .buffer in the publisher chain.

2 Likes
10

I see.
Is this behavior a bug though?

Thank you very much for the help!

11

The bug isn't in the AsyncPublisher but in ObservableObjectPublisher; it breaks the rules normally enforced by the rest of Combine (particularly it expects the downstream to request .unlimited). So I would highly suggest that in that case where you are connecting to a objectWillChangePublisher to put a buffer in there to ensure the proper demand.

3 Likes
12

For anyone who will encounter this issue in the future, here is a simple fix

extension ObservableObject {
    typealias BufferedObjectWillChangePublisher = Publishers.Buffer<ObjectWillChangePublisher>

    var bufferedObjectWillChange: BufferedObjectWillChangePublisher {
        objectWillChange
            .buffer(size: 1, prefetch: .byRequest, whenFull: .dropOldest)
    }

    var asyncObjectWillChange: AsyncPublisher<BufferedObjectWillChangePublisher> {
        bufferedObjectWillChange.values
    }
}

Usage:

for await _ in viewModel.asyncObjectWillChange {
    // update the ViewController
}
3 Likes