I'm borrowing a pattern from Paul's UltimatePortfolio app in which I have DataController that handles all things CoreData and it has a @Published var selectedLiftEvent: LiftEvent? (similar to @Published var selectedIssue: Issue? in Paul's project)

I know this error typically occurs when you're trying to update a @Published property within a SwiftUI view's lifecycle methods, such as init(), onAppear(), or onChange() of another @ObservedObject, but I don't think I'm doing any of this in CalculatorView. Am I wrong?

I'm using the 'LiftEvent' in CalculatorView like this:

struct CalculatorView: View {
    @EnvironmentObject var dataController: DataController
    @ObservedObject var liftEvent: LiftEvent

    @State private var showPercentages = false
    @State var units: String = "kg"
    @State private var oneRepMax: String = "0.0"

    private let weightCharacterLimit = 3
    private let repsCharacterLimit = 2

    let calculator = Calculator()

    var body: some View {

        NavigationView {
            ZStack {
                VStack {
                    Text(oneRepMax)
                        .font(.system(size: 90, weight: .light))
                        .foregroundColor(.yellow)

                    HStack {
                            HStack() {
                                TextField("Weight", text: $liftEvent.eventWeight)
                                    .keyboardType(.decimalPad)
                                    .onChange(of: liftEvent.eventWeight) { _, newValue in
                                        if liftEvent.eventWeight.count > weightCharacterLimit {
                                            liftEvent.eventWeight = String(liftEvent.eventWeight.prefix(weightCharacterLimit))
                                        }
                                        updateOneRepMax()
                                    }

                                Text($units.wrappedValue)
                            }
                        }

                        VStack {
                            Text("Reps")
                                .font(.system(size: 20, weight: .medium))
                                .foregroundColor(.white)

                            TextField("Reps", text: $liftEvent.eventReps)
                                .keyboardType(.decimalPad)
                                .onChange(of: liftEvent.eventReps) { _, newValue in
                                    if liftEvent.eventReps.count > repsCharacterLimit {
                                        liftEvent.eventReps = String(liftEvent.eventReps.prefix(repsCharacterLimit))
                                    }
                                    updateOneRepMax()
                                }

                        }
                    }
                    .padding(.top, 20)
                    .sheet(isPresented: $showPercentages) {
                        PercentagesView()
                            .presentationDetents([.medium, .large])
                    }
                    Spacer()
                }
                .onChange(of: liftEvent.eventWeight) { _, newValue in
                    updateOneRepMax()
                }
                .onChange(of: liftEvent.eventReps) { _, newValue in
                    updateOneRepMax()
                }
            }
            .toolbar(content: CalculatorViewToolbar.init)
        }
    }

    private func updateOneRepMax() {
        oneRepMax = String(format: "%.1f", calculator.calculateOneRepMax(weight: Double(liftEvent.eventWeight) ?? 0, reps: Int(liftEvent.eventReps) ?? 0, formula: UserDefaults.formulaName()))
    }

}

The problem is that I get the error here:

    func newLiftEvent() {
        let liftEvent = LiftEvent(context: container.viewContext)
        liftEvent.uuid = NSUUID().uuidString
        liftEvent.date = .now
        liftEvent.formula = defaultFormula()
        liftEvent.lift = defaultLift()
        liftEvent.weightUnit = UserDefaults.unitsNotation().weightUnit

        selectedLiftEvent = liftEvent // ERROR: Publishing changes from within view updates is not allowed, this will cause undefined behavior.
    }

But this function is doing essentially the same thing as Paul's function for creating a new Issue:

    func newIssue() {
        let issue = Issue(context: container.viewContext)
        issue.title = NSLocalizedString("New issue", comment: "Create a new issue")
        issue.creationDate = .now
        issue.priority = 1

        // If we are currently browsing a user-created tag, immediately
        // add this new issue to the tag otherwise it won't appear in
        // the list of issues they see.
        if let tag = selectedFilter?.tag {
            issue.addToTags(tag)
        }

        save()

        selectedIssue = issue // No error
    }

Thanks for the suggestions, @Obelix. I added 'save()' (and the functions it needs to work) so it exactly matches what Paul is doing but, as I suspected, it didn't fix the problem.

About the .onChange(of: liftEvent.eventWeight), I actually didn't miss what Paul was saying about the job of views. I believe that he's absolutely correct. The code in that modifier limits the number of characters a user can input (which you probably figured out) and I didn't consider that business logic.

I don't think it makes sense to move it to the LiftEvent because LiftEvent can accept much larger numbers for the eventWeight and eventReps properties. Limiting the characters a user can input is purely a UI thing. I do think it might make sense to have that code in a ViewModifier though.

Let me know if you think otherwise.

You're in the elite HWS+ community. Us plebes in the lower decks don't get to see the articles for his Ultimate Portfolio. Nor can we view the vids he posts as solutions to his HWS+ Ultimate Portfolio.

To be honest, I was just taking a wild guess as I can't see his code, nor his solutions. Since it didn't work, I deleted my unhelpful post.

I can't see the context in which newLiftEvent is being called. Are you sure you're on the main thread? I have had similar problems in one of my apps and the solution was to save the changed object, then set the selected item inside a DispatchQueue.main.async callback. It then forces the update onto the main thread where the UI updates. Might be worth. try.

@barnettsab, I'm not sure if I'm on the main thread. I'm not very knowledgeable about multi-threading to be honest. Is there an easy way for me to tell?

I encourage the OP to buy Paul's "Swift Concurrency by Example". I think it is the clearest book he's written. It explains the basis of @barnettsab's advice.

@JackAward, since the LiftEvent managed object properties are binded to the TextFields, is it possible to separate view updates from data changes?

Thanks, @Barnettsab. That made the warning go away so it must have not been running on the main thread.

No worries. I suspect you are calling this code from a closure and that closure is from a background activity. Without seeing the code where your method is called, I can only guess (and offer the same fix that I did in my own code).