Infinite loop when using onAppear in SwiftUI

Question

I have a strange infinite loop when using onAppear and I cannot identify the root of the problem. It only happens when the view is the detail view of a navigation view, but it works fine when it's the root view. Another interesting thing is that if I wrap the detail view in a NavigationView (so, now we have a navigation view inside a navigation view), then the issue does not appear anymore. Is this a bug in SwiftUI? Is conceptually my design OK? I mean, using onAppear like viewDidLoad to trigger the initial sequence. Thanks for suggestions.

Here is the source code. ContentView.swift:

import SwiftUI

struct ContentView: View {

    @StateObject var viewModel = ContentViewModel()

    var body: some View {
        NavigationView {
            VStack {
                Group {
                    switch viewModel.state {
                    case .loading:
                        Text("Loading...")
                    case .loaded:
                        HStack {
                            Text("Loaded")
                            Button("Retry") {
                                viewModel.fetchData()
                            }
                        }
                    }
                }
                .padding(.bottom, 20)
                NavigationLink("Go to detail screen", destination: DetailView())
            }
        }
        .onAppear() {
            viewModel.fetchData()
        }
    }
}

class ContentViewModel: ObservableObject  {

    enum State {
        case loading
        case loaded
    }

    @Published var state: State = .loading

    func fetchData() {
        state = .loading
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            self.state = .loaded
        }
    }
}

And here the code of the detail view:

import SwiftUI

struct DetailView: View {

    @StateObject var viewModel = DetailViewModel()

    var body: some View {
        Group {
            switch viewModel.state {
            case .loading:
                Text("Loading...")
            case .loaded:
                HStack {
                    Text("Loaded")
                    Button("Retry") {
                        viewModel.fetchData()
                    }
                }
            }
        }
        .onAppear() {
            print("infinite loop here")
            viewModel.fetchData()
        }
    }
}

class DetailViewModel: ObservableObject  {

    enum State {
        case loading
        case loaded
    }

    @Published var state: State = .loading

    func fetchData() {
        state = .loading
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            self.state = .loaded
        }
    }
}

Here I attach the project: https://www.dropbox.com/s/5alokj3q81jbpj7/TestBug.zip?dl=0

I'm using Xcode Version 12.5.1 (12E507) and iOS 14.5

Thanks a lot.

Adding another voice to the chorus. 13RC, iOS 14.4 & iOS 15. Tested fine on both. Are you sure this code caused an infinite loop? — Yrb, Oct 04 '21 at 23:52
Everyone keeps commenting about how it's working on *the wrong Xcode version*. Just tested and the issue is as-described with Xcode 12.5.1 and iOS 14.5. — Charles Maria, Oct 05 '21 at 00:31
OK, i just realised the problem is Group and xcode 12. If I use VStack, it works. So, this is an apple bug. — Ricardo, Oct 05 '21 at 14:45

Charles Maria · Accepted Answer · 2021-10-06T02:54:47.073

This issue is a bug in iOS 14 with Group, and will still happen even when building with Xcode 13.

What's happening is the SwiftUI runtime is messing up its view diff, so it believes that it "appeared" when really it only reloaded, and thus did not disappear (which is a requirement for onAppear to be called, hence a bug).

To compensate for this issue, you'll need to use initialisers rather than relying on onAppear.

I've amended your code, this amended version does not infinitely loop.

struct DetailView: View {

    @StateObject var viewModel = DetailViewModel()

    var body: some View {
        Group {
            switch viewModel.state {
            case .loading:
                Text("Loading...")
            case .loaded:
                HStack {
                    Text("Loaded")
                    Button("Retry") {
                        viewModel.fetchData()
                    }
                }
            }
        }
    }
}

class DetailViewModel: ObservableObject  {

    enum State {
        case loading
        case loaded
    }

    @Published var state: State = .loading

    init() {
        self.fetchData()
    }

    func fetchData() {
        state = .loading
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            self.state = .loaded
        }
    }
}

Hope the project goes well!

Perhaps the bug is with Group. I just realised that if I use VStack instead of Group, it works. — Ricardo, Oct 05 '21 at 14:45
I approve your answer for testing it, but the bug is with Group (and Xcode 12). Thank you! — Ricardo, Oct 05 '21 at 14:47
Oh I totally missed that you were using a group! Great catch, I'll update my answer :) — Charles Maria, Oct 06 '21 at 02:54
Thanks for this answer! This also happens in iOS 15 and was struggling for ages until I found this. — Tometoyou, Jan 11 '22 at 10:33

score 1 · Answer 2 · answered Feb 08 '22 at 19:58

I saw something similar with iOS 15 and moving all the onAppear stuff to an initializer fixed things. No more infinite looping trying to make the dialog appear. Weirdly this only happens with the settings in a certain state.

In onAppear I had code like:

useSecondary = data.settings.showSecondaryAxis
...

But in the init function needs this to do the job:

   init(data: Binding<PlotData> {
      self._data = data
      _useSecondary = State(initialValue: data.wrappedValue.settings.showSecondaryAxis)
...

}

… Cumbersome but it works.

Infinite loop when using onAppear in SwiftUI

2 Answers2

Linked