SwiftUI View reads stale ObservableObject property value - swiftui

Question

I am seeing a very strange state behavior when updating ObservableObjects on iOS 14.5 under a very particular condition. I suspect this is a SwiftUI bug - I extracted code to reproduce this from my application for further investigation.
The question is: Is this actually a bug in SwiftUI or is there an explanation for the behavior?
The bug is hard to reproduce, so let's start simple: Assume we have two models FlipModel (storing a value flipped) and CounterModel (keeping the number the value has been flipped). The flipped value is observed via onReceive and if a value change occurs, the counter is incremented. This is just an arbitrary, simplified example to be able to reproduce the bug.
class FlipModel: ObservableObject {
#Published var flipped = true {
didSet {
print("new value for flipped: \(self.flipped)")
}
}
}
class CounterModel: ObservableObject {
#Published var counter = 0
}
struct StrangeStateGlitchExampleView: View {
#StateObject var flipModel = FlipModel()
#StateObject var counterModel = CounterModel()
// MARK: - View
var body: some View {
VStack(spacing: 10) {
Button("Toggle value from SwiftUI") {
self.flipModel.flipped.toggle()
}
Text("Flipped: \(String(describing: flipModel.flipped))")
Text("Flipped \(self.counterModel.counter) times")
}
.onReceive(self.flipModel.$flipped, perform: { _ in
withAnimation {
self.counterModel.counter += 1
}
})
}
}
struct StrangeStateGlitchExampleView_Previews: PreviewProvider {
static var previews: some View {
StrangeStateGlitchExampleView()
}
}
This works fine:
Now let's add three things to the example that are all necessary for the bug to occur:
The change is not done from SwiftUI, but from a background thread via DispatchQueue.main.async.
FlipModel is also observed from another view.
The counter change is done in an withAnimation block.
Example code:
class FlipModel: ObservableObject {
#Published var flipped = true {
didSet {
print("new value for flipped: \(self.flipped)")
}
}
}
class CounterModel: ObservableObject {
#Published var counter = 0
}
struct StrangeStateGlitchExampleView: View {
#StateObject var flipModel = FlipModel()
#StateObject var counterModel = CounterModel()
// MARK: - View
var body: some View {
VStack(spacing: 10) {
Button("Toggle value from SwiftUI") {
self.flipModel.flipped.toggle()
}
Button("Toggle value from external thread") {
// --> 1) The change is not done from SwiftUI.
// but from a background thread via `DispatchQueue.main.async`.
DispatchQueue.main.async {
self.flipModel.flipped.toggle()
}
}
// -- 2) FlipModel is also observed from another view.
OtherView(flipModel: self.flipModel)
Text("Flipped (ExampleView): \(String(describing: flipModel.flipped))")
Text("Flipped \(self.counterModel.counter) times")
}
.onReceive(self.flipModel.$flipped, perform: { _ in
// --> 3) The counter change is done in an `withAnimation` block.
withAnimation {
self.counterModel.counter += 1
}
})
}
}
struct OtherView: View {
#ObservedObject var flipModel: FlipModel
var body: some View {
Text("Flipped (OtherView): \(String(describing: flipModel.flipped))")
}
}
struct StrangeStateGlitchExampleView_Previews: PreviewProvider {
static var previews: some View {
StrangeStateGlitchExampleView()
}
}
Now, if the change is triggered like that, the OtherView reads an old value for flipped: