SwiftUI, can't change state from within delegate callback? (code attached) - swiftui

Question

Anyone able to spot why when I set the "firstPass" state variable in the "updateUIView" callback it does NOT set the state? As output I see:
First Pass1: true
First Pass2: true. // <= not set to false as expected
Also I do not in Xcode where I set this state "firstPass = false" that there is an Xcode warning here: "Modifying state during view update, this will cause undefined behavior."
import SwiftUI
import MapKit
struct GCMapView {
#State var firstPass : Bool = true
func makeCoordinator() -> Coordinator {
return Coordinator(self)
}
class Coordinator: NSObject, MKMapViewDelegate {
var parent: GCMapView
init(_ parent: GCMapView) {
self.parent = parent
super.init()
}
}
}
extension GCMapView : UIViewRepresentable {
func makeUIView(context: Context) -> MKMapView {
let map = MKMapView()
map.delegate = context.coordinator
map.showsUserLocation = true
return map
}
func updateUIView(_ view: MKMapView, context: Context) {
print("--- updateUIView ---")
if firstPass {
print("First Pass1: ", firstPass)
firstPass = false. // <<=== *** THIS DOES NOT WORK ****
print("First Pass2: ", firstPass)
} else {
print("Subsequent Passes")
}
}
}

Answer 1

This operation causes cycling, because it invalidates SwiftUI view that result in calling updateUIView and so forth, so SwiftUI rendering engine drops it (auto-fix).
Not really sure you need such if there at all, but possible solution would be to separate update on next event loop, like
if firstPass {
print("First Pass1: ", firstPass)
DispatchQueue.main.async {
firstPass = false
print("First Pass2: ", firstPass)
}
} else {
Tested with Xcode 13 / iOS 15