Add a preference key to NSViewPepresentable/UIViewRepresentable in SwiftUI - swiftui

Question

I have an instance of a custom AppKit/UIKit view, wrapped into an **ViewRepresentable that needs to communicate some diagnostic information up the SwiftUI hierarchy. Since this information is not of primary importance, I would like to use the PreferenceKey API to do it, so that parent views can subscribe to it only when needed. Unfortunately, I can't figure a way to set a preference key on my view, since I don't have assess to the view struct itself and thus can't use the preference() API. Is there some way how I can set the preference key from the coordinator?

Answer 1

Here is a simple demo of possible approach. Tested with Xcode 12 / iOS 14. The idea is to construct UIViewRepresentable with injected preference key.
struct DemoView: View {
#State private var diag = CGFloat.zero
var body: some View {
VStack {
Text("Diagnostic >> \(diag)")
.background(MyUIRep.trackingRep)
}
.onPreferenceChange(MyPrefKey.self) { value in
self.diag = value
}
}
}
struct TestPreferenceKeyInRep_Previews: PreviewProvider {
static var previews: some View {
TestPreferenceKeyInRep()
}
}
struct MyUIRep: UIViewRepresentable {
static var trackingRep: some View {
MyUIRep().preference(key: MyPrefKey.self, value: 13.31)
}
func makeUIView(context: Context) -> UIView {
return UIView()
}
func updateUIView(_ uiView: UIView, context: Context) {
}
}
struct MyPrefKey : PreferenceKey {
static var defaultValue: CGFloat = .zero
static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
value += nextValue()
}
}