In SwiftUI, how to implement something like `navigationBarItems(...)` - swiftui

Question

I've had some problems with SwiftUI's navigation API, so I'm experimenting with implementing my own. Parts of this are relatively easy: I create a class NavModel that is basically a stack. Depending on what's on the top of that stack, I can display different views.
But I can't see how to implement something like SwiftUI's .navigationBarItems(...). That view modifier seems to use something like the Preferences API to pass its argument View up the hierarchy to the containing navigation system. Eg:
VStack {
...
}.navigationBarItems(trailing: Button("Edit") { startEdit() })
Anything that goes through onPreferenceChange(...) has to be Equatable, so if I want to pass an AnyView? for the navigation bar items, I need to somehow may it Equatable, and I don't see how to do that.
Here's some sample code that shows a basic push and pop navigation. I'm wondering: how could I make the navBarItems(...) work? (The UI is ugly, but that's not important now.)
struct ContentView: View {
#StateObject var navModel: NavModel = .shared
var body: some View {
NavView(model: navModel) { node in
switch node {
case .root: rootView
case .foo: fooView
}
}
}
var rootView: some View {
VStack {
Text("This is the root")
Button {
navModel.push(.foo)
} label: {
Text("Push a view")
}
}
}
var fooView: some View {
VStack {
Text("Foo")
Button {
navModel.pop()
} label: {
Text("Pop nav stack")
}
}.navBarItems(trailing: Text("Test"))
}
}
struct NavView<Content: View>: View {
#ObservedObject var model: NavModel
let makeViews: (NavNode) -> Content
init(model: NavModel, #ViewBuilder makeViews: #escaping (NavNode) -> Content) {
self.model = model
self.makeViews = makeViews
}
#State var navItems: AnyView? = nil
var body: some View {
VStack {
let node = model.stack.last!
navBar
Divider()
makeViews(node)
.frame(maxHeight: .infinity)
// This doesn't compile
.onPreferenceChange(NavBarItemsPrefKey.self) { v in
navItems = v
}
}
}
var navBar: some View {
HStack {
if model.stack.count > 1 {
Button {
model.pop()
} label: { Text("Back") }
}
Spacer()
if let navItems = self.navItems {
navItems
}
}
}
}
enum NavNode {
case root
case foo
}
class NavModel: ObservableObject {
static let shared = NavModel()
#Published var stack: [NavNode]
init() {
stack = [.root]
}
func push(_ node: NavNode) { stack.append(node) }
func pop() {
if stack.count > 1 {
stack.removeLast()
}
}
}
struct NavBarItemsPrefKey: PreferenceKey {
typealias Value = AnyView?
static var defaultValue: Value = nil
static func reduce(value: inout Value, nextValue: () -> Value) {
let n = nextValue()
if n != nil { // ???
value = n
}
}
}
// Is this the right way? But then anything passed to navBarItems(...) would need
// to be Equatable. The common case - Buttons - are not.
struct AnyEquatableView: Equatable {
???
init<T>(_ ev: EquatableView<T>) {
???
}
static func == (lhs: AnyEquatableView, rhs: AnyEquatableView) -> Bool {
???
}
}
struct NavBarItemsModifier<T>: ViewModifier where T: View {
let trailing: T
func body(content: Content) -> some View {
content.preference(key: NavBarItemsPrefKey.self, value: AnyView(trailing))
}
}
extension View {
func navBarItems<T>(trailing: T) -> some View where T: View {
return self.modifier(NavBarItemsModifier(trailing: trailing))
}
}