Use cached navigation title

Author: carson-katri

Sources/LiveViewNative/Modifiers/NavigationTitleModifier.swift

@@ -8,32 +8,36 @@
 import SwiftUI
 
 struct NavigationTitleModifier: ViewModifier, Decodable, Equatable {
-    private let title: String
+    fileprivate let title: String
+    private let cached: Bool
 
     init(from decoder: Decoder) throws {
         let container = try decoder.container(keyedBy: CodingKeys.self)
         self.title = try container.decode(String.self, forKey: .title)
+        self.cached = false
     }
 
-    init(title: String) {
+    init(title: String, cached: Bool = false) {
         self.title = title
+        self.cached = cached
     }
 
     func body(content: Content) -> some View {
         content
             .navigationTitle(title)
-            .preference(key: NavigationTitleModifierKey.self, value: self)
+            .preference(key: NavigationTitleModifierKey.self, value: cached ? nil : self)
     }
 
     enum CodingKeys: String, CodingKey {
         case title
     }
 }
 
+/// A key that passes the navigation title up the View hierarchy via preferences.
 enum NavigationTitleModifierKey: PreferenceKey {
     static var defaultValue: NavigationTitleModifier?
 
     static func reduce(value: inout NavigationTitleModifier?, nextValue: () -> NavigationTitleModifier?) {
-        value = nextValue() ?? value
+        value = nextValue().flatMap({ .init(title: $0.title, cached: true) }) ?? value
     }
 }

Sources/LiveViewNative/NavStackEntryView.swift

@@ -41,33 +41,48 @@ struct NavStackEntryView<R: CustomRegistry>: View {
 
     @ViewBuilder
     private var elementTree: some View {
-        switch coordinator.state {
-        case .connected:
-            if let doc = coordinator.document {
-                coordinator.builder.fromNodes(doc[doc.root()].children(), coordinator: coordinator, url: coordinator.url)
-                    .environment(\.coordinatorEnvironment, CoordinatorEnvironment(coordinator, document: doc))
-            } else {
-                fatalError("State is `.connected`, but no `Document` was found.")
-            }
-        default:
-            if R.LoadingView.self == Never.self {
-                switch coordinator.state {
-                case .connected:
-                    fatalError()
-                case .notConnected:
-                    SwiftUI.Text("Not Connected")
-                case .connecting:
-                    SwiftUI.Text("Connecting")
-                case .connectionFailed(let error):
-                    SwiftUI.VStack {
-                        SwiftUI.Text("Connection Failed")
-                            .font(.subheadline)
-                        SwiftUI.Text(error.localizedDescription)
+        if coordinator.url == entry.url {
+            switch coordinator.state {
+            case .connected:
+                if let doc = coordinator.document {
+                    coordinator.builder.fromNodes(doc[doc.root()].children(), coordinator: coordinator, url: coordinator.url)
+                        .environment(\.coordinatorEnvironment, CoordinatorEnvironment(coordinator, document: doc))
+                        .onPreferenceChange(NavigationTitleModifierKey.self) { navigationTitle in
+                            self.liveViewModel.cachedNavigationTitle = navigationTitle
+                            print("Nav title changed")
+                        }
+                } else {
+                    fatalError("State is `.connected`, but no `Document` was found.")
+                }
+            default:
+                let content = Group {
+                    if R.LoadingView.self == Never.self {
+                        switch coordinator.state {
+                        case .connected:
+                            fatalError()
+                        case .notConnected:
+                            SwiftUI.Text("Not Connected")
+                        case .connecting:
+                            SwiftUI.Text("Connecting")
+                        case .connectionFailed(let error):
+                            SwiftUI.VStack {
+                                SwiftUI.Text("Connection Failed")
+                                    .font(.subheadline)
+                                SwiftUI.Text(error.localizedDescription)
+                            }
+                        }
+                    } else {
+                        R.loadingView(for: coordinator.url, state: coordinator.state)
                     }
                 }
-            } else {
-                R.loadingView(for: coordinator.url, state: coordinator.state)
+                if let cachedNavigationTitle = liveViewModel.cachedNavigationTitle {
+                    content.modifier(cachedNavigationTitle)
+                } else {
+                    content
+                }
             }
+        } else if let cachedNavigationTitle = liveViewModel.cachedNavigationTitle {
+            SwiftUI.Text("").modifier(cachedNavigationTitle)
         }
     }
 }

Sources/LiveViewNative/ViewModel.swift

@@ -14,6 +14,7 @@ import LiveViewNativeCore
 /// In a view in the LiveView tree, a model can be obtained using `@EnvironmentObject`.
 public class LiveViewModel<R: CustomRegistry>: ObservableObject {
     private var forms = [String: FormModel]()
+    var cachedNavigationTitle: NavigationTitleModifier?
 
     /// Get or create a ``FormModel`` for the `<form>` element with the given ID.
     public func getForm(elementID id: String) -> FormModel {