Update properties based on previous state

Sources/Model/User Interface/Scene/SceneStorage.swift

@@ -22,6 +22,8 @@ public class SceneStorage {
     public var destroy = false
     /// Show the scene element (including moving into the foreground, if possible).
     public var show: () -> Void
+    /// The previous state of the scene element.
+    public var previousState: SceneElement?
 
     /// The pointer as an opaque pointer, as this may be needed with backends interoperating with C or C++.
     public var opaquePointer: OpaquePointer? {

Sources/Model/User Interface/View/ViewStorage.swift

@@ -18,6 +18,8 @@ public class ViewStorage {
     var state: [String: StateProtocol] = [:]
     /// Various properties of a widget.
     public var fields: [String: Any] = [:]
+    /// The previous state of the widget.
+    public var previousState: Widget?
 
     /// The pointer as an opaque pointer, as this is needed with backends interoperating with C or C++.
     public var opaquePointer: OpaquePointer? {
@@ -35,10 +37,12 @@ public class ViewStorage {
     ///   - content: The view's content for container widgets.
     public init(
         _ pointer: Any?,
-        content: [String: [ViewStorage]] = [:]
+        content: [String: [ViewStorage]] = [:],
+        state: Widget? = nil
     ) {
         self.pointer = pointer
         self.content = content
+        self.previousState = state
     }
 
 }

Tests/SampleBackends/Backend1.swift

@@ -62,7 +62,9 @@ public enum Backend1 {
         public func update<Data>(_ storage: ViewStorage, modifiers: [(any AnyView) -> any AnyView], updateProperties: Bool, type: Data.Type) {
             storage.fields["action"] = action
             if updateProperties {
-                print("Update button (label = \(label))")
+                if (storage.previousState as? Self)?.label != label {
+                    print("Update button (label = \(label))")
+                }
             } else {
                 print("Do not update button (label = \(label))")
             }