GTK: `background` modifier, support widget updates in `WidgetView` (#344)

Re-created #340.

* Added background modifier support for color backgrounds

* Fix indentation

* Allow WidgetView to be initialized with an update closure in order to fix updates to children of WidgetViews

* Fix indentation

Co-authored-by: Morten Bek Ditlevsen <morten@ka-ching.dk>

Sources/TokamakGTK/Modifiers/LayoutModifiers.swift

@@ -43,3 +43,14 @@ extension _FlexFrameLayout: WidgetModifier {
     gtk_widget_set_size_request(widget, Int32(idealWidth ?? -1), Int32(idealHeight ?? -1))
   }
 }
+
+extension _BackgroundModifier: WidgetModifier where Background == Color {
+    public func modify(widget: UnsafeMutablePointer<GtkWidget>) {
+        let resolved = _ColorProxy(self.background).resolve(in: environment)
+        var color = GdkRGBA(red: resolved.red,
+                            green: resolved.green,
+                            blue: resolved.blue,
+                            alpha: resolved.opacity)
+        gtk_widget_override_background_color(widget, GtkStateFlags(rawValue: 0), &color)
+    }
+}

Sources/TokamakGTK/Modifiers/WidgetModifier.swift

@@ -24,38 +24,44 @@ protocol WidgetModifier {
 
 extension ModifiedContent: ViewDeferredToRenderer where Content: View {
   public var deferredBody: AnyView {
-    if let widgetModifier = modifier as? WidgetModifier {
+    guard let widgetModifier = modifier as? WidgetModifier else {
+      return AnyView(content)
+    }
+    let anyWidget: AnyWidget
     if let anyView = content as? ViewDeferredToRenderer,
-         let anyWidget = mapAnyView(
+       let _anyWidget = mapAnyView(
         anyView.deferredBody,
         transform: { (widget: AnyWidget) in widget }
        )
     {
-        return AnyView(WidgetView {
-          let contentWidget = anyWidget.new($0)
-          widgetModifier.modify(widget: contentWidget)
-          return contentWidget
-        } content: {
-          if let parentView = anyWidget as? ParentView {
-            ForEach(Array(parentView.children.enumerated()), id: \.offset) { _, view in
-              view
-            }
-          }
-        })
-      } else if let anyWidget = content as? AnyWidget {
-        return AnyView(WidgetView {
-          let contentWidget = anyWidget.new($0)
-          widgetModifier.modify(widget: contentWidget)
-          return contentWidget
-        } content: {
-          if let parentView = anyWidget as? ParentView {
-            ForEach(Array(parentView.children.enumerated()), id: \.offset) { _, view in
-              view
-            }
-          }
-        })
-      }
-    }
+      anyWidget = _anyWidget
+    } else if let _anyWidget = content as? AnyWidget {
+      anyWidget = _anyWidget
+    } else {
       return AnyView(content)
     }
+    return AnyView(WidgetView {
+      let contentWidget = anyWidget.new($0)
+      widgetModifier.modify(widget: contentWidget)
+      return contentWidget
+    }
+    update: { widget in
+      anyWidget.update(widget: widget)
+
+      // Is it correct to apply the modifier again after updating?
+      // I assume so since the modifier parameters may have changed.
+      if case .widget(let w) = widget.storage {
+        widgetModifier.modify(widget: w)
+      }
+    }
+    content: {
+      if let parentView = anyWidget as? ParentView, parentView.children.count > 1 {
+        ForEach(Array(parentView.children.enumerated()), id: \.offset) { _, view in
+          view
+        }
+      } else if let parentView = anyWidget as? ParentView, parentView.children.count == 1 {
+        parentView.children[0]
+      }
+    })
+  }
 }

Sources/TokamakGTK/Widget.swift

@@ -27,16 +27,19 @@ extension AnyWidget {
 
 struct WidgetView<Content: View>: View, AnyWidget, ParentView {
   let build: (UnsafeMutablePointer<GtkApplication>) -> UnsafeMutablePointer<GtkWidget>
+  let update: (Widget) -> Void
   let content: Content
   let expand: Bool
 
   init(build: @escaping (UnsafeMutablePointer<GtkApplication>) -> UnsafeMutablePointer<GtkWidget>,
+       update: @escaping (Widget) -> Void = { _ in },
        expand: Bool = false,
        @ViewBuilder content: () -> Content)
   {
     self.build = build
     self.expand = expand
     self.content = content()
+    self.update = update
   }
 
   func new(_ application: UnsafeMutablePointer<GtkApplication>) -> UnsafeMutablePointer<GtkWidget> {
@@ -44,9 +47,8 @@ struct WidgetView<Content: View>: View, AnyWidget, ParentView {
   }
 
   func update(widget: Widget) {
-    // Rebuild from scratch
-    if case let .widget(w) = widget.storage {
-      widget.destroy()
+    if case .widget = widget.storage {
+      update(widget)
     }
   }
 

Sources/TokamakGTKDemo/main.swift

@@ -22,7 +22,7 @@ struct Counter: View {
   @State private var count: Int = 0
   var body: some View {
     VStack {
-      Text("\(count)")
+      Text("\(count)").background(Color(red: 0.5, green: 1, blue: 0.5))
       HStack {
         Button("Decrement") { count -= 1 }
         Button("Increment") { count += 1 }