Add typed widget references

Tests/PorticoTests/WidgetRefTests.swift

@@ -0,0 +1,104 @@
+import Testing
+@_spi(SGTKInternal) import Gtk
+@_spi(SGTKInternal) import Adw
+@_spi(Portico) import Portico
+
+@MainActor @Suite(.serialized) struct WidgetRefTests {
+    @Test func backwardReference() {
+        guard Gtk.initCheck() else { return }
+        let ref = WidgetRef<Adw.Carousel>()
+        let root = AnyView(VStack {
+            Carousel { Label(str: "a") }.ref(ref)
+            CarouselIndicatorDots().carousel(ref.projectedValue)
+        }).makeWidget(MountContext()) as! Gtk.Box
+
+        let first = root.getFirstChild()!
+        let carousel = Adw.Carousel(retaining: first.pointer)
+        let dots = Adw.CarouselIndicatorDots(retaining: first.getNextSibling()!.pointer)
+        #expect(dots.getCarousel()?.pointer == carousel.pointer)
+    }
+
+    @Test func forwardReference() {
+        guard Gtk.initCheck() else { return }
+        let ref = WidgetRef<Adw.Carousel>()
+        let root = AnyView(VStack {
+            CarouselIndicatorDots().carousel(ref.projectedValue)
+            Carousel { Label(str: "a") }.ref(ref)
+        }).makeWidget(MountContext()) as! Gtk.Box
+
+        let first = root.getFirstChild()!
+        let dots = Adw.CarouselIndicatorDots(retaining: first.pointer)
+        let carousel = Adw.Carousel(retaining: first.getNextSibling()!.pointer)
+        #expect(dots.getCarousel()?.pointer == carousel.pointer)
+    }
+
+    @Test func latePublishFromForEach() {
+        guard Gtk.initCheck() else { return }
+        let ref = WidgetRef<Adw.Carousel>()
+        let items = StateBox<[Int]>([])
+        let root = AnyView(VStack {
+            CarouselIndicatorDots().carousel(ref.projectedValue)
+            ForEach(Binding(items), id: \.self) { _ in
+                Carousel().ref(ref)
+            }
+        }).makeWidget(MountContext()) as! Gtk.Box
+
+        let first = root.getFirstChild()!
+        let dots = Adw.CarouselIndicatorDots(retaining: first.pointer)
+        #expect(dots.getCarousel() == nil)
+
+        items.set([1])
+        let rows = Gtk.Box(retaining: first.getNextSibling()!.pointer)
+        let carousel = Adw.Carousel(retaining: rows.getFirstChild()!.pointer)
+        #expect(dots.getCarousel()?.pointer == carousel.pointer)
+    }
+
+    @Test func teardownClearsPublishedWidget() {
+        guard Gtk.initCheck() else { return }
+        let ref = WidgetRef<Adw.Carousel>()
+        let items = StateBox<[Int]>([])
+        let ctx = MountContext()
+        _ = AnyView(VStack {
+            CarouselIndicatorDots().carousel(ref.projectedValue)
+            ForEach(Binding(items), id: \.self) { _ in
+                Carousel().ref(ref)
+            }
+        }).makeWidget(ctx)
+
+        items.set([1])
+        #expect(ref.wrappedValue != nil)
+        ctx.registry.teardown()
+        #expect(ref.wrappedValue == nil)
+    }
+
+    /// A `WidgetRef` declared at the concrete subclass feeds a modifier whose property
+    /// is declared `Gtk.Widget?`; the generated overload is generic, so `Binding`'s
+    /// invariance does not reject it.
+    @Test func widgetTypedBindingOverload() {
+        guard Gtk.initCheck() else { return }
+        let entryRef = WidgetRef<Gtk.Entry>()
+        let root = AnyView(VStack {
+            Entry().ref(entryRef)
+            Label(str: "N").mnemonicWidget(entryRef.projectedValue)
+        }).makeWidget(MountContext()) as! Gtk.Box
+
+        let first = root.getFirstChild()!
+        let entry = Gtk.Entry(retaining: first.pointer)
+        let label = Gtk.Label(retaining: first.getNextSibling()!.pointer)
+        #expect(label.getMnemonicWidget()?.pointer == entry.pointer)
+    }
+
+    @Test func loweredNullableBindingOverload() {
+        guard Gtk.initCheck() else { return }
+        let stackRef = WidgetRef<Gtk.Stack>()
+        let root = AnyView(VStack {
+            Stack().ref(stackRef)
+            StackSidebar().stack(stackRef.projectedValue)
+        }).makeWidget(MountContext()) as! Gtk.Box
+
+        let first = root.getFirstChild()!
+        let stack = Gtk.Stack(retaining: first.pointer)
+        let sidebar = Gtk.StackSidebar(retaining: first.getNextSibling()!.pointer)
+        #expect(sidebar.getStack()?.pointer == stack.pointer)
+    }
+}