Add dirty UI element visualization to UISystem, fix Frame being constantly updated.

Voile/Source/Resources/Resource.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using Voile.Resources;
 
 namespace Voile
@@ -32,7 +33,7 @@ namespace Voile
         /// </summary>
         /// <param name="value">An instance of a retrieved <see cref="Resource"/>.</param>
         /// <returns><c>true</c> if the resource was successfully retrieved, otherwise <c>false</c>.</returns>
-        public bool TryGetValue(out T? value)
+        public bool TryGetValue([NotNullWhen(true)] out T? value)
         {
             value = ResourceManager.GetResource<T>(Guid);
             return value != null;

Voile/Source/UI/Containers/Container.cs

@@ -44,7 +44,6 @@ public abstract class Container : UIElement, IParentableElement
         {
             if (child is not IUpdatableElement updatable) continue;
 
-            updatable.MarkDirty();
             updatable.Update();
 
             if (child is IAnchorableElement anchorable)

Voile/Source/UI/Containers/Frame.cs

@@ -50,7 +50,12 @@ public class Frame : Container
     protected override void OnUpdate()
     {
         base.OnUpdate();
+
+        if (Size == _lastParentSize) return;
+
         Size = _lastParentSize;
+
+        Console.WriteLine("Updating");
     }
 
     private Rect _lastParentSize = Rect.Zero;

Voile/Source/UI/UISystem.cs

@@ -9,6 +9,8 @@ public class UISystem : IUpdatableSystem, IRenderableSystem
     public IReadOnlyList<IElement> Elements => _elements;
 
     public bool RenderDebugRects { get; set; }
+    public Color DebugSizeRectColor { get; set; } = Color.Red;
+    public Color DebugDirtyRectColor { get; set; } = new Color(1.0f, 1.0f, 0.0f, 0.5f);
 
     public UISystem(InputSystem inputSystem)
     {
@@ -50,21 +52,43 @@ public class UISystem : IUpdatableSystem, IRenderableSystem
         {
             if (element is IRenderableElement renderable)
             {
-                renderable.Render(renderer, _style.Value);
-
-                if (!RenderDebugRects) return;
-                renderable.DrawSize(renderer);
-            }
-
-            if (element is IParentableElement parentable)
-            {
-                foreach (var child in parentable.Children)
+                // TODO: normally you'd load a default style if the one supplied is empty,
+                // but for now this will do.
+                if (!_style.TryGetValue(out var value))
                 {
-                    if (child is not IRenderableElement renderableChild) continue;
-
-                    if (!RenderDebugRects) return;
-                    renderableChild.DrawSize(renderer);
+                    value = new Style(string.Empty);
                 }
+
+                renderable.Render(renderer, value);
+            }
+        }
+
+        if (!RenderDebugRects) return;
+
+        foreach (var element in _elements)
+        {
+            if (element is not UIElement uiElement) continue;
+            DrawDebugForElement(renderer, uiElement);
+        }
+    }
+
+    private void DrawDebugForElement(RenderSystem renderer, UIElement element)
+    {
+        var size = new Vector2(element.Size.Width, element.Size.Height);
+        renderer.SetTransform(element.GlobalPosition, Vector2.Zero);
+        renderer.DrawRectangleOutline(size, DebugSizeRectColor);
+
+        if (element.Dirty)
+        {
+            renderer.DrawRectangle(size, DebugDirtyRectColor);
+        }
+
+        if (element is IParentableElement parentableElement)
+        {
+            foreach (var child in parentableElement.Children)
+            {
+                if (child is not UIElement childElement) continue;
+                DrawDebugForElement(renderer, childElement);
             }
         }
     }