WIP: dirty flag for UI.

Voile/Source/UI/Widgets/Widget.cs

@@ -22,11 +22,14 @@ public abstract class Widget : IElement, IRenderableElement, IInputElement, IRes
     public Widget(Vector2 position)
     {
         Position = position;
+        Size = MinimumRect;
     }
 
     /// </inheritdoc>
     public abstract Rect MinimumRect { get; }
 
+    public bool Dirty => _isDirty;
+
     /// <summary>
     /// Called when its time to draw this widget.
     /// </summary>
@@ -41,15 +44,22 @@ public abstract class Widget : IElement, IRenderableElement, IInputElement, IRes
 
     public void Update()
     {
+        if (!_isDirty) return;
+        _isDirty = false;
+
         if (Size == Rect.Zero)
         {
             Size = MinimumRect;
         }
     }
 
+    public void MarkDirty() => _isDirty = true;
+
     public void DrawSize(RenderSystem renderer)
     {
         renderer.SetTransform(Position, Vector2.Zero);
         renderer.DrawRectangleOutline(new Vector2(Size.Width, Size.Height), Color.Red, 2.0f);
     }
+
+    private bool _isDirty = true;
 }