Initial implementation of UI anchors, update TestGame.

Voile/Source/UI/Widgets/Widget.cs

@@ -1,4 +1,5 @@
 using System.Numerics;
+using System.Xml.Serialization;
 using Voile.Input;
 using Voile.Rendering;
 
@@ -7,7 +8,7 @@ namespace Voile.UI.Widgets;
 /// <summary>
 /// A base class for all UI widgets.
 /// </summary>
-public abstract class Widget : IElement, IRenderableElement, IInputElement, IResizeableElement, IUpdatableElement
+public abstract class Widget : IElement, IRenderableElement, IInputElement, IResizeableElement, IUpdatableElement, IAnchorableElement
 {
     public bool Visible { get; set; } = true;
     public bool IgnoreInput { get; set; }
@@ -36,6 +37,36 @@ public abstract class Widget : IElement, IRenderableElement, IInputElement, IRes
 
     public bool Dirty => _isDirty;
 
+    public Anchor Anchor { get; set; }
+    public Vector2 AnchorOffset { get; set; }
+
+    public void ApplyAnchor(Vector2 parentPosition, Rect parentRect)
+    {
+        switch (Anchor)
+        {
+            case Anchor.TopLeft:
+                Position = parentPosition + AnchorOffset;
+                break;
+            case Anchor.TopCenter:
+                var topCenterX = parentRect.Width / 2;
+                var topCenterY = parentPosition.Y;
+
+                Position = new Vector2(parentPosition.X + topCenterX, topCenterY) + AnchorOffset;
+                break;
+            case Anchor.TopRight:
+                Position = new Vector2(parentPosition.X + parentRect.Width, parentPosition.Y);
+                break;
+
+            case Anchor.Fill:
+                Position = parentPosition;
+                Size = parentRect;
+                break;
+
+            default:
+                throw new NotImplementedException("This anchor type is not implemented!");
+        }
+    }
+
     /// <summary>
     /// Called when its time to draw this widget.
     /// </summary>