Initial implementation of UI anchors, update TestGame.

Voile/Source/UI/Containers/Container.cs

@@ -3,10 +3,11 @@ using Voile.Rendering;
 
 namespace Voile.UI.Containers;
 
+// TODO: make Container extend Widget, it already implements similar behaviors. 
 /// <summary>
 /// A base class for all UI containers, used to position and rendering child <see cref="IElement">s.
 /// </summary>
-public abstract class Container : IElement, IParentableElement, IUpdatableElement, IResizeableElement, IRenderableElement
+public abstract class Container : IElement, IParentableElement, IUpdatableElement, IResizeableElement, IRenderableElement, IAnchorableElement
 {
     /// <inheritdoc />
     public IReadOnlyList<IElement> Children => _children;
@@ -44,6 +45,8 @@ public abstract class Container : IElement, IParentableElement, IUpdatableElemen
     /// Specifies if this <see cref="Container"/>'s minimum size will be confined to contents.
     /// </summary>
     public bool ConfineToContents { get; set; } = false;
+     public Anchor Anchor { get; set; }
+    public Vector2 AnchorOffset { get; set; }
 
     public Container()
     {
@@ -75,6 +78,11 @@ public abstract class Container : IElement, IParentableElement, IUpdatableElemen
         {
             if (child is not IUpdatableElement updatable) continue;
             updatable.Update();
+
+            if (child is IAnchorableElement anchorable)
+            {
+                anchorable.ApplyAnchor(Position, Size);
+            }
         }
 
         Arrange();
@@ -161,6 +169,38 @@ public abstract class Container : IElement, IParentableElement, IUpdatableElemen
         renderer.DrawRectangleOutline(new Vector2(Size.Width, Size.Height), Color.Red, 2.0f);
     }
 
+    public void ApplyAnchor(Vector2 parentPosition, Rect parentRect)
+    {
+        var absoluteOffset = AnchorOffset * new Vector2(Size.Width, Size.Height);
+
+        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) - absoluteOffset;
+                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!");
+        }
+
+        MarkDirty();
+        Update();
+    }
+
     private List<IElement> _children = new();
     private bool _isDirty;
     private Rect _size = Rect.Zero;