Add container nesting.

Voile/Source/UI/Containers/Container.cs

@@ -1,27 +1,41 @@
 using System.Numerics;
-using Voile.UI.Widgets;
 
 namespace Voile.UI.Containers;
 
 /// <summary>
-/// A base class for all UI containers, used to position and rendering child <see cref="Widget">s.
+/// A base class for all UI containers, used to position and rendering child <see cref="IElement">s.
 /// </summary>
-public abstract class Container : IElement, IParentableElement
+public abstract class Container : IElement, IParentableElement, IUpdatableElement
 {
     public IReadOnlyList<IElement> Children => _children;
     public Vector2 Position { get; set; }
 
+    public Container()
+    {
+
+    }
+
     public Container(List<IElement> children)
     {
         _children = children;
     }
 
+    public void Update()
+    {
+        Arrange();
+        foreach (var child in _children)
+        {
+            if (child is not IUpdatableElement updatable) continue;
+            updatable.Update();
+        }
+    }
     public abstract void Arrange();
 
     public void AddChild(IElement child)
     {
         _children.Add(child);
+        Update();
     }
 
-    private List<IElement> _children;
+    private List<IElement> _children = new();
 }

Voile/Source/UI/Containers/HorizontalContainer.cs

@@ -0,0 +1,42 @@
+using Voile.Rendering;
+
+namespace Voile.UI.Containers;
+
+public class HorizontalContainer : Container, IRenderableElement
+{
+    public float Spacing { get; set; } = 16.0f;
+    public bool Visible { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
+
+    public HorizontalContainer(List<IElement> children, float spacing = 16.0f) : base(children)
+    {
+        Spacing = spacing;
+    }
+
+    public HorizontalContainer(float spacing = 16.0f)
+    {
+        Spacing = spacing;
+    }
+
+    public override void Arrange()
+    {
+        int i = 0;
+        foreach (var child in Children)
+        {
+            var pos = Position;
+            pos.X += i * Spacing;
+
+            child.Position = pos;
+
+            i++;
+        }
+    }
+
+    public void Render(RenderSystem renderer, Style style)
+    {
+        foreach (var child in Children)
+        {
+            if (child is not IRenderableElement renderable) continue;
+            renderable.Render(renderer, style);
+        }
+    }
+}

Voile/Source/UI/Containers/VerticalContainer.cs

@@ -1,5 +1,4 @@
 using Voile.Rendering;
-using Voile.UI.Widgets;
 
 namespace Voile.UI.Containers;
 
@@ -8,7 +7,12 @@ public class VerticalContainer : Container, IRenderableElement
     public float Spacing { get; set; } = 16.0f;
     public bool Visible { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
 
-    public VerticalContainer(List<IElement> children, float spacing) : base(children)
+    public VerticalContainer(List<IElement> children, float spacing = 16.0f) : base(children)
+    {
+        Spacing = spacing;
+    }
+
+    public VerticalContainer(float spacing = 16.0f)
     {
         Spacing = spacing;
     }

Voile/Source/UI/IElement.cs

@@ -15,6 +15,11 @@ public interface IParentableElement
     public void AddChild(IElement child);
 }
 
+public interface IUpdatableElement
+{
+    void Update();
+}
+
 public interface IRenderableElement
 {
     public bool Visible { get; set; }

Voile/Source/UI/UISystem.cs

@@ -25,10 +25,8 @@ public class UISystem : IUpdatableSystem, IRenderableSystem
     {
         foreach (var element in _elements)
         {
-            if (element is Container container)
-            {
-                container.Arrange();
-            }
+            if (element is not IUpdatableElement updatable) continue;
+            updatable.Update();
         }
     }
 

Voile/Source/UI/Widgets/RectangleWidget.cs

@@ -0,0 +1,27 @@
+using System.Numerics;
+using Voile.Input;
+using Voile.Rendering;
+
+namespace Voile.UI.Widgets;
+
+public class RectangleWidget : Widget
+{
+    public override Rect MinimumRect { get; }
+    public Color Color { get; set; } = Color.White;
+    public RectangleWidget(Rect minimumRect, Color color)
+    {
+        MinimumRect = minimumRect;
+        Color = color;
+    }
+
+    public override void Input(IInputAction action)
+    {
+        throw new NotImplementedException();
+    }
+
+    public override void Render(RenderSystem renderer, Style style)
+    {
+        renderer.SetTransform(Position, Vector2.Zero);
+        renderer.DrawRectangle(new Vector2(MinimumRect.Width, MinimumRect.Height), Color);
+    }
+}

Voile/Source/Utils/MathUtils.cs

@@ -35,6 +35,15 @@ namespace Voile
             return new Vector2((float)x, (float)y);
         }
 
+        public static Color RandomColor()
+        {
+            var r = _random.NextDouble(0.0f, 1.0f);
+            var g = _random.NextDouble(0.0f, 1.0f);
+            var b = _random.NextDouble(0.0f, 1.0f);
+
+            return new Color((float)r, (float)g, (float)b);
+        }
+
         public static float EaseOutBack(float x)
         {
             var c1 = 1.70158f;