Mark UIElement as dirty in SetParent.

Voile/Source/Rendering/RaylibRenderSystem.cs

@@ -81,7 +81,7 @@ namespace Voile.Rendering
                 Raylib.InitWindow((int)_windowSize.X, (int)_windowSize.Y, windowSettings.Title);
             }
 
-            // Raylib.SetWindowState(windowFlags);
+            Raylib.SetWindowState(windowFlags);
         }
 
         // TODO

Voile/Source/Rendering/RenderSystem.cs

@@ -256,7 +256,7 @@ namespace Voile.Rendering
     {
         public string Title;
         public Vector2 Size = new Vector2(1280, 720);
-        public bool Resizable { get; set; }
+        public bool Resizable { get; set; } = true;
 
         public WindowSettings(string title, Vector2 size)
         {

Voile/Source/Resources/Resource.cs

@@ -13,11 +13,35 @@ namespace Voile
         /// </summary>
         public readonly Guid Guid = Guid.Empty;
         public bool HasValue => Guid != Guid.Empty;
-        /// <summary>
-        /// Retrieve a reference.
-        /// </summary>
-        public T Value => ResourceManager.GetResource<T>(Guid);
 
+        /// <summary>
+        /// Retrieves a <see cref="Resource"/>.<br />
+        /// This will throw an <see cref="InvalidOperationException"/> if the resource wasn't loaded or is invalid. <br />
+        /// You can check if resource was loaded with <see cref="HasValue"/>, or consider using <see cref="TryGetValue"/>.
+        /// </summary> 
+        public T Value => ResourceManager.GetResource<T>(Guid)
+            ?? throw new InvalidOperationException($"Resource with GUID {Guid} is not loaded or invalid.");
+
+        /// <summary>
+        /// Retrieves a resource or <c>null</c> if the resource wasn't loaded or is invalid.
+        /// </summary>
+        public T? ValueOrNull => ResourceManager.GetResource<T>(Guid);
+
+        /// <summary>
+        /// Tries to retrieve a <see cref="Resource"/>.
+        /// </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)
+        {
+            value = ResourceManager.GetResource<T>(Guid);
+            return value != null;
+        }
+
+        /// <summary>
+        /// Create an empty <see cref="ResourceRef"/>.
+        /// </summary>
+        /// <returns></returns>
         public static ResourceRef<T> Empty()
         {
             return new ResourceRef<T>(Guid.Empty);

Voile/Source/UI/Anchor.cs

@@ -2,23 +2,76 @@ using System.Numerics;
 
 namespace Voile.UI;
 
+/// <summary>
+/// Specifies predefined anchor points used to position UI elements relative to their parent container.
+/// </summary>
 public enum Anchor
 {
+    /// <summary>
+    /// Anchors the element to the top-left corner of the parent.
+    /// </summary>
     TopLeft,
+
+    /// <summary>
+    /// Anchors the element to the top-center of the parent.
+    /// </summary>
     TopCenter,
+
+    /// <summary>
+    /// Anchors the element to the top-right corner of the parent.
+    /// </summary>
     TopRight,
+
+    /// <summary>
+    /// Anchors the element to the center-left edge of the parent.
+    /// </summary>
     CenterLeft,
+
+    /// <summary>
+    /// Anchors the element to the exact center of the parent.
+    /// </summary>
     Center,
+
+    /// <summary>
+    /// Anchors the element to the center-right edge of the parent.
+    /// </summary>
     CenterRight,
+
+    /// <summary>
+    /// Anchors the element to the bottom-left corner of the parent.
+    /// </summary>
     BottomLeft,
+
+    /// <summary>
+    /// Anchors the element to the bottom-center of the parent.
+    /// </summary>
     BottomCenter,
+
+    /// <summary>
+    /// Anchors the element to the bottom-right corner of the parent.
+    /// </summary>
     BottomRight,
     Fill
 }
 
-
+/// <summary>
+/// Provides extension methods for calculating anchored positions of UI elements.
+/// </summary>
 public static class AnchorExtensions
 {
+    /// <summary>
+    /// Calculates the offset position for an element based on the specified <see cref="Anchor"/>.
+    /// </summary>
+    /// <param name="anchor">The anchor mode to use.</param>
+    /// <param name="parentPosition">The absolute position of the parent container (top-left corner).</param>
+    /// <param name="parentRect">The bounding rectangle of the parent container.</param>
+    /// <param name="elementRect">The size of the element being anchored.</param>
+    /// <returns>
+    /// A <see cref="Vector2"/> representing the local offset position where the element should be placed inside the parent.
+    /// </returns>
+    /// <remarks>
+    /// The result is the relative offset from the parent's origin, not a global position.
+    /// </remarks>
     public static Vector2 Calculate(this Anchor anchor, Vector2 parentPosition, Rect parentRect, Rect elementRect)
     {
         var size = new Vector2(elementRect.Width, elementRect.Height);

Voile/Source/UI/Containers/Container.cs

@@ -3,7 +3,6 @@ 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>

Voile/Source/UI/Containers/Frame.cs

@@ -1,5 +1,10 @@
+using Voile.Rendering;
+
 namespace Voile.UI.Containers;
 
+/// <summary>
+/// A frame is a special container that occupies the entire available size of the parent.
+/// </summary>
 public class Frame : Container
 {
     public Frame()
@@ -9,11 +14,44 @@ public class Frame : Container
 
     public Frame(Rect minimumSize) : base(minimumSize)
     {
-        
+
     }
 
     public override void Arrange()
     {
 
     }
+
+    public override void Render(RenderSystem renderer, Style style)
+    {
+        base.Render(renderer, style);
+
+        Rect parentSize;
+
+        if (Parent != null)
+        {
+            parentSize = Parent.Size;
+        }
+        else
+        {
+            var windowSize = renderer.WindowSize;
+            var windowRect = new Rect(windowSize.X, windowSize.Y);
+
+            parentSize = windowRect;
+        }
+
+        if (_lastParentSize != parentSize)
+        {
+            Size = parentSize;
+            _lastParentSize = parentSize;
+        }
+    }
+
+    protected override void OnUpdate()
+    {
+        base.OnUpdate();
+        Size = _lastParentSize;
+    }
+
+    private Rect _lastParentSize = Rect.Zero;
 }

Voile/Source/UI/IElement.cs

@@ -1,9 +1,11 @@
 using System.Numerics;
-using Voile.Input;
 using Voile.Rendering;
 
 namespace Voile.UI;
 
+/// <summary>
+/// Represents a basic UI element with position and size information.
+/// </summary>
 public interface IElement
 {
     /// <summary>
@@ -16,6 +18,9 @@ public interface IElement
     public Rect Size { get; set; }
 }
 
+/// <summary>
+/// Represents a UI element that can contain child elements.
+/// </summary>
 public interface IParentableElement
 {
     /// <summary>
@@ -34,6 +39,10 @@ public interface IParentableElement
     public void RemoveChild(UIElement child);
 }
 
+/// <summary>
+/// Represents a UI element that can provide a minimum size constraint.<br />
+/// Implement this interface if your UI element is expected to be resizeable.
+/// </summary>
 public interface IResizeableElement
 {
     /// <summary>
@@ -42,10 +51,13 @@ public interface IResizeableElement
     public abstract Rect MinimumSize { get; }
 }
 
+/// <summary>
+/// Represents a UI element that supports updates when its state changes.
+/// </summary>
 public interface IUpdatableElement
 {
     /// <summary>
-    /// Specifies if this element's properties have changed, making it necessary to update it.
+    /// Gets a value indicating whether the element's state has changed and needs to be updated.
     /// </summary>
     public bool Dirty { get; }
     /// <summary>
@@ -58,6 +70,9 @@ public interface IUpdatableElement
     void MarkDirty();
 }
 
+/// <summary>
+/// Represents a UI element that can be rendered to the screen.
+/// </summary>
 public interface IRenderableElement
 {
     /// <summary>
@@ -77,6 +92,9 @@ public interface IRenderableElement
     public void DrawSize(RenderSystem renderer);
 }
 
+/// <summary>
+/// Represents a UI element that can receive and process user input.
+/// </summary>
 public interface IInputElement
 {
     /// <summary>
@@ -90,9 +108,23 @@ public interface IInputElement
     void Input(UIInputContext action);
 }
 
+/// <summary>
+/// Represents a UI element that supports positional anchoring within a parent.
+/// </summary>
 public interface IAnchorableElement
 {
+    /// <summary>
+    /// Gets or sets the anchor point relative to the parent container.
+    /// </summary>
     public Anchor Anchor { get; set; }
+    /// <summary>
+    /// Gets or sets an additional offset to apply after anchoring, in pixels.
+    /// </summary>
     public Vector2 AnchorOffset { get; set; }
+    /// <summary>
+    /// Applies the current anchor settings based on the parent's position and size.
+    /// </summary>
+    /// <param name="parentPosition">The parent's top-left global position.</param>
+    /// <param name="parentRect">The bounding rectangle of the parent container.</param>
     public void ApplyAnchor(Vector2 parentPosition, Rect parentRect);
 }

Voile/Source/UI/UIElement.cs

@@ -3,6 +3,9 @@ using Voile.Rendering;
 
 namespace Voile.UI;
 
+/// <summary>
+/// Base class for all UI elements. 
+/// </summary>
 public abstract class UIElement : IElement, IRenderableElement, IResizeableElement, IUpdatableElement, IAnchorableElement
 {
     public bool Visible { get; set; } = true;
@@ -10,6 +13,11 @@ public abstract class UIElement : IElement, IRenderableElement, IResizeableEleme
     public Vector2 LocalPosition { get; set; } = Vector2.Zero;
     public Vector2 GlobalPosition => _parent?.GlobalPosition + LocalPosition ?? LocalPosition;
 
+    /// <summary>
+    /// Parent <see cref="UIElement"/> of this element.
+    /// </summary>
+    public UIElement? Parent => _parent;
+
     public Rect Size
     {
         get => _size;
@@ -38,9 +46,14 @@ public abstract class UIElement : IElement, IRenderableElement, IResizeableEleme
 
     public virtual void MarkDirty() => _dirty = true;
 
+    /// <summary>
+    /// Sets a parent element for this <see cref="UIElement"/>.
+    /// </summary>
+    /// <param name="parent">Element to parent this <see cref="UIElement"/> to.</param>
     public void SetParent(UIElement parent)
     {
         _parent = parent;
+        MarkDirty();
     }
 
     public void Update()