WIP: dirty flag for UI.

Voile/Source/UI/Containers/Container.cs

@@ -8,41 +8,65 @@ namespace Voile.UI.Containers;
 /// </summary>
 public abstract class Container : IElement, IParentableElement, IUpdatableElement, IResizeableElement, IRenderableElement
 {
+    /// <inheritdoc />
     public IReadOnlyList<IElement> Children => _children;
+    /// <inheritdoc />
     public Vector2 Position { get; set; }
+    /// <inheritdoc />
     public Rect MinimumRect { get; set; } = Rect.Zero;
+    /// <inheritdoc />
     public Rect Size { get; set; } = Rect.Zero;
+    /// <inheritdoc />
     public bool Visible { get; set; } = true;
+    /// <inheritdoc />
+    public bool Dirty => _isDirty;
 
     public Container()
     {
-
+        MarkDirty();
     }
 
     public Container(Rect minimumSize)
     {
         MinimumRect = minimumSize;
+
+        MarkDirty();
     }
 
     public Container(Rect minimumSize, List<IElement> children)
     {
         MinimumRect = minimumSize;
         _children = children;
+
+        MarkDirty();
     }
 
     public void Update()
     {
-        Arrange();
-        CalculateMinimumSize();
+        if (!_isDirty) return;
+        _isDirty = false;
+
 
         foreach (var child in _children)
         {
             if (child is not IUpdatableElement updatable) continue;
             updatable.Update();
         }
+
+        Arrange();
+        CalculateMinimumSize();
     }
+
+    public void MarkDirty() => _isDirty = true;
+
+    /// <summary>
+    /// Called when this <see cref="Container"/> has to rearrange its children.
+    /// </summary>
     public abstract void Arrange();
 
+    /// <summary>
+    /// Recalculates a minimum size for this <see cref="Container"/>.
+    /// </summary>
     public void CalculateMinimumSize()
     {
         if (Children.Count == 0)
@@ -81,12 +105,16 @@ public abstract class Container : IElement, IParentableElement, IUpdatableElemen
     public void AddChild(IElement child)
     {
         _children.Add(child);
+
+        MarkDirty();
         Update();
     }
 
     public void RemoveChild(IElement child)
     {
         _children.Remove(child);
+
+        MarkDirty();
         Update();
     }
 
@@ -106,4 +134,5 @@ public abstract class Container : IElement, IParentableElement, IUpdatableElemen
     }
 
     private List<IElement> _children = new();
+    private bool _isDirty;
 }