Custom Label element for Unity / UI Toolkit stack

.LabelAutoFit.cs.md

this code is NOT production-ready, it's just a sketch to test out a possible solution for a problem

IMPORTANT NOTE: This elemeent doesn't work with flexGrow as it leads to undefined behaviour (recursion). Use Size/Width[%] and Size/Height attributes instead

LabelAutoFit.cs

// src* = https://gist.github.com/andrew-raphael-lukasik/8f65a4d7055e29f80376bcb4f9b500af
using UnityEngine;
using UnityEngine.UIElements;

// IMPORTANT NOTE:
// This elemeent doesn't work with flexGrow as it leads to undefined behaviour (recursion).
// Use Size/Width[%] and Size/Height attributes</b> instead

[UnityEngine.Scripting.Preserve]
public class LabelAutoFit : UnityEngine.UIElements.Label
{

	public Axis axis { get; set; }
	public float ratio { get; set; }

	[UnityEngine.Scripting.Preserve]
	public new class UxmlFactory : UxmlFactory<LabelAutoFit,UxmlTraits> {}
	
	[UnityEngine.Scripting.Preserve]
	public new class UxmlTraits : Label.UxmlTraits// VisualElement.UxmlTraits
	{
		UxmlFloatAttributeDescription _ratio = new UxmlFloatAttributeDescription{
			name = "ratio" ,
			defaultValue = 0.1f ,
			restriction = new UxmlValueBounds{ min="0.0" , max="0.9" , excludeMin=false , excludeMax=true }
		};
		UxmlEnumAttributeDescription<Axis> _axis = new UxmlEnumAttributeDescription<Axis>{
			name = "ratio-axis" ,
			defaultValue=Axis.Vertical
		};
		public override void Init ( VisualElement ve , IUxmlAttributes bag , CreationContext cc )
		{
			base.Init( ve , bag , cc );

			LabelAutoFit instance = ve as LabelAutoFit;
			instance.RegisterCallback<GeometryChangedEvent>( instance.OnGeometryChanged );

			instance.ratio = _ratio.GetValueFromBag( bag , cc );
			instance.axis = _axis.GetValueFromBag( bag , cc );
			instance.style.fontSize = 1;// triggers GeometryChangedEvent
		}
	}

	void OnGeometryChanged ( GeometryChangedEvent evt )
	{
		float oldRectSize = this.axis==Axis.Vertical ? evt.oldRect.height : evt.oldRect.width;
		float newRectLenght = this.axis==Axis.Vertical ? evt.newRect.height : evt.newRect.width;
		
		float oldFontSize = this.style.fontSize.value.value;
		float newFontSize = newRectLenght * this.ratio;
		
		float fontSizeDelta = Mathf.Abs( oldFontSize - newFontSize );
		float fontSizeDeltaNormalized = fontSizeDelta / Mathf.Max(oldFontSize,1);

		if( fontSizeDeltaNormalized>0.01f )
			this.style.fontSize = newFontSize;
	}

	public enum Axis { Horizontal , Vertical }

}