User Interface Behaviour. Inherit from this class to create a User Interface Behaviour which will have a reference to the RectTransform.

UserInterfaceBehaviour.cs

namespace UnityEngine
{
    using EventSystems;

    /// <summary>
    /// User Interface Behaviour.
    /// Inherit from this class to create a User Interface Behaviour
    /// which will have a reference to the <see cref="RectTransform"/>.
    /// </summary>
    public abstract class UserInterfaceBehaviour : UIBehaviour
    {
        /// <summary>
        /// Get the <see cref="RectTransform"/> of the current <see cref="GameObject"/>.
        /// </summary>
        public ref RectTransform rectTransform
        {
            get
            {
                if (m_rectTransform is null)
                {
                    m_rectTransform = GetComponent<RectTransform>();
                }
                return ref m_rectTransform;
            }
        }

        private RectTransform m_rectTransform;

        protected override void OnBeforeTransformParentChanged()
        {
            m_rectTransform = GetComponent<RectTransform>();
        }

        protected override void OnRectTransformDimensionsChange()
        {
            m_rectTransform = GetComponent<RectTransform>();
        }

        protected override void OnTransformParentChanged()
        {
            m_rectTransform = GetComponent<RectTransform>();
        }

        protected override void Reset()
        {
            m_rectTransform = GetComponent<RectTransform>();
        }
    }
}