Some extensions to Unity's VisualElement class.

VisualElementExtensions.cs

/* Original code[1] Copyright (c) 2021 Shane Celis[2]
   Licensed under the MIT License[3]

   This comment generated by code-cite[4].

   [1]: https://gist.github.com/shanecelis/343e3159b6dab44b18b53f47a05fd7c7/edit
   [2]: https://twitter.com/shanecelis
   [3]: https://opensource.org/licenses/MIT
   [4]: https://github.com/shanecelis/code-cite
*/

using UnityEngine;
using UnityEngine.UIElements;

/** Some extensions to Unity's VisualElement class. */
public static class VisualElementExtensions {
  /** Add a ChangeEvent<Visibility> that sends an event when visibility changes.
      This is based off whether the rect of the element becomes zero.

   ```
   element.AddChangeEventVisibility();
   element.RegisterCallback<ChangeEvent<Visibility>>(evt => {
     if (evt.newValue)
       Debug.Log($"{evt.target} is visible");
     else
       Debug.Log($"{evt.target} is not visible");
   });
   ```
   */
  public static void AddChangeEventVisibility(this VisualElement element) {
    element.RegisterCallback<GeometryChangedEvent>(OnGeometryChanged);
  }


  private static void OnGeometryChanged(GeometryChangedEvent evt) {
    // Thanks to Midiphony-panda for his comments.
    // https://forum.unity.com/threads/visualelement-lacking-equivalent-to-monobehaviour-messages.1082570/
    if (evt.newRect == Rect.zero) {
      // "Likely", DisplayStyle was set to None
      using var rectZero = ChangeEvent<Visibility>.GetPooled(evt.oldRect != Rect.zero, false);
      rectZero.target = evt.target;
      evt.target.SendEvent(rectZero);
    } else if(evt.oldRect == Rect.zero) {
      // "Likely", DisplayStyle was set to None and is back to Flex
      using var rectZero = ChangeEvent<Visibility>.GetPooled(false, true);
      rectZero.target = evt.target;
      evt.target.SendEvent(rectZero);
    }
  }

  /** Query up the list of parents. */
  public static VisualElement QUp(this VisualElement element,
                                  string name = null,
                                  string className = null) {
    while (element != null
           && (name == null || (name != null && element.name != name))
           && (className == null || (className != null && ! element.ClassListContains(className))))
      element = element.parent;
    return element;
  }

}

public class Visibility : IChangeEvent {
  public bool isVisible;
  // XXX: I don't normally care for implicits but this class is just a 1bit wrapper.
  public static implicit operator Visibility(bool b) => new Visibility { isVisible = b };
  public static implicit operator bool(Visibility rz) => rz.isVisible;
}