Adding persistent events in Unity

AddKeyComponents.cs

#if UNITY_EDITOR

using UnityEngine;
using UnityEngine.UI;
using UnityEngine.Events;
using UnityEditor;
using UnityEditor.Events;

public class AddKeyComponents : MonoBehaviour {
	public GameObject virtualKeyboard;

	public void OnExecute() {
		Debug.Log("Adding key components...");

		if (virtualKeyboard == null) {
			Debug.LogError("No VirtualKeyboard game object assigned!");
			return;
		}

		foreach (Transform child in virtualKeyboard.transform) {
			var key = child.gameObject;

			var oldTextKey = key.GetComponent<VirtualKeyboardTextKey>();
			if (oldTextKey != null)
				DestroyImmediate(oldTextKey);

			var textKey = key.AddComponent<VirtualKeyboardTextKey>();
			textKey.lowerText = child.name.ToLower();
			textKey.upperText = child.name.ToUpper();

			var button = key.GetComponent<Button>() ?? key.AddComponent<Button>();
			var action = new UnityAction(textKey.OnClick);

			// Remove all listeners.
			button.onClick = new Button.ButtonClickedEvent();

			UnityEventTools.AddVoidPersistentListener(
				button.onClick,
				action
			);

			Debug.Log("Added key component to " + key.name);
		}

		Debug.Log("Finished!");
	}
}

[CanEditMultipleObjects]
[CustomEditor(typeof(AddKeyComponents))]
public class AddKeyComponentsEditor : Editor {
	public override void OnInspectorGUI() {
		base.OnInspectorGUI();

		if (GUILayout.Button("Execute"))
			foreach (Object obj in targets)
				((AddKeyComponents)obj).OnExecute();
	}
}

#endif