Permite agregarle a un botón la posibilidad de llamar una función mientras se mantenga presionado.

UI_HoldButton.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.EventSystems;
using UnityEngine.Events;

public class UI_HoldButton : MonoBehaviour,  IPointerDownHandler, IPointerUpHandler
{

    public UnityEvent OnHold;
    bool IsHolding;

    public void OnPointerDown( PointerEventData eventData )
    {
        IsHolding = true;
    }

    public void OnPointerUp( PointerEventData eventData )
    {
        IsHolding = false;
    }
	

	void Update ()
    {
        if (IsHolding)
        {
            OnHold.Invoke();
        }
	}
}