ButtonClickOnHold.cs

using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystem;

[RequireComponent(typeof(Button))]
public class ButtonClickOnHold : MonoBehaviour, IPointerDownHandler, IPointerUpHandler
{
	public float HoldDurationThreshold = 0.5f;
	
	
	private Button m_button;
	private float m_lastButtonClickTime;
	private bool m_isDown;
	
	private void Awake()
	{
		m_button = GetComponent<Button>();
	}
	
	private void OnEnable()
	{
		m_lastButtonClickTime = 0f;
	}
	
	private void Update()
	{
		if(!m_isDown || m_lastButtonClickTime < 0f)
			return;
			
		if(Time.time - m_lastButtonClickTime > HoldDurationThreshold)
		{
			// This invokes whatever is registered to the button's OnClick handler. It won't however cause the button to release its visible
			// press state. If you want to control that too you'll need to derive from Button, make a similar set of logic counting and force
			// its state up/down.
			
			// Also er... the button will fire OnClick a second time when the mouse is actually released, unless you set m_button.interactable = false.
			// this has the side effect of making the button suddenly grey out. An alternative solution would be to make your own UnityEvent in this class,
			// and listen for that event instead of the buttons... or go with deriving from the Button class again.
			m_button.onClick.Invoke();
			
			// Invalidate our last click time so that it doesn't try to invoke the button click every frame after they've passed the threshold.
			m_lastButtonClickTime = -1f;
		}
	}
	
	private void OnPointerDown (PointerEventData eventData)
	{
		m_isDown = true;
		m_lastButtonClickTime = 0f;
	}
	
	private void OnPointerUp(PointerEventData eventData)
	{
		m_isDown = false;
	}
}