class XXX : MonoBehaviour, IPointerClickHandler
{
    bool block = false;

    void IPointerClickHandler.OnPointerClick(PointerEventData eventData)
    {
        if (!block)
        {
            block = true;
            StartCoroutine(c_Hide());
        }
    }

    IEnumerator c_Hide()
    {
        var g = GetComponent<Graphic>();
        var endColor = g.color;
        endColor.a = 0.0f;

        while (true)
        {
            g.color = Color.Lerp(g.color, endColor, Time.deltaTime);

            if (g.color.a <= 0.001f)
            {
                g.color = endColor;
                break;
            }

            yield return null;
        }
    }
}