Unity字代的Slider，有些功能不全，这里简单做了一个扩展，继承Slider，加入了开始拖拽、结束拖拽、点击抬起、点击落下事件。

替换Slider组件，使用下面的代码即可。 

using UnityEngine.UI;
using UnityEngine.EventSystems;
using System;


public class SliderExtension : Slider, IBeginDragHandler, IEndDragHandler
{
    public Action BeiginDrag { get; set; }
    public Action EndDrag { get; set; }

    public Action<PointerEventData> Drag { get; set; }
    
    public Action<PointerEventData> OnSliderPointerDown{ get; set; }
    public Action<PointerEventData> OnSliderPointerUp{ get; set; }

    public void OnBeginDrag(PointerEventData eventData)
    {
        BeiginDrag?.Invoke();
    }

    public void OnEndDrag(PointerEventData eventData)
    {
        EndDrag?.Invoke();
    }

    public override void OnDrag(PointerEventData eventData)
    {
        base.OnDrag(eventData);
        Drag?.Invoke(eventData);
    }

    public override void OnPointerDown(PointerEventData eventData)
    {
        base.OnPointerDown(eventData);
        OnSliderPointerDown?.Invoke(eventData);
    }
    
    public override void OnPointerUp(PointerEventData eventData)
    {
        base.OnPointerUp(eventData);
        OnSliderPointerUp?.Invoke(eventData);
    }
    
}