当Canvas需要重绘UI时，Unity内部会调用Canvas.SendWillRenderCanvas，进行网格重建，但是我们不知道是由于哪个UI元素引起了哪个Canvas的网格重建。
通过查看UGUI源码，我们发现所有待重建的UI元素都被保存在CanvasUpdateRegistry.cs类中。

private readonly IndexedSet<ICanvasElement> m_LayoutRebuildQueue = new IndexedSet<ICanvasElement>();
private readonly IndexedSet<ICanvasElement> m_GraphicRebuildQueue = new IndexedSet<ICanvasElement>();

因此，我们可以通过反射的方式，获取这俩个列表中的数据。代码如下：

using System.Collections.Generic;
using System.Reflection;
using UnityEngine;
using UnityEngine.UI;
using Object = UnityEngine.Object;

public class UGUIUtil : MonoBehaviour
{
    private IList<ICanvasElement> m_LayoutRebuildQueue;
    private IList<ICanvasElement> m_GraphicRebuildQueue;

    private void Awake()
    {
        System.Type type = typeof(CanvasUpdateRegistry);
        FieldInfo field = type.GetField("m_LayoutRebuildQueue", BindingFlags.NonPublic | BindingFlags.Instance);
        m_LayoutRebuildQueue = (IList<ICanvasElement>) field.GetValue(CanvasUpdateRegistry.instance);
        field = type.GetField("m_GraphicRebuildQueue", BindingFlags.NonPublic | BindingFlags.Instance);
        m_GraphicRebuildQueue = (IList<ICanvasElement>) field.GetValue(CanvasUpdateRegistry.instance);
    }

    private void Update()
    {
        for (int j = 0; j < m_LayoutRebuildQueue.Count; j++)
        {
            var rebuild = m_LayoutRebuildQueue[j];
            if (ObjectValidFofUpdate(rebuild))
            {
                Debug.LogFormat("m_LayoutRebuildQueue中{0}引起{1}网格重建", rebuild.transform.name, 
rebuild.transform.GetComponent<Graphic>().canvas.name);
            }
        }

        for (int j = 0; j < m_GraphicRebuildQueue.Count; j++)
        {
            var element = m_GraphicRebuildQueue[j];
            if (ObjectValidFofUpdate(element))
            {
                Debug.LogFormat("m_GraphicRebuildQueue中{0}引起{1}网格重建",element.transform.name,
element.transform.GetComponent<Graphic>().canvas.name);
            }
        }
    }

    private bool ObjectValidFofUpdate(ICanvasElement element)
    {
        var valid = element != null;
        var isUnityObject = element is Object;
        if (isUnityObject)
            valid = (element as Object) != null;
        return valid;
    }
}