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最近在项目开发过程中，出现了一个问题，同样的配置，在背包中的DropDown点击展开后，点击非DropDown区域外可以自动关闭，但是好友的确关闭不了，查看UGUI的DropDown源码后发现UGUI的大坑，看源码：

GameObject CreateBlocker(Canvas rootCanvas)
{
      // Create blocker GameObject.
      GameObject blocker = new GameObject("Blocker");

      // Setup blocker RectTransform to cover entire root canvas area.
      RectTransform blockerRect = blocker.AddComponent<RectTransform>();
      blockerRect.SetParent(rootCanvas.transform, false);
      blockerRect.anchorMin = Vector3.zero;
      blockerRect.anchorMax = Vector3.one;
      blockerRect.sizeDelta = Vector2.zero;

      // Make blocker be in separate canvas in same layer as dropdown and in layer just below it.
      Canvas blockerCanvas = blocker.AddComponent<Canvas>();
      blockerCanvas.overrideSorting = true;
      Canvas dropdownCanvas = m_Dropdown.GetComponent<Canvas>();
      blockerCanvas.sortingLayerID = dropdownCanvas.sortingLayerID;
      blockerCanvas.sortingOrder = dropdownCanvas.sortingOrder - 1;

      // Add raycaster since it's needed to block.
      blocker.AddComponent<GraphicRaycaster>();

      // Add image since it's needed to block, but make it clear.
      Image blockerImage = blocker.AddComponent<Image>();
      blockerImage.color = Color.clear;

      // Add button since it's needed to block, and to close the dropdown when blocking area is clicked.
      Button blockerButton = blocker.AddComponent<Button>();
      blockerButton.onClick.AddListener(Hide);

      return blocker;
}

看以看出点击区域外关闭主要是通过创建了一个大的透明按钮，点击这个按钮就会调用自己的Hide函数关闭，并且自适应是根据Canvas自适应的，那么这个Canvas是怎么来的呢，再看代码：

list = ListPool<Canvas>.Get();
gameObject.GetComponentsInParent(false, list);
if (list.Count == 0)
    return;
Canvas rootCanvas = list[0];
ListPool<Canvas>.Release(list);

坑的地方来了，它是取父的Canvas中最近的那个Canvas来做参照的，在我们的游戏项目总中，这个Canvas的大小是0,0，所以你怎么点都点不到

我们的解决方案是重新设置这个blocker的父到响应的面板下，这个这个bloker的大小会自动适应全屏，当然解决方案有很多，找一个最适合自己项目