「Unity3D」UGUI运行时设置元素的锚点Anchor，维持元素Rect的显示不变，即待在原处

在编辑器中，通过设置Raw edit mode，可以切换两种，元素锚点的改变模式：

• 一种是锚点单独改变，即：不开启原始模式，保持原样，改变anchoredPosition与sizeDelta。
• 一种是锚点联动显示，即：开启原始模式，不保持原样，不改变anchoredPosition与sizeDelta。

原理很简单，anchoredPosition与sizeDelta都是相对于锚点Anchor的，所以Anchor变动，元素rect保持原样，就需要改变pos和delta，而元素rect跟着变动，就可以维持pos和delta不变。

那么，在运行时用代码设置anchorMin与anchorMax，只有联动显示的模式，即相当于开启原始模式——不改变anchoredPosition与sizeDelta，改变元素rect的显示。

但有时候，我们需要只改变锚点，而保持rect显示不变——这是利用锚点适配不同分辨率后的结果——这样其父类的改变，就可以不影响子类的缩放，如：将子类锚点设置为中心点。

解决方案，就是用offsetMin与offsetMax，来反向抵消anchorMin与anchorMax的变化，从而维持元素rect的显示不变。

代码实现如下：

/// <summary>
/// Set the anchorMin [v2] without changing the [rectTransform] display.
/// Assume [rectTransform] has a parent, because the root is rarely operated on.
/// </summary>
public static void SetAnchorMinOnly(this RectTransform rectTransform, in Vector2 v2)
{var offsetOriginal = rectTransform.anchorMin - v2;if (offsetOriginal == Vector2.zero){Debug.LogWarning($"[SetAnchorMinOnly]: The [v2 = {v2}] is the same as the [anchorMin].");return;}else if (offsetOriginal.x == 0.0f){Debug.LogWarning($"[SetAnchorMinOnly]: The [v2.x = {v2.x}] is the same as the [anchorMin.x].");rectTransform.SetAnchorMinYOnly(v2.y);return;}else if (offsetOriginal.y == 0.0f){Debug.LogWarning($"[SetAnchorMinOnly]: The [v2.y = {v2.y}] is the same as the [anchorMin.y].");rectTransform.SetAnchorMinXOnly(v2.x);return;}rectTransform.anchorMin = v2;var parentSize          = (rectTransform.parent as RectTransform).rect.size;rectTransform.offsetMin = parentSize * offsetOriginal;
}  /// <summary>
/// Set the anchorMax [v2] without changing the [rectTransform] display.
/// Assume [rectTransform] has a parent, because the root is rarely operated on.
/// </summary>
public static void SetAnchorMaxOnly(this RectTransform rectTransform, in Vector2 v2)
{var offsetOriginal = rectTransform.anchorMax - v2;if (offsetOriginal == Vector2.zero){Debug.LogWarning($"[SetAnchorMaxOnly]: The [v2 = {v2}] is the same as the [anchorMax].");return;}else if (offsetOriginal.x == 0.0f){Debug.LogWarning($"[SetAnchorMaxOnly]: The [v2.x = {v2.x}] is the same as the [anchorMax.x].");rectTransform.SetAnchorMaxYOnly(v2.y);return;}else if (offsetOriginal.y == 0.0f){Debug.LogWarning($"[SetAnchorMaxOnly]: The [v2.y = {v2.y}] is the same as the [anchorMax.y].");rectTransform.SetAnchorMaxXOnly(v2.x);return;}rectTransform.anchorMax = v2;var parentSize          = (rectTransform.parent as RectTransform).rect.size;rectTransform.offsetMax = parentSize * offsetOriginal;
}