Unity 重写GridLayoutGroup使居中对齐
使用GridLayoutGroup的时候，以StartAxis设置为Horizontal为例，当我们设置ChildAlignment为***Center时，我们会发现，如果显示内容只有一行，那么元素排列是居中的，但是如果显示内容不止一行，那么最后一行的元素排列是靠左或者靠右（由StartCornner决定）。这就导致假设有4个子物体，固定为3列，第2列只有一个子物体，但是不是居中的，是靠左/右的，不符合美术的要求。
要解决这个问题有一种方案是把GridLayout分割成HorizontalLayout和VerticalLayout，然后分行/列显示，这样保证每一行/列都居中对齐，但是有点麻烦的是需要对数据进行分割，相对来讲比较麻烦。
另一种方案就是重写GridLayoutGroup，实现居中的需求。通过阅读源码+ 断点数据，发现逻辑还是比较好理解的。以StartAxis设置为Horizontal为例 ，原逻辑是通过CellSize与Spacing计算一行所占的长度与Layout对象本身的size做对比，然后再根据设置的布局方式计算出坐标偏移值，以此来确定每个子物体的坐标，出现上述的问题是因为，计算行所占的长度时，只计算了一次最大长度，这是不合理的，因为当最后一行没有填充满的时候，所占的长度与上面的行是不一样的，以此为基础修改源码。代码如下，修改内容从116行开始：
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
namespace UnityEngine.UI
{
public class GridLayoutGroupCenter : GridLayoutGroup
{
/// <summary>
/// Called by the layout system
/// Also see ILayoutElement
/// </summary>
public override void SetLayoutHorizontal()
{
SetCellsAlongAxis(0);
}
/// <summary>
/// Called by the layout system
/// Also see ILayoutElement
/// </summary>
public override void SetLayoutVertical()
{
SetCellsAlongAxis(1);
}
private void SetCellsAlongAxis(int axis)
{
// Normally a Layout Controller should only set horizontal values when invoked for the horizontal axis
// and only vertical values when invoked for the vertical axis.
// However, in this case we set both the horizontal and vertical position when invoked for the vertical axis.
// Since we only set the horizontal position and not the size, it shouldn't affect children's layout,
// and thus shouldn't break the rule that all horizontal layout must be calculated before all vertical layout.
var rectChildrenCount = rectChildren.Count;
if (axis == 0)
{
// Only set the sizes when invoked for horizontal axis, not the positions.
for (int i = 0; i < rectChildrenCount; i++)
{
RectTransform rect = rectChildren[i];
m_Tracker.Add(this, rect,
DrivenTransformProperties.Anchors |
DrivenTransformProperties.AnchoredPosition |
DrivenTransformProperties.SizeDelta);
rect.anchorMin = Vector2.up;
rect.anchorMax = Vector2.up;
rect.sizeDelta = cellSize;
}
return;
}
float width = rectTransform.rect.size.x;
float height = rectTransform.rect.size.y;
int cellCountX = 1;
int cellCountY = 1;
if (m_Constraint == Constraint.FixedColumnCount)
{
cellCountX = m_ConstraintCount;
if (rectChildrenCount > cellCountX)
cellCountY = rectChildrenCount / cellCountX + (rectChildrenCount % cellCountX > 0 ? 1 : 0);
}
else if (m_Constraint == Constraint.FixedRowCount)
{
cellCountY = m_ConstraintCount;
if (rectChildrenCount > cellCountY)
cellCountX = rectChildrenCount / cellCountY + (rectChildrenCount % cellCountY > 0 ? 1 : 0);
}
else
{
if (cellSize.x + spacing.x <= 0)
cellCountX = int.MaxValue;
else
cellCountX = Mathf.Max(1, Mathf.FloorToInt((width - padding.horizontal + spacing.x + 0.001f) / (cellSize.x + spacing.x)));
if (cellSize.y + spacing.y <= 0)
cellCountY = int.MaxValue;
else
cellCountY = Mathf.Max(1, Mathf.FloorToInt((height - padding.vertical + spacing.y + 0.001f) / (cellSize.y + spacing.y)));
}
//startCorner = 0
int cornerX = (int)startCorner % 2;//0
int cornerY = (int)startCorner / 2;//0
int cellsPerMainAxis, actualCellCountX, actualCellCountY;
if (startAxis == Axis.Horizontal)
{
cellsPerMainAxis = cellCountX;
actualCellCountX = Mathf.Clamp(cellCountX, 1, rectChildrenCount);
actualCellCountY = Mathf.Clamp(cellCountY, 1, Mathf.CeilToInt(rectChildrenCount / (float)cellsPerMainAxis));
}
else
{
cellsPerMainAxis = cellCountY;
actualCellCountY = Mathf.Clamp(cellCountY, 1, rectChildrenCount);
actualCellCountX = Mathf.Clamp(cellCountX, 1, Mathf.CeilToInt(rectChildrenCount / (float)cellsPerMainAxis));
}
//总长度 & 总高度
Vector2 requiredSpace = new Vector2(
actualCellCountX * cellSize.x + (actualCellCountX - 1) * spacing.x,
actualCellCountY * cellSize.y + (actualCellCountY - 1) * spacing.y
);
Vector2 startOffset = new Vector2(
GetStartOffset(0, requiredSpace.x),
GetStartOffset(1, requiredSpace.y)
);
//最后一行/列的偏移参数
Vector2 requiredSpaceNew = Vector2.zero;
Vector2 startOffsetNew = Vector2.zero;
//水平顺序布局，计算是否为整行
if (startAxis == Axis.Horizontal)
{
//少于一行,按照原数据计算
if (rectChildrenCount <= actualCellCountX)
{
requiredSpaceNew = requiredSpace;
startOffsetNew = startOffset;
}
else
{
//整行
var rem = rectChildrenCount % actualCellCountX;
if (rem == 0)
{
requiredSpaceNew = requiredSpace;
startOffsetNew = startOffset;
}
else
{
requiredSpaceNew = new Vector2(
rem * cellSize.x + (rem - 1) * spacing.x,
actualCellCountY * cellSize.y + (actualCellCountY - 1) * spacing.y);
startOffsetNew = new Vector2(
GetStartOffset(0, requiredSpaceNew.x),
GetStartOffset(1, requiredSpaceNew.y)
);
}
}
}
else
{
//少于一列,按照原数据计算
if (rectChildrenCount <= actualCellCountY)
{
requiredSpaceNew = requiredSpace;
startOffsetNew = startOffset;
}
else
{
//整列
var rem = rectChildrenCount % actualCellCountY;
if (rem == 0)
{
requiredSpaceNew = requiredSpace;
startOffsetNew = startOffset;
}
else
{
requiredSpaceNew = new Vector2(
actualCellCountX * cellSize.x + (actualCellCountX - 1) * spacing.x,
rem * cellSize.y + (rem - 1) * spacing.y);
startOffsetNew = new Vector2(
GetStartOffset(0, requiredSpaceNew.x),
GetStartOffset(1, requiredSpaceNew.y)
);
}
}
}
//最后一行/列的起始索引
int lastRowIndex = rectChildrenCount - rectChildrenCount % actualCellCountX;
int lastColIndex = rectChildrenCount - rectChildrenCount % actualCellCountY;
for (int i = 0; i < rectChildrenCount; i++)
{
int positionX;
int positionY;
//最后一行/列
bool lastRowOrCol = false;
if (startAxis == Axis.Horizontal)
{
positionX = i % cellsPerMainAxis;
positionY = i / cellsPerMainAxis;
lastRowOrCol = i >= lastRowIndex;
}
else
{
positionX = i / cellsPerMainAxis;
positionY = i % cellsPerMainAxis;
lastRowOrCol = i >= lastColIndex;
}
if (cornerX == 1)
positionX = actualCellCountX - 1 - positionX;
if (cornerY == 1)
positionY = actualCellCountY - 1 - positionY;
if (lastRowOrCol)
{
SetChildAlongAxis(rectChildren[i], 0, startOffsetNew.x + (cellSize[0] + spacing[0]) * positionX, cellSize[0]);
SetChildAlongAxis(rectChildren[i], 1, startOffsetNew.y + (cellSize[1] + spacing[1]) * positionY, cellSize[1]);
}
else
{
SetChildAlongAxis(rectChildren[i], 0, startOffset.x + (cellSize[0] + spacing[0]) * positionX, cellSize[0]);
SetChildAlongAxis(rectChildren[i], 1, startOffset.y + (cellSize[1] + spacing[1]) * positionY, cellSize[1]);
}
}
}
}
}
实际要添加的逻辑很少，代码中加了比较多的ifelse是为了减少运算量，因为部分情况下，是不需要计算新的offset的。