首先看一个例子，新建一个Panel，在下面添加两个Button，分别命名为Button、Button2。
1、给Panel添加一个VerticalLayoutGroup组件，ChildForceExpand属性中勾上Width。
2、给Button、Button2添加LayoutElement组件，其中Button的FlexibleHeight设置为0.3，Button2的FlexibleHeight设置为0.1
3、将Panel的高度设置为100
这时我们发现，Button的高度是70，Button2的高度是30。奇怪，这个高度是怎么算出来的呢？
网上搜索一番，竟然很少有人讨论uGUI的AutoLayout，尤其是flexibleWidth/Height属性的意义，官方文档也语焉不详。这时只能放大招了，uGUI已经开源，索性把代码拉下来看看到底怎么实现的。下面是托管代码的地址：
https://bitbucket.org/Unity-Technologies/ui
uGUI的AutoLayout有三个核心接口，定义在ILayoutElement.cs文件中：
ILayoutElement
ILayoutController
ILayoutIgnorer
结构很清晰，由ILayoutElement提供布局信息，ILayoutController来控制布局，ILayoutIgnore提供给UI忽略AutoLayout的能力。
例子中使用的VerticalLayoutGroup继承自HorizontalOrVerticalLayoutGroup，这个类实现了布局的核心逻辑，代码量不多，我就直接贴上来了
protected void CalcAlongAxis(int axis, bool isVertical)
{
float combinedPadding = (axis == 0 ? padding.horizontal : padding.vertical);
float totalMin = combinedPadding;
float totalPreferred = combinedPadding;
float totalFlexible = 0;
bool alongOtherAxis = (isVertical ^ (axis == 1));
for (int i = 0; i < rectChildren.Count; i++)
{
RectTransform child = rectChildren[i];
float min = LayoutUtility.GetMinSize(child, axis);
float preferred = LayoutUtility.GetPreferredSize(child, axis);
float flexible = LayoutUtility.GetFlexibleSize(child, axis);
if ((axis == 0 ? childForceExpandWidth : childForceExpandHeight))
flexible = Mathf.Max(flexible, 1);
if (alongOtherAxis)
{
totalMin = Mathf.Max(min + combinedPadding, totalMin);
totalPreferred = Mathf.Max(preferred + combinedPadding, totalPreferred);
totalFlexible = Mathf.Max(flexible, totalFlexible);
}
else
{
totalMin += min + spacing;
totalPreferred += preferred + spacing;
// Increment flexible size with element's flexible size.
totalFlexible += flexible;
}
}
if (!alongOtherAxis && rectChildren.Count > 0)
{
totalMin -= spacing;
totalPreferred -= spacing;
}
totalPreferred = Mathf.Max(totalMin, totalPreferred);
SetLayoutInputForAxis(totalMin, totalPreferred, totalFlexible, axis);
}
protected void SetChildrenAlongAxis(int axis, bool isVertical)
{
float size = rectTransform.rect.size[axis];
bool alongOtherAxis = (isVertical ^ (axis == 1));
if (alongOtherAxis)
{
float innerSize = size - (axis == 0 ? padding.horizontal : padding.vertical);
for (int i = 0; i < rectChildren.Count; i++)
{
RectTransform child = rectChildren[i];
float min = LayoutUtility.GetMinSize(child, axis);
float preferred = LayoutUtility.GetPreferredSize(child, axis);
float flexible = LayoutUtility.GetFlexibleSize(child, axis);
if ((axis == 0 ? childForceExpandWidth : childForceExpandHeight))
flexible = Mathf.Max(flexible, 1);
float requiredSpace = Mathf.Clamp(innerSize, min, flexible > 0 ? size : preferred);
float startOffset = GetStartOffset(axis, requiredSpace);
SetChildAlongAxis(child, axis, startOffset, requiredSpace);
}
}
else
{
float pos = (axis == 0 ? padding.left : padding.top);
if (GetTotalFlexibleSize(axis) == 0 && GetTotalPreferredSize(axis) < size)
pos = GetStartOffset(axis, GetTotalPreferredSize(axis) - (axis == 0 ? padding.horizontal : padding.vertical));
float minMaxLerp = 0;
if (GetTotalMinSize(axis) != GetTotalPreferredSize(axis))
minMaxLerp = Mathf.Clamp01((size - GetTotalMinSize(axis)) / (GetTotalPreferredSize(axis) - GetTotalMinSize(axis)));
float itemFlexibleMultiplier = 0;
if (size > GetTotalPreferredSize(axis))
{
if (GetTotalFlexibleSize(axis) > 0)
itemFlexibleMultiplier = (size - GetTotalPreferredSize(axis)) / GetTotalFlexibleSize(axis);
}
for (int i = 0; i < rectChildren.Count; i++)
{
RectTransform child = rectChildren[i];
float min = LayoutUtility.GetMinSize(child, axis);
float preferred = LayoutUtility.GetPreferredSize(child, axis);
float flexible = LayoutUtility.GetFlexibleSize(child, axis);
if ((axis == 0 ? childForceExpandWidth : childForceExpandHeight))
flexible = Mathf.Max(flexible, 1);
float childSize = Mathf.Lerp(min, preferred, minMaxLerp);
childSize += flexible * itemFlexibleMultiplier;
SetChildAlongAxis(child, axis, pos, childSize);
pos += childSize + spacing;
}
}
}
其中SetChildrenAlongAxis方法清晰地阐释了minHeight,preferredHeight,flexibleHeight的涵义。
为了帮助理解，我们先定义几个概念。我们把当前UI所有同级并参与自动布局的组件的preferredHeight总和称为totalPreferredHeight，minHeight的总和称为totalMinHeight，父UI的真实高度称为realHeight。总结如下：
1、minHeight
在自动布局中，此UI最小高度不会小于minHeight。这个参数定义了realHeight < totalMinHeight时，当前子UI的height为minHeight。
2、preferredHeight
可以理解为，UI自身希望的高度。
当totalMinHeight < realHeight < totalPreferredHeight时，realHeight处于totalMinHeight和totalPreferredHeight之间一定百分比，把这个比例应用到每一个接受自动布局的子UI上，即是我们最终得到的效果
3、flexibleHeight
当realHeight > totalPreferredHeight时，父UI会剩下一部分高度。flexibleHeight就是告诉AutoLayout系统，应该怎么瓜分剩下的高度，使子UI填充满父UI。flexibleHeight默认是-1，不会进行扩充。当flexibleHeight > 0时，flexibleHeight值作为权重来计算当前子UI最终的高度，公式如下：
height = preferredHeight + (flexibleHeight / totalFlexibleHeight) * (realHeight - totalPreferredHeight)
弄清楚这些概念后，我们再看一下文章开头的例子。
button1的flexibleHeight=0.3，button2的flexibleHeight=0.1，minHeight和preferredHeight都没有设置，按道理高度应该分别是75、25。为什么会出现70、30？
查一下ILayoutElement的实现类
发现Image和Text实现了ILayoutElement，而我们的按钮中默认是有一个Image组件的，用脚本获取这个Image然后打印它的preferredHeight，发现等于10
再套用flexibleHeight的计算公式：
Button1:10 + 0.3/0.4 * (100 - 20) = 70
Button2:10 + 0.1/0.4 * (100 - 20) = 30
这里有个问题，一个GameObject上挂载两个ILayoutElement组件，是怎么决定用哪个的？这个可以在LayoutUtility.cs中找到答案：
LayoutUtility.cs
public static float GetLayoutProperty(RectTransform rect, System.Func property, float defaultValue, out ILayoutElement source)
{
source = null;
if (rect == null)
return 0;
float min = defaultValue;
int maxPriority = System.Int32.MinValue;
var components = ComponentListPool.Get();
rect.GetComponents(typeof(ILayoutElement), components);
for (int i = 0; i < components.Count; i++)
{
var layoutComp = components[i] as ILayoutElement;
if (layoutComp is Behaviour && (!(layoutComp as Behaviour).enabled || !(layoutComp as Behaviour).isActiveAndEnabled))
continue;
int priority = layoutComp.layoutPriority;
// If this layout components has lower priority than a previously used, ignore it.
if (priority < maxPriority)
continue;
float prop = property(layoutComp);
// If this layout property is set to a negative value, it means it should be ignored.
if (prop < 0)
continue;
// If this layout component has higher priority than all previous ones,
// overwrite with this one's value.
if (priority > maxPriority)
{
min = prop;
maxPriority = priority;
source = layoutComp;
}
// If the layout component has the same priority as a previously used,
// use the largest of the values with the same priority.
else if (prop > min)
{
min = prop;
source = layoutComp;
}
}
ComponentListPool.Release(components);
return min;
}
原来LayoutElement有一个layoutPriority属性用来决定优先级，这个属性暂时还没有在编辑器中暴露，也许后续版本会加强这方面的能力。
AutoLayout系统会选用优先级最高的ILayoutElement里相应属性返回。Image和Text的优先级默认是0，LayoutElement默认优先级是1。所以正常情况会使用LayoutElement中的设置，但我们的例子中，LayoutElement没有设置preferredHeight，LayoutElement里布局相关的初始值都是-1，所以还是使用了Image的preferredHeight:10。
【结语】
其实，只要官方文档描述详细一些，根本没必要浪费时间去查这个来龙去脉。这几天在学习swift，苹果人性化的Programming Guide加上iBooks的配合，使得学习这门语言真是件轻松愉快的事情。相比之下，Unity简直是在虐待开发者。Unity、Unreal、Cryengine等最近也为争市场弄得头破血流，除了降价开源提供新特性之外，完善文档也是不容忽视的工作。