Unity UGUI ContentSizeFitter源码浅析

using UnityEngine.EventSystems;

namespace UnityEngine.UI
{
    [AddComponentMenu("Layout/Content Size Fitter", 141)]
    [ExecuteAlways]
    [RequireComponent(typeof(RectTransform))]
    /// 

    /// Resizes a RectTransform to fit the size of its content.
    /// 
    /// 
    /// The ContentSizeFitter can be used on GameObjects that have one or more ILayoutElement components, such as Text, Image, HorizontalLayoutGroup, VerticalLayoutGroup, and GridLayoutGroup.
    /// 
    public class ContentSizeFitter : UIBehaviour, ILayoutSelfController
    {
        /// 

        /// The size fit modes avaliable to use.
        /// 
        public enum FitMode
        {
            /// 

            /// Don't perform any resizing.
            /// 
            Unconstrained,
            /// 

            /// Resize to the minimum size of the content.
            /// 
            MinSize,
            /// 

            /// Resize to the preferred size of the content.
            /// 
            PreferredSize
        }

        [SerializeField] protected FitMode m_HorizontalFit = FitMode.Unconstrained;

        /// 

        /// The fit mode to use to determine the width.
        /// 
        public FitMode horizontalFit { get { return m_HorizontalFit; } set { if (SetPropertyUtility.SetStruct(ref m_HorizontalFit, value)) SetDirty(); } }

        [SerializeField] protected FitMode m_VerticalFit = FitMode.Unconstrained;

        /// 

        /// The fit mode to use to determine the height.
        /// 
        public FitMode verticalFit { get { return m_VerticalFit; } set { if (SetPropertyUtility.SetStruct(ref m_VerticalFit, value)) SetDirty(); } }

        [System.NonSerialized] private RectTransform m_Rect;
        private RectTransform rectTransform
        {
            get
            {
                if (m_Rect == null)
                    m_Rect = GetComponent<RectTransform>();
                return m_Rect;
            }
        }

        private DrivenRectTransformTracker m_Tracker;

        protected ContentSizeFitter()
        {}

        protected override void OnEnable()
        {
            base.OnEnable();
            SetDirty();    //标记为脏
        }

        protected override void OnDisable()
        {
            m_Tracker.Clear();
            LayoutRebuilder.MarkLayoutForRebuild(rectTransform);  //标记此rectTransform需要布局重建
            base.OnDisable();
        }

        protected override void OnRectTransformDimensionsChange()  //rectTransform维度发生变化，也标记为脏
        {
            SetDirty();
        }

        private void HandleSelfFittingAlongAxis(int axis)
        {
            FitMode fitting = (axis == 0 ? horizontalFit : verticalFit);
            if (fitting == FitMode.Unconstrained)
            {
                // Keep a reference to the tracked transform, but don't control its properties:
                m_Tracker.Add(this, rectTransform, DrivenTransformProperties.None);
                return;
            }

            m_Tracker.Add(this, rectTransform, (axis == 0 ? DrivenTransformProperties.SizeDeltaX : DrivenTransformProperties.SizeDeltaY));

            // Set size to min or preferred size
            if (fitting == FitMode.MinSize)
            	//设置最小适应
                rectTransform.SetSizeWithCurrentAnchors((RectTransform.Axis)axis, LayoutUtility.GetMinSize(m_Rect, axis));  
            else
            	//设置完美适应
                rectTransform.SetSizeWithCurrentAnchors((RectTransform.Axis)axis, LayoutUtility.GetPreferredSize(m_Rect, axis));  
        }

        /// 

        /// Calculate and apply the horizontal component of the size to the RectTransform
        /// 
        public virtual void SetLayoutHorizontal()
        {
            m_Tracker.Clear();
            HandleSelfFittingAlongAxis(0);
        }

        /// 

        /// Calculate and apply the vertical component of the size to the RectTransform
        /// 
        public virtual void SetLayoutVertical()
        {
            HandleSelfFittingAlongAxis(1);
        }

        protected void SetDirty()
        {
            if (!IsActive())
                return;

            LayoutRebuilder.MarkLayoutForRebuild(rectTransform);  标记此rectTransform需要布局重建
        }

    #if UNITY_EDITOR
        protected override void OnValidate()
        {
            SetDirty();
        }

    #endif
    }
}

首先看MarkLayoutForRebuild方法，调用自LayoutRebuilder的MarkLayoutForRebuild方法

public static void MarkLayoutForRebuild(RectTransform rect)
        {
            if (rect == null || rect.gameObject == null)
                return;

            var comps = ListPool<Component>.Get();
            bool validLayoutGroup = true;
            RectTransform layoutRoot = rect;
            var parent = layoutRoot.parent as RectTransform;
            while (validLayoutGroup && !(parent == null || parent.gameObject == null))
            {
                validLayoutGroup = false;
                parent.GetComponents(typeof(ILayoutGroup), comps);

                for (int i = 0; i < comps.Count; ++i)
                {
                    var cur = comps[i];
                    if (cur != null && cur is Behaviour && ((Behaviour)cur).isActiveAndEnabled)
                    {
                        validLayoutGroup = true;
                        layoutRoot = parent;
                        break;
                    }
                }

                parent = parent.parent as RectTransform;
            }

            // We know the layout root is valid if it's not the same as the rect,
            // since we checked that above. But if they're the same we still need to check.
            if (layoutRoot == rect && !ValidController(layoutRoot, comps))
            {
                ListPool<Component>.Release(comps);
                return;
            }

            MarkLayoutRootForRebuild(layoutRoot);
            ListPool<Component>.Release(comps);
        }

这里往当前RectTransform 的父亲上寻找符合条件的RectTransform，继续调用MarkLayoutRootForRebuild

private static void MarkLayoutRootForRebuild(RectTransform controller)
        {
            if (controller == null)
                return;

            var rebuilder = s_Rebuilders.Get();
            rebuilder.Initialize(controller);
            if (!CanvasUpdateRegistry.TryRegisterCanvasElementForLayoutRebuild(rebuilder))
                s_Rebuilders.Release(rebuilder);
        }

这里初始化rebuilder，同时将其注册到布局重绘中

再看SetLayoutHorizontal和SetLayoutVertical两个方法，往上溯源，发现来自LayoutRebuilder的Rebuild方法

public void Rebuild(CanvasUpdate executing)
        {
            switch (executing)
            {
                case CanvasUpdate.Layout:
                    // It's unfortunate that we'll perform the same GetComponents querys for the tree 2 times,
                    // but each tree have to be fully iterated before going to the next action,
                    // so reusing the results would entail storing results in a Dictionary or similar,
                    // which is probably a bigger overhead than performing GetComponents multiple times.
                    PerformLayoutCalculation(m_ToRebuild, e => (e as ILayoutElement).CalculateLayoutInputHorizontal());
                    PerformLayoutControl(m_ToRebuild, e => (e as ILayoutController).SetLayoutHorizontal());
                    PerformLayoutCalculation(m_ToRebuild, e => (e as ILayoutElement).CalculateLayoutInputVertical());
                    PerformLayoutControl(m_ToRebuild, e => (e as ILayoutController).SetLayoutVertical());
                    break;
            }
        }

查看PerformLayoutControl

private void PerformLayoutControl(RectTransform rect, UnityAction<Component> action)
        {
            if (rect == null)
                return;

            var components = ListPool<Component>.Get();
            rect.GetComponents(typeof(ILayoutController), components);
            StripDisabledBehavioursFromList(components);   //移除所有非Behaviour的和未激活的

            // If there are no controllers on this rect we can skip this entire sub-tree
            // We don't need to consider controllers on children deeper in the sub-tree either,
            // since they will be their own roots.
            if (components.Count > 0)
            {
                // Layout control needs to executed top down with parents being done before their children,
                // because the children rely on the sizes of the parents.

                // 首先调用布局控制器，它们可能会改变自己的RectTransform
                for (int i = 0; i < components.Count; i++)
                    if (components[i] is ILayoutSelfController)
                        action(components[i]);

                // 然后调用其余的，比如改变它们的子布局组，考虑它们自己的RectTransform大小
                for (int i = 0; i < components.Count; i++)
                    if (!(components[i] is ILayoutSelfController))
                        action(components[i]);
				
				//递归向下
                for (int i = 0; i < rect.childCount; i++)   
                    PerformLayoutControl(rect.GetChild(i) as RectTransform, action);
            }

            ListPool<Component>.Release(components);
        }

这里ILayoutSelfController继承自ILayoutController，先执行了ILayoutSelfController类的设置，在执行了非ILayoutSelfController的设置，最后递归遍历子结点