项目上需要对Canvas中的控件添加调整大小功能，即能在控件的四个角和四条边上可进行相应的拖动，类似Windows窗口那种。于是在参考以前同事写的代码基础上，完成了该功能。

我们是给现有的控件添加功能，属于装饰功能。当然首先想到的就是Adorner。在MSDN中Adorner的介绍如下：

装饰器是一个绑定到 UIElement 的自定义 FrameworkElement。 装饰器呈现在装饰器层中，它是一个呈现图面，始终位于装饰元素或装饰元素集合的顶部；呈现装饰器独立于呈现该装饰器绑定到的 UIElement。 装饰器通常相对于其绑定到的元素进行定位，且使用位于装饰元素的左上部的标准 2-D 坐标原点进行定位。

关于Adorner更详细的信息，可参考 WPF - Adorner - loveis715 - 博客园 。Adorner是一个抽象类，我们可以继承自该类来实现自己的装饰功能。

WPF中存在支持拖动的Thumb控件，而且Thumb控件继承自Control，可以定义控件模板。Thumb最重要的三个事件如下：

Thumb 提供 DragStarted, DragCompleted 和 DragDelta 事件来管理与鼠标指针相关的拖动操作。 当用户按下鼠标左键时，Thumb 控件接收逻辑焦点和鼠标捕获，并引发 DragStarted 事件。 在 Thumb 控件具有焦点和鼠标捕获的同时，可以无限制地多次引发 DragDelta 事件。 当用户释放鼠标左键时，Thumb 控件失去鼠标捕获，并引发 DragCompleted 事件。

思路很明确，就是自定义一个Adorner，在四条边和四个角上添加相应的Thumb，处理相应的事件实现改变大小。值得注意的是，在左上角、右上角、左下角、上边、左边这些地方实际上不仅是改变大小，同时也会改变控件在宿主中的位置，所以我更愿意称之为调整布局。

主要类及其关系如下：

添加CanvasArrangementAdorner之后控件效果如下（浅蓝色为控件）：

因为将Thumb设为透明了，看不出来是由8个Thumb组成的，如果改下颜色，会更容易理解些。

可以很明显的看出，在四个角和四条边上各有4个Thumb，我重新定义了Thumb的控件模板，控件模板内部是一个Rectangle。

各个主要类如下，因代码较简单，就不多解释了。

ArrangementDirection

using System;
/// <summary>
/// 布局方向
/// </summary>
[Flags]
public enum ArrangementDirection
    None = 0,
    LeftTop = 1,
    Top = 2,
    RightTop = 4,
    Right = 8,
    RightBottom = 16,
    Bottom = 32,
    LeftBottom = 64,
    Left = 128,
    All = LeftTop | Top | RightTop | Right | RightBottom | Bottom | LeftBottom | Left,
ArrangementChangedEventArgs
using System;
using System.Windows;
/// <summary>
/// 布局变化的事件
/// </summary>
public class ArrangementChangedEventArgs : EventArgs
    public ArrangementChangedEventArgs(Rect oldArrangement, Rect newArrangement)
        this.OldArrangement = oldArrangement;
        this.NewArrangement = newArrangement;
    /// <summary>
    /// 旧布局信息
    /// </summary>
    public Rect OldArrangement { get; private set; }
    /// <summary>
    /// 新布局信息
    /// </summary>
    public Rect NewArrangement { get; private set; }
ArrangementDirection
using System;
/// <summary>
/// 布局方向
/// </summary>
[Flags]
public enum ArrangementDirection
    None = 0,
    LeftTop = 1,
    Top = 2,
    RightTop = 4,
    Right = 8,
    RightBottom = 16,
    Bottom = 32,
    LeftBottom = 64,
    Left = 128,
    All = LeftTop | Top | RightTop | Right | RightBottom | Bottom | LeftBottom | Left,
ArrangementAdorner
using System;
    using System.Diagnostics.Contracts;
    using System.Windows;
    using System.Windows.Controls;
    using System.Windows.Controls.Primitives;
    using System.Windows.Documents;
    using System.Windows.Input;
    using System.Windows.Media;
    using System.Windows.Shapes;
    /// <summary>
    /// 布局装饰器
    /// </summary>
    public abstract class ArrangementAdorner : Adorner
        #region Fields
        /// <summary>
        /// 拖动方块的边长
        /// </summary>
        private const double ThumbSideLength = 6;
        /// <summary>
        /// 可视化对象集合
        /// </summary>
        private readonly VisualCollection visualCollection;
        /// <summary>
        /// 对齐方向
        /// </summary>
        private readonly ArrangementDirection direction;
        /// <summary>
        /// 各个方向的拖动方块
        /// </summary>
        private readonly Thumb topThumb,
                               leftTopthumb,
                               rightTopThumb,
                               righThumb,
                               rightBottomThumb,
                               bottomThumb,
                               leftBottomThumb,
                               leftThumb;
        /// <summary>
        /// 当前位置
        /// </summary>
        private Point currentLocation;
        /// <summary>
        /// 拖动前的大小
        /// </summary>
        private Size oldSize;
        /// <summary>
        /// 拖动前左边缘的值
        /// </summary>
        private double oldLeft;
        /// <summary>
        /// 拖动前上边缘的值
        /// </summary>
        private double oldTop;
        #endregion Fields
        #region Constructors
        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="adornedElement">装饰器所要绑定到的元素。</param>
        /// <param name="arrangementDirection">布局方向</param>
        protected ArrangementAdorner(FrameworkElement adornedElement, ArrangementDirection arrangementDirection = ArrangementDirection.All)
            : base(adornedElement)
            this.direction = arrangementDirection;
            this.visualCollection = new VisualCollection(this);
            this.AddThumbIfNeeded(
                ref this.leftTopthumb,
                ArrangementDirection.LeftTop,
                HorizontalAlignment.Left,
                VerticalAlignment.Top,
                Cursors.SizeNWSE);
            this.AddThumbIfNeeded(
                ref this.topThumb,
                ArrangementDirection.Top,
                HorizontalAlignment.Stretch,
                VerticalAlignment.Top,
                Cursors.SizeNS);
            this.AddThumbIfNeeded(
                ref this.rightTopThumb,
                ArrangementDirection.RightTop,
                HorizontalAlignment.Right,
                VerticalAlignment.Top,
                Cursors.SizeNESW);
            this.AddThumbIfNeeded(
                ref this.righThumb,
                ArrangementDirection.Right,
                HorizontalAlignment.Right,
                VerticalAlignment.Stretch,
                Cursors.SizeWE);
            this.AddThumbIfNeeded(
                ref this.rightBottomThumb,
                ArrangementDirection.RightBottom,
                HorizontalAlignment.Right,
                VerticalAlignment.Bottom,
                Cursors.SizeNWSE);
            this.AddThumbIfNeeded(
                ref this.bottomThumb,
                ArrangementDirection.Bottom,
                HorizontalAlignment.Stretch,
                VerticalAlignment.Bottom,
                Cursors.SizeNS);
            this.AddThumbIfNeeded(
                ref this.leftBottomThumb,
                ArrangementDirection.LeftBottom,
                HorizontalAlignment.Left,
                VerticalAlignment.Bottom,
                Cursors.SizeNESW);
            this.AddThumbIfNeeded(
                ref this.leftThumb,
                ArrangementDirection.Left,
                HorizontalAlignment.Left,
                VerticalAlignment.Stretch,
                Cursors.SizeWE);
        #endregion Constructors
        public event EventHandler<ArrangementChangedEventArgs> ArrangementChanged;
        #region Protected Methods
        #region Overrides
        /// <summary>
        /// 获取此元素内的可视化子元素的数目。
        /// </summary>
        /// <returns>
        /// 此元素内的可视化子元素的数目。
        /// </returns>
        protected override int VisualChildrenCount
                return this.visualCollection.Count;
        /// <summary>
        /// 定位子元素并确定大小。
        /// </summary>
        /// <returns>
        /// 所用的实际大小。
        /// </returns>
        /// <param name="finalSize">排列自身及其子元素的最终区域。</param>
        protected override Size ArrangeOverride(Size finalSize)
            this.ArrangeThumbIfNeeded(
                this.leftTopthumb,
                new Point(-ThumbSideLength, -ThumbSideLength),
                new Size(ThumbSideLength, ThumbSideLength));
            this.ArrangeThumbIfNeeded(
                this.topThumb,
                new Point(0, -ThumbSideLength),
                new Size(finalSize.Width, ThumbSideLength));
            this.ArrangeThumbIfNeeded(
                this.rightTopThumb,
                new Point(finalSize.Width, -ThumbSideLength),
                new Size(ThumbSideLength, ThumbSideLength));
            this.ArrangeThumbIfNeeded(
                this.righThumb,
                new Point(finalSize.Width, 0),
                new Size(ThumbSideLength, finalSize.Height));
            this.ArrangeThumbIfNeeded(
                this.rightBottomThumb,
                new Point(finalSize.Width, finalSize.Height),
                new Size(ThumbSideLength, ThumbSideLength));
            this.ArrangeThumbIfNeeded(
                this.bottomThumb,
                new Point(0, finalSize.Height),
                new Size(finalSize.Width, ThumbSideLength));
            this.ArrangeThumbIfNeeded(
                this.leftBottomThumb,
                new Point(-ThumbSideLength, finalSize.Height),
                new Size(ThumbSideLength, ThumbSideLength));
            this.ArrangeThumbIfNeeded(
                this.leftThumb,
                new Point(-ThumbSideLength, 0),
                new Size(ThumbSideLength, finalSize.Height));
            return base.ArrangeOverride(finalSize);
        /// <summary>
        /// 从子元素集合返回指定索引处的子级。
        /// </summary>
        /// <returns>
        /// 所请求的子元素。它不应返回 null；如果提供的索引超出范围，将引发异常。
        /// </returns>
        /// <param name="index">集合中所请求子元素从零开始的索引。</param>
        protected override Visual GetVisualChild(int index)
            return this.visualCollection[index];
        #endregion Overrides
        #region Virtuals
        /// <summary>
        /// 创建布局方块
        /// </summary>
        /// <param name="horizontalAlignment">方块的水平对齐方向</param>
        /// <param name="verticalAlignment">方块的垂直对齐方向</param>
        /// <param name="cursor">方块的光标</param>
        /// <returns>创建好的方块</returns>
        protected virtual Thumb CreateResizeThumb(
            HorizontalAlignment horizontalAlignment,
            VerticalAlignment verticalAlignment,
            Cursor cursor)
            var thumb = new Thumb
                HorizontalAlignment = horizontalAlignment,
                VerticalAlignment = verticalAlignment,
                Cursor = cursor,
                Template = this.GetResizeThumbControlTemplate()
            return thumb;
        /// <summary>
        /// 获取框架元素的位置
        /// </summary>
        /// <param name="element">框架元素</param>
        /// <returns>框架元素所在的位置</returns>
        protected abstract Point GetLocation(FrameworkElement element);
        /// <summary>
        /// 判断框架元素的位置偏移是否合法
        /// </summary>
        /// <param name="element">框架元素</param>
        /// <param name="offset">偏移向量</param>
        /// <returns>合法返回true,否则返回false</returns>
        protected virtual bool IsLocationOffsetLegal(FrameworkElement element, Vector offset)
            var targetLocation = this.currentLocation + offset;
            if (targetLocation.X < 0)
                return false;
            return true;
        /// <summary>
        /// 设置框架元素的位置
        /// </summary>
        /// <param name="element">框架元素</param>
        /// <param name="location">新位置</param>
        protected abstract void SetLocation(FrameworkElement element, Point location);
        /// <summary>
        /// 获取框架元素的宽度
        /// </summary>
        /// <param name="element">框架元素</param>
        /// <returns>宽度</returns>
        protected virtual double GetWidth(FrameworkElement element)
            return element.Width;
        /// <summary>
        /// 获取框架元素的高度
        /// </summary>
        /// <param name="element">框架元素</param>
        /// <returns>高度</returns>
        protected virtual double GetHeight(FrameworkElement element)
            return element.Height;
        /// <summary>
        /// 判断框架元素的宽度变化是否合法
        /// </summary>
        /// <param name="element">框架元素</param>
        /// <param name="widthDelta">宽度变化</param>
        /// <returns>变化是否合法</returns>
        protected virtual bool IsWidthDeltaLegal(FrameworkElement element, double widthDelta)
            double newWidth = this.GetWidth(element) + widthDelta;
            return this.IsInRange(element.MaxWidth, element.MinWidth, newWidth);
        /// <summary>
        /// 判断框架元素的高度变化是否合法
        /// </summary>
        /// <param name="element">框架元素</param>
        /// <param name="heightDelta">高度变化</param>
        /// <returns>变化是否合法</returns>
        protected virtual bool IsHeightDeltaLegal(FrameworkElement element, double heightDelta)
            double newHeight = this.GetHeight(element) + heightDelta;
            return this.IsInRange(element.MaxHeight, element.MinHeight, newHeight);
        /// <summary>
        /// 设置框架元素的宽度变化
        /// </summary>
        /// <param name="element">框架元素</param>
        /// <param name="widthDelta">宽度变化</param>
        protected virtual void SetWidthDelta(FrameworkElement element, double widthDelta)
            element.Width += widthDelta;
        /// <summary>
        /// 设置框架元素的高度变化
        /// </summary>
        /// <param name="element">框架元素</param>
        /// <param name="heightDelta">高度变化</param>
        protected virtual void SetHeightDelta(FrameworkElement element, double heightDelta)
            element.Height += heightDelta;
        #endregion Virtuals
        #endregion  Protected Methods
        #region Private Methods
        /// <summary>
        /// 在需要时添加拖动方块
        /// </summary>
        /// <param name="thumb">类中对应的方块</param>
        /// <param name="arrangementDirection">方块对应的布局方向</param>
        /// <param name="horizontalAlignment">方块的水平对齐方向</param>
        /// <param name="verticalAlignment">方块的垂直对齐方向</param>
        /// <param name="cursor">方块的光标</param>
        private void AddThumbIfNeeded(
            ref Thumb thumb,
            ArrangementDirection arrangementDirection,
            HorizontalAlignment horizontalAlignment,
            VerticalAlignment verticalAlignment,
            Cursor cursor)
            if (this.HasDirectionFlagSet(arrangementDirection))
                thumb = this.CreateResizeThumb(horizontalAlignment, verticalAlignment, cursor);
                thumb.DragStarted += this.ThumbDragStarted;
                thumb.DragDelta += this.ThumbDragDelta;
                thumb.DragCompleted += this.ThumbDragCompleted;
                this.visualCollection.Add(thumb);
        /// <summary>
        /// 判断布局方向是否被设置
        /// </summary>
        /// <param name="arrangementDirection">布局方向</param>
        /// <returns>被设置返回true,否则返回false</returns>
        private bool HasDirectionFlagSet(ArrangementDirection arrangementDirection)
            return (this.direction & arrangementDirection) == arrangementDirection;
        /// <summary>
        /// 获取布局方块的控件模板
        /// </summary>
        /// <returns>控件模板</returns>
        private ControlTemplate GetResizeThumbControlTemplate()
            var factory = new FrameworkElementFactory(typeof(Rectangle));
            factory.SetValue(Shape.FillProperty, Brushes.Transparent);
            factory.SetValue(Shape.StrokeProperty, Brushes.Transparent);
            var controlTemplate = new ControlTemplate { TargetType = typeof(Thumb), VisualTree = factory };
            return controlTemplate;
        /// <summary>
        /// 在需要时定位并确定方块大小
        /// </summary>
        /// <param name="thumb">方块</param>
        /// <param name="location">方块位置</param>
        /// <param name="size">方块大小</param>
        private void ArrangeThumbIfNeeded(Thumb thumb, Point location, Size size)
            if (thumb != null)
                if (thumb.HorizontalAlignment != HorizontalAlignment.Stretch)
                    thumb.Width = size.Width;
                if (thumb.VerticalAlignment != VerticalAlignment.Stretch)
                    thumb.Height = size.Height;
                thumb.Arrange(new Rect(location, size));
        /// <summary>
        /// 判断一个值是否在范围中
        /// </summary>
        /// <param name="maximum">最大值,可取</param>
        /// <param name="minimum">最小值,可取</param>
        /// <param name="value">值</param>
        /// <returns>值在范围中返回true,否则返回false</returns>
        private bool IsInRange(double maximum, double minimum, double value)
            return (value >= minimum) && (value <= maximum);
        #endregion Private Methods
        #region Events Handler
        /// <summary>
        /// 拖动开始的响应
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void ThumbDragStarted(object sender, DragStartedEventArgs e)
            var frameworkElement = this.AdornedElement as FrameworkElement;
            this.currentLocation = this.GetLocation(frameworkElement);
            this.oldLeft = this.currentLocation.X;
            this.oldTop = this.currentLocation.Y;
            var width = this.GetWidth(frameworkElement);
            var height = this.GetHeight(frameworkElement);
            this.oldSize = new Size(width, height);
        /// <summary>
        /// 拖动变化的响应
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void ThumbDragDelta(object sender, DragDeltaEventArgs e)
            var frameworkElement = this.AdornedElement as FrameworkElement;
            var thumb = sender as Thumb;
            Contract.Assert(thumb != null);
            switch (thumb.HorizontalAlignment)
                case HorizontalAlignment.Left:
                        var offset = new Vector(e.HorizontalChange, 0);
                        if (this.IsLocationOffsetLegal(frameworkElement, offset))
                            this.currentLocation.Offset(e.HorizontalChange, 0);
                            if (this.IsWidthDeltaLegal(frameworkElement, -e.HorizontalChange))
                                this.SetWidthDelta(frameworkElement, -e.HorizontalChange);
                        break;
                case HorizontalAlignment.Right:
                        if (this.IsWidthDeltaLegal(frameworkElement, e.HorizontalChange))
                            this.SetWidthDelta(frameworkElement, e.HorizontalChange);
                        break;
            switch (thumb.VerticalAlignment)
                case VerticalAlignment.Top:
                        var offset = new Vector(0, e.VerticalChange);
                        if (this.IsLocationOffsetLegal(frameworkElement, offset))
                            this.currentLocation.Offset(0, e.VerticalChange);
                            if (this.IsHeightDeltaLegal(frameworkElement, -e.VerticalChange))
                                this.SetHeightDelta(frameworkElement, -e.VerticalChange);
                        break;
                case VerticalAlignment.Bottom:
                        if (this.IsHeightDeltaLegal(frameworkElement, e.VerticalChange))
                            this.SetHeightDelta(frameworkElement, e.VerticalChange);
                        break;
            this.SetLocation(frameworkElement, this.currentLocation);
        /// <summary>
        /// 拖动结束的响应
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void ThumbDragCompleted(object sender, DragCompletedEventArgs e)
            if (this.ArrangementChanged != null)
                var frameworkElement = this.AdornedElement as FrameworkElement;
                var oldArrangement = new Rect(new Point(this.oldLeft, this.oldTop), this.oldSize);
                var newArrangement = new Rect(
                    this.GetLocation(frameworkElement),
                    new Size(this.GetWidth(frameworkElement), this.GetHeight(frameworkElement)));
                this.ArrangementChanged(this, new ArrangementChangedEventArgs(oldArrangement, newArrangement));
        #endregion Events Handler
CanvasArrangementAdorner
using System.Windows;
    using System.Windows.Controls;
    /// <summary>
    /// 画布布局装饰器
    /// </summary>
    public class CanvasArrangementAdorner : ArrangementAdorner
        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="adornedElement">装饰器所要绑定到的元素。</param>
        /// <param name="arrangementDirection">布局方向</param>
        public CanvasArrangementAdorner(FrameworkElement adornedElement, ArrangementDirection arrangementDirection = ArrangementDirection.All)
            : base(adornedElement, arrangementDirection)
        #region Overrides of ArrangementAdorner
        /// <summary>
        /// 获取框架元素的位置
        /// </summary>
        /// <param name="element">框架元素</param>
        /// <returns>框架元素所在的位置</returns>
        protected override Point GetLocation(FrameworkElement element)
            return new Point(Canvas.GetLeft(element), Canvas.GetTop(element));
        /// <summary>
        /// 设置框架元素的位置
        /// </summary>
        /// <param name="element">框架元素</param>
        /// <param name="location">新位置</param>
        protected override void SetLocation(FrameworkElement element, Point location)
            Canvas.SetLeft(element, location.X);
            Canvas.SetTop(element, location.Y);
        #endregion
博客园：ControlResize