wpf wrapPanel居中并从左到右排列

public class AlignableWrapPanel : Panel
    {
        /// <summary>
        /// 注册新的属性 HorizontalContentAlignment
        /// </summary>
        public HorizontalAlignment HorizontalContentAlignment
        {
            get { return (HorizontalAlignment)GetValue(HorizontalContentAlignmentProperty); }
            set { SetValue(HorizontalContentAlignmentProperty, value); }
        }

        public static readonly DependencyProperty HorizontalContentAlignmentProperty =
            DependencyProperty.Register("HorizontalContentAlignment", 
                typeof(HorizontalAlignment), 
                typeof(AlignableWrapPanel), 
                new FrameworkPropertyMetadata(HorizontalAlignment.Left, FrameworkPropertyMetadataOptions.AffectsArrange));
        /// <summary>
        /// panel尺寸的计算
        /// </summary>
        /// <param name="constraint"></param>
        /// <returns></returns>
        protected override Size MeasureOverride(Size constraint)
        {
            //当前行的尺寸
            Size curLineSize = new Size();

            //整个panel的尺寸
            Size panelSize = new Size();

            //panel里的item
            UIElementCollection children = base.InternalChildren;

            for (int i = 0; i < children.Count; i++)
            {
                UIElement child = children[i] as UIElement;

                // 更新子元素在容器中的尺寸
                child.Measure(constraint);
                Size sz = child.DesiredSize;

                //当需要换行时
                if (curLineSize.Width + sz.Width > constraint.Width) 
                {
                    panelSize.Width = Math.Max(curLineSize.Width, panelSize.Width);
                    panelSize.Height += curLineSize.Height;
                    curLineSize = sz;

                    //当子元素宽度大于容器时，给予新的一行
                    if (sz.Width > constraint.Width)                 
                    {
                        panelSize.Width = Math.Max(sz.Width, panelSize.Width);
                        panelSize.Height += sz.Height;
                        curLineSize = new Size();
                    }
                }
                else //不需要换行时，也继续加一行
                {
                    curLineSize.Width += sz.Width;
                    curLineSize.Height = Math.Max(sz.Height, curLineSize.Height);
                }
            }

            // 最后一行的尺寸
            panelSize.Width = Math.Max(curLineSize.Width, panelSize.Width);
            panelSize.Height += curLineSize.Height;

            return panelSize;
        }

        /// <summary>
        /// 换行的方法
        /// </summary>
        /// <param name="arrangeBounds"></param>
        /// <returns></returns>
        protected override Size ArrangeOverride(Size arrangeBounds)
        {
            int firstInLine = 0;
            Size curLineSize = new Size();
            double accumulatedHeight = 0;
            UIElementCollection children = this.InternalChildren;

            for (int i = 0; i < children.Count; i++)
            {
                Size sz = children[i].DesiredSize;

                if (curLineSize.Width + sz.Width > arrangeBounds.Width) 
                {
                    //安排元素在行中的位置
                    ArrangeLine(accumulatedHeight, curLineSize, arrangeBounds.Width, firstInLine, i);

                    
                    accumulatedHeight += curLineSize.Height;
                    curLineSize = sz;

                    if (sz.Width > arrangeBounds.Width)                   
                    {
                        //安排元素在行中的位置
                        ArrangeLine(accumulatedHeight, sz, arrangeBounds.Width, i, ++i);
                        
                        accumulatedHeight += sz.Height;
                        curLineSize = new Size();
                    }
                    firstInLine = i;
                }
                else 
                {
                    curLineSize.Width += sz.Width;
                    curLineSize.Height = Math.Max(sz.Height, curLineSize.Height);
                }
            }

            if (firstInLine < children.Count)
            {
                //安排元素在行中的位置
                ArrangeLine(accumulatedHeight, curLineSize, arrangeBounds.Width, firstInLine, children.Count);
            }

            return arrangeBounds;
        }
        /// <summary>
        /// 行中item的排列方式
        /// </summary>
        /// <param name="y"></param>
        /// <param name="lineSize"></param>
        /// <param name="boundsWidth"></param>
        /// <param name="start"></param>
        /// <param name="end"></param>
        private void ArrangeLine(double y, Size lineSize, double boundsWidth, int start, int end)
        {
            var children = InternalChildren;

            var x = 0D;
            if (HorizontalContentAlignment == HorizontalAlignment.Center)
            {
                //item起点的位置
                x = 0D;
            }

            //安排每个item在一行中的位置
            for (var i = start; i < end; i++)
            {
                var child = children[i];
                double itemWidth;
                itemWidth = child.DesiredSize.Width;
                

                child.Arrange(new Rect(x, y, itemWidth, lineSize.Height));
                //x每多一个item就加上自己本身的距离，作为起点
                x += itemWidth;
            }
        }
       
    }