WPF DrawingContext Pen

<Window x:Class="WPFDrawing.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WPFDrawing"
        mc:Ignorable="d"
        Title="MainWindow"
        Height="600"
        Width="800">
    <Grid>
        <Grid.RowDefinitions>

            <RowDefinition Height="*" />
            <RowDefinition Height="50" />
        </Grid.RowDefinitions>
        <local:DrawingCanvas x:Name="drawCanvas"
                             Background="LightGray"
                             MouseLeftButtonDown="drawCanvas_MouseLeftButtonDown"
                             MouseMove="drawCanvas_MouseMove">

        </local:DrawingCanvas>
        <Button Content="test"
                Width="100"
                Grid.Row="1"
                Click="Button_Click"></Button>
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WPFDrawing
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            Button_Click(null, null);
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            var pts = new PointCollection();
            pts.Add(new Point() { X = 10, Y = 10 });
            pts.Add(new Point() { X = 300, Y = 300 });
            Visual vs = drawCanvas.Polyline(pts, Brushes.Red, 1);
            drawCanvas.AddVisual(vs);
        }

        private void drawCanvas_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.LeftButton == MouseButtonState.Pressed)
            {
                var p = e.GetPosition(null);
                if (vs != null)
                {
                    drawCanvas.NewLine(vs, new Point() { X = 10, Y = 10 }, p);
                }
                Console.WriteLine(p.X + "    " + p.Y);
            }
        }

        DrawingVisual vs = null;

        private void drawCanvas_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            var p = e.GetPosition(null);
            Console.WriteLine(p.X + "    " + p.Y);
            vs = drawCanvas.GetVisual(p);
            if (vs != null)
            {
                Console.WriteLine("已经选中了。。。。。");
            }
        }
    }
}

using System.Collections.Generic;
using System.Windows.Controls;
using System.Windows.Media;
using System.Windows;

namespace WPFDrawing
{
    public class DrawingCanvas : Canvas
    {
        private List<Visual> visuals = new List<Visual>();

        //获取Visual的个数
        protected override int VisualChildrenCount
        {
            get { return visuals.Count; }
        }

        //获取Visual
        protected override Visual GetVisualChild(int index)
        {
            return visuals[index];
        }

        //添加Visual
        public void AddVisual(Visual visual)
        {
            visuals.Add(visual);

            base.AddVisualChild(visual);
            base.AddLogicalChild(visual);
        }

        //删除Visual
        public void RemoveVisual(Visual visual)
        {
            visuals.Remove(visual);

            base.RemoveVisualChild(visual);
            base.RemoveLogicalChild(visual);
        }

        //命中测试
        public DrawingVisual GetVisual(Point point)
        {
            HitTestResult hitResult = VisualTreeHelper.HitTest(this, point);
            return hitResult.VisualHit as DrawingVisual;
        }

        //使用DrawVisual画Polyline
        public Visual Polyline(PointCollection points, Brush color, double thinkness)
        {
            DrawingVisual visual = new DrawingVisual();
            DrawingContext dc = visual.RenderOpen();
            Pen pen = new Pen(Brushes.Red, 10);
            pen.Freeze();  //冻结画笔，这样能加快绘图速度

            for (int i = 0; i < points.Count - 1; i++)
            {
                dc.DrawLine(pen, points[i], points[i + 1]);
            }

            dc.Close();
            return visual;
        }

        public void NewLine(DrawingVisual visual, Point ps, Point pd)
        {
            DrawingContext dc = visual.RenderOpen();
            Pen pen = new Pen(Brushes.Red, 2);
            pen.Freeze();  //冻结画笔，这样能加快绘图速度
            dc.DrawLine(pen, ps, pd);
            dc.Close();
        }

    }
}