用WPF的Polyline做十字线（橡皮筋），需要在xaml里加个另外的控件才能显示

[梁子哥]

如果不加那个 <TextBlock />就一片空白，想不明白什么原理，求老师讲解

public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = this;
        }

        #region API方式取鼠标坐标
        [StructLayout(LayoutKind.Sequential)]
        public struct MyPoint
        {
            public int X;
            public int Y;

            public MyPoint(int x, int y)
            {
                this.X = x;
                this.Y = y;
            }
        }
        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        public static extern bool GetCursorPos(out MyPoint pt);
        #endregion


        private void Grid_MouseMove(object sender, MouseEventArgs e)
        {
            MyPoint myPoint = new MyPoint();
            GetCursorPos(out myPoint);
            Point userPoint = this.PointFromScreen(new Point(myPoint.X, myPoint.Y));

            PointCollection points = new PointCollection();
            points.Add(new Point(0, userPoint.Y));
            points.Add(new Point(MyGd.ActualWidth, userPoint.Y));
            points.Add(new Point(userPoint.X, userPoint.Y));
            points.Add(new Point(userPoint.X, 0));
            points.Add(new Point(userPoint.X, MyGd.ActualHeight));
           
            Pl1.Points = points;
            Pl1.StrokeThickness = 1;
            Pl1.Stroke = new SolidColorBrush(Colors.Black);
        }
    }

<Window x:Class="简单橡皮筋.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:简单橡皮筋"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid x:Name="MyGd" MouseMove="Grid_MouseMove">
        <Polyline x:Name="Pl1"/>
        <TextBlock />
    </Grid>
</Window>

[梁子哥]

刚测试的，如果在外层嵌套一个PANEL则十字光标会非常的卡顿

        <StackPanel>
        <Polyline x:Name="Pl1"/>
        <TextBlock />
        </StackPanel>

[E次奥]

是不是因为没有背景色导致的，尝试设置Grid背景色看看；