如何使用WPF从用户输入绘制矩形

<Canvas x:Name="canvas" Background="White" MouseLeftButtonDown="Canvas_MouseLeftButtonDown" MouseLeftButtonUp="Canvas_MouseLeftButtonUp" MouseMove="Canvas_MouseMove"/>

private bool isDrawing = false;
private Point startPoint;

private void Canvas_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    isDrawing = true;
    startPoint = e.GetPosition(canvas);
}

private void Canvas_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
    if (isDrawing)
    {
        isDrawing = false;
        Point endPoint = e.GetPosition(canvas);

        // 计算矩形的位置和大小
        double left = Math.Min(startPoint.X, endPoint.X);
        double top = Math.Min(startPoint.Y, endPoint.Y);
        double width = Math.Abs(startPoint.X - endPoint.X);
        double height = Math.Abs(startPoint.Y - endPoint.Y);

        // 创建矩形并添加到Canvas控件中
        Rectangle rectangle = new Rectangle()
        {
            Width = width,
            Height = height,
            Stroke = Brushes.Black,
            StrokeThickness = 2
        };
        Canvas.SetLeft(rectangle, left);
        Canvas.SetTop(rectangle, top);
        canvas.Children.Add(rectangle);
    }
}

private void Canvas_MouseMove(object sender, MouseEventArgs e)
{
    if (isDrawing)
    {
        Point currentPoint = e.GetPosition(canvas);

        // 更新矩形的位置和大小
        double left = Math.Min(startPoint.X, currentPoint.X);
        double top = Math.Min(startPoint.Y, currentPoint.Y);
        double width = Math.Abs(startPoint.X - currentPoint.X);
        double height = Math.Abs(startPoint.Y - currentPoint.Y);

        Rectangle rectangle = canvas.Children.OfType<Rectangle>().LastOrDefault();
        if (rectangle != null)
        {
            rectangle.Width = width;
            rectangle.Height = height;
            Canvas.SetLeft(rectangle, left);
            Canvas.SetTop(rectangle, top);
        }
    }
}