DrawingVisual和DrawingContext的简单应用（在Image上画图）

1.新建一个继承Image的基类，因为要在Image对象上画图，所以下面的代码必须放入基类中，如果放在加载图片的主窗体上，背景会变成黑色。

  protected override Visual GetVisualChild(int index)
  {
            return visuals[index];
   }
   protected override int VisualChildrenCount
   {
        get
            {
                return visuals.Count;
            }
    }

下面是新建的基类代码

 public class DrawingImages : Image
    {
        public List<Visual> visuals = new List<Visual>();

        protected override Visual GetVisualChild(int index)
        {
            return this.visuals[index];
        }

        protected override int VisualChildrenCount
        {
            get
            {
                return visuals.Count;
            }
        }

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

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

        public void DeleteVisual(Visual visual)
        {
            this.visuals.Remove(visual);

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

        public void DeleteVisualAll()
        {
            foreach (var item in visuals)
            {
                base.RemoveVisualChild(item);
                base.RemoveLogicalChild(item);
            }
            this.visuals.Clear();
        }
    }

2.主窗体代码

　xaml窗体代码默认即可。后台代码如下：

 /// <summary>
    /// DrawingImage.xaml 的交互逻辑
    /// </summary>
    public partial class DrawingImage : Window
    {
        private DrawingImages img_ = new DrawingImages();
        public DrawingImage()
        {
            InitializeComponent();
            this.Content = img_;
            img_.Stretch = Stretch.Fill;
            img_.MouseLeftButtonDown += new MouseButtonEventHandler(img__MouseLeftButtonDown);
            img_.MouseMove += new MouseEventHandler(img__MouseMove);
            img_.MouseLeftButtonUp += new MouseButtonEventHandler(img__MouseLeftButtonUp);
            img_.MouseDown += new MouseButtonEventHandler(img__MouseDown);
            InitDrawImage();
           
        }

        void img__MouseDown(object sender, MouseButtonEventArgs e)
        {
            if (e.ChangedButton==MouseButton.Right)
               img_.DeleteVisualAll();
        }

        private void InitDrawImage()
        {
            int X = (int)this.Width;
            int Y = (int)this.Height;
            DrawingVisual dv = new DrawingVisual();
            using (DrawingContext dc = dv.RenderOpen())
            {
                Rect rect = new Rect(new Point(0, 0), new Size(X, Y));
                Brush bc = new SolidColorBrush(Colors.LightBlue);
                Pen pen = new Pen(Brushes.Pink, 1);
                dc.DrawRectangle(bc, pen, rect);
                dc.DrawText(new FormattedText("LightBlue", CultureInfo.CurrentCulture, FlowDirection.LeftToRight, new Typeface("微软雅黑"), (int)Y / 32, Brushes.White), new Point((int)X / 25, (int)Y / 21));
            }
            var rtbitmap = new RenderTargetBitmap(X, Y, 0.0, 0.0, PixelFormats.Default);
            rtbitmap.Render(dv);
            img_.Source = rtbitmap;
        }

        private Point? _startPoint;
        private DrawingVisual selectionSquare;
        void img__MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            _startPoint = e.GetPosition(this.img_);
            selectionSquare = new DrawingVisual();
            this.img_.AddVisual(selectionSquare);
            this.img_.CaptureMouse();
 
        }

        private Brush selectionSquareBrush = Brushes.Transparent;
        private Pen selectionSquarePen = new Pen(Brushes.LightPink, 2);
        private void DrawSelectionSquare(Point point1, Point point2)
        {
            if (selectionSquare == null)
                return;
            using (DrawingContext dc = selectionSquare.RenderOpen())
            {
                dc.DrawRectangle(selectionSquareBrush, selectionSquarePen,
                    new Rect(point1, point2));
            }
        }

        void img__MouseMove(object sender, MouseEventArgs e)
        {
            if (_startPoint == null)
                return;
            Point endPoint = e.GetPosition(this.img_);
            DrawSelectionSquare((Point)_startPoint, endPoint);
            
        }

        void img__MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
        {
            this.img_.ReleaseMouseCapture();
            _startPoint = null;
        }
      
    }