WPF Canvas 画区域

有时候需要实现类似于QQ截图那样的选择区域功能，这里的区域可以是一条线，圆，矩形等等

实现原理就是一个Canvas做蒙板，然后canvas的三个事件，MouseLeftButtonDown，MouseMove，MouseLeftButtonUp。非常easy！

首先，你要有个canvas

<Canvas Name="videocanvas" Height="" Width="" Background="Transparent" MouseMove="image_MouseMove" MouseLeftButtonDown="Mask_MouseLeftButtonDown" MouseLeftButtonUp="Mask_MouseLeftButtonUp"/>

然后实现他的事件，在之前我们先决定一下到底是什么形状的区域。

 public static Shape CreateShape()
 {
     //矩形区域
     return new System.Windows.Shapes.Rectangle() { Fill = null, Stroke = System.Windows.Media.Brushes.Red, StrokeThickness =  };
     //圆形区域
     //return new System.Windows.Shapes.Ellipse() { Fill = null, Stroke = System.Windows.Media.Brushes.Red, StrokeThickness = 1 };

 }

然后实现三个事件

bool drawFlag = false;
Shape insertShape;
System.Windows.Point startPosition;

private void Mask_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    insertShape = CreateShape();
    if (insertShape != null)
    {
        drawFlag = true;
        Canvas board = sender as Canvas;
        board.Children.Clear();
        startPosition = e.GetPosition(board);
        insertShape.Opacity = ;
        Canvas.SetLeft(insertShape, e.GetPosition(board).X);
        Canvas.SetTop(insertShape, e.GetPosition(board).Y);
        board.Children.Add(insertShape);
    }
}

private void image_MouseMove(object sender, MouseEventArgs e)
{
    Canvas board = sender as Canvas;
    if (drawFlag && insertShape != null)
    {
        if (e.GetPosition(board).X > startPosition.X)
        {
            insertShape.Width = e.GetPosition(board).X - startPosition.X;

        }
        else
        {
            insertShape.Width = startPosition.X - e.GetPosition(board).X;
            Canvas.SetLeft(insertShape, e.GetPosition(board).X);
        }
        if (e.GetPosition(board).Y > startPosition.Y)
        {
            insertShape.Height = e.GetPosition(board).Y - startPosition.Y;
        }
        else
        {
            insertShape.Height = startPosition.Y - e.GetPosition(board).Y;
            Canvas.SetTop(insertShape, e.GetPosition(board).Y);
        }
    }
}

private void Mask_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
    drawFlag = false;
    if (insertShape != null)
    {
        insertShape.Opacity = ;
        System.Windows.Point p = e.GetPosition(sender as Canvas);
        Canvas.SetLeft(insertShape, e.GetPosition(board).X);
        Canvas.SetTop(insertShape, e.GetPosition(board).Y);
    }
}

本来还想做个多边形的例子，水平有限，没达到我想要的效果。研究研究再发。