WPF开发之利用DrawingVisual绘制高性能曲线图

前言

项目中涉及到了心率检测，而且数据量达到了百万级别，通过WPF实现大数据曲线图时，尝试过最基础的Canvas来实现，但是性能堪忧，而且全部画出来也不实际。同时也尝试过找第三方的开源库，但是因为曲线图涉及到很多细节功能，第三方的肯定也没法满足。没办法，只能自己实现，上网查找后发现DrawingVisual这个玩意可以实现高性能画图，同时再搭配局部显示，这样就能实现自己想要的效果。话不多说，今天把大致的实现思路写一下，就不直接把项目的源码贴出来，写个简单的Demo就好了。

正文

1、首先新建个项目，然后创建个自定义控件，命名为CurveChartDrawingVisual，然后让它继承FrameworkElement。因为要使用DrawingVisual对象的话，需要为它创建一个主机容器。关于其他相关DrawingVisual的细节这里不做过多阐述，不明白的可以去微软官网看。

2、实现的具体代码如下，相关细节有备注标注了。这里记得要重写一下VisualChildrenCount属性和重写GetVisualChild()方法，不然图画不出来

public class CruveChartDrawingVisual : FrameworkElement
{
    private List<Visual> visuals = new List<Visual>();
    private DrawingVisual Layer;

    private double offset_x = 0;//滑动条偏移值
    private double y_scale;//y轴放心缩放比例

    private List<int> list_points;//曲线数据

    private static int Top_Val_Max = 100;//y轴最大值
    private static int Top_Val_Min = 0;//y轴最小值
    private static int X_Sex = 20;//x轴分度值
    private static int Y_Sex = 20;//x轴分度值
    private static int Bottom = 30;//底部x轴坐标显示高度

    Pen pen = new Pen(Brushes.Green, 2);
    Pen primarygrid_pen = new Pen(Brushes.Black, 1);
    Pen secondgrid_pen = new Pen(Brushes.Gray, 1);

    public CruveChartDrawingVisual()
    {
        pen.Freeze();//冻结笔，提高性能关键所在
        primarygrid_pen.Freeze();
        secondgrid_pen.Freeze();

        Layer = new DrawingVisual();

        visuals.Add(Layer);
    }

    public void SetupData(List<int> points)
    {
        list_points = points;
        offset_x = 0;
        DrawContent();
    }

    public void OffsetX(double offset)
    {
        offset_x = offset;
        DrawContent();
        InvalidateVisual();
    }

    private void DrawContent()
    {
        var dc = Layer.RenderOpen();
        y_scale = (RenderSize.Height - Bottom) / (Top_Val_Max - Top_Val_Min);

        var mat = new Matrix();
        mat.ScaleAt(1, -1, 0, RenderSize.Height / 2);

        mat.OffsetX = -offset_x;
        dc.PushTransform(new MatrixTransform(mat));

        //横线
        for (int y = 0; y <= Top_Val_Max - Top_Val_Min; y += 10)
        {
            Point point1 = new Point(offset_x, y * y_scale + Bottom);
            Point point2 = new Point(offset_x + RenderSize.Width, y * y_scale + Bottom);
            if (y % Y_Sex == 0)
            {
                dc.DrawLine(primarygrid_pen, point1, point2);
                continue;
            }
            dc.DrawLine(secondgrid_pen, point1, point2);
        }

        //竖线与文字
        for (int i = 0; i <= (offset_x + RenderSize.Width); i += X_Sex * 2)
        {
            if (i < offset_x)
            {
                continue;
            }
            var point1 = new Point(i, Bottom);
            var point2 = new Point(i, (Top_Val_Max - Top_Val_Min) * y_scale + Bottom);

            //y轴文字
            if (i % 100 == 0)
            {
                var text1 = new FormattedText(i + "", CultureInfo.CurrentCulture, FlowDirection.LeftToRight, new Typeface("Verdana"), 16, Brushes.Black);
                var mat3 = new Matrix();
                mat3.ScaleAt(1, -1, i - text1.Width / 2, 8 + text1.Height / 2);
                dc.PushTransform(new MatrixTransform(mat3));
                dc.DrawText(text1, new Point(i - text1.Width / 2, 8));
                dc.Pop();
            }

            //表格刻度文字
            if (i % 100 == 0)
            {
                for (int y = Top_Val_Min; y <= Top_Val_Max; y += 10)
                {
                    if (y % Y_Sex == 0)
                    {
                        var text1 = new FormattedText(y + "", CultureInfo.CurrentCulture, FlowDirection.LeftToRight, new Typeface("Verdana"), 12, Brushes.Black);
                        var mat3 = new Matrix();
                        mat3.ScaleAt(1, -1, i + 1, (y - Top_Val_Min) * y_scale + Bottom + text1.Height / 2);
                        dc.PushTransform(new MatrixTransform(mat3));
                        dc.DrawText(text1, new Point(i + 1, (y - Top_Val_Min) * y_scale + Bottom));
                        dc.Pop();
                    }
                }
                //深色竖线
                dc.DrawLine(primarygrid_pen, point1, point2);
                continue;
            }
            //浅色竖线
            dc.DrawLine(secondgrid_pen, point1, point2);
        }

        if (list_points != null)
        {
            for (int i = (int)offset_x; i < list_points.Count - 1; i++)
            {
                if (i > offset_x + RenderSize.Width)
                {
                    break;
                }
                dc.DrawLine(pen, new Point(i, list_points[i] * y_scale + Bottom), new Point(i + 1, list_points[i + 1] * y_scale + Bottom));
            }
        }

        dc.Pop();
        dc.Close();
    }

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

    protected override void OnRenderSizeChanged(SizeChangedInfo sizeInfo)
    {
        DrawContent();
        base.OnRenderSizeChanged(sizeInfo);
    }

    protected override void OnRender(DrawingContext drawingContext)
    {
        drawingContext.DrawRectangle(Brushes.White, null, new Rect(0, 0, RenderSize.Width, RenderSize.Height));
        base.OnRender(drawingContext);
    }
}

3、接着测试一下，打开MainWindow，添加我们的自定义控件，这里局部显示需要搭配一个ScrollViewer来实现，记得这里没有将我们的自定义控件嵌入ScrollViewer，而是放一个Canvas来填充，代码如下

<Grid>
    <local:CruveChartDrawingVisual x:Name="curve" Margin="0,15,0,20" />
    <ScrollViewer
        Name="scroll"
        HorizontalScrollBarVisibility="Auto"
        ScrollChanged="ScrollViewer_ScrollChanged"
        VerticalScrollBarVisibility="Disabled">
        <Canvas x:Name="canvas" Height="1" />
    </ScrollViewer>
</Grid>

4、接着就是后台代码，比较简单，就是自动生成一个List，然后传给自定义控件，Canvas的宽度直接设置为List的长度，因为这里是水平方向一个像素点画一个点，然后滑动条滑动时再将对应的偏移值传递到控件，再通过偏移值更新视图

public partial class MainWindow : Window
{
    private bool isAdd = true;

    public MainWindow()
    {
        InitializeComponent();
    }

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        List<int> lists = new List<int>();
        int temp = 20;
        for (int i = 0; i < 60 * 60; i++)
        {
            if (isAdd)
            {
                lists.Add(temp);
                temp ++;
            }
            else
            {
                lists.Add(temp);
                temp --;
            }

            if (temp == 90) isAdd = false;
            if (temp == 10) isAdd = true;
        }

        canvas.Width = lists.Count;

        curve.SetupData(lists);
    }

    private void ScrollViewer_ScrollChanged(object sender, ScrollChangedEventArgs e)
    {
        curve.OffsetX(scroll.HorizontalOffset);
    }
}

5、运行效果如下，滑动条拖到哪里就显示哪里，这样就算数据量再大也没问题，这种曲线图跟常规的曲线图有点差别，这里更多的是提供一种思路

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