一、前言

專案中涉及到了心率檢測，而且資料量達到了百萬級別，通過WPF實現大資料曲線圖時，嘗試過最基礎的Canvas來實現，但是效能堪憂，而且全部畫出來也不實際。同時也嘗試過找第三方的開源庫，但是因為曲線圖涉及到很多細節功能，第三方的肯定也沒法滿足。沒辦法，只能自己實現，上網查詢後發現DrawingVisual這個玩意可以實現高效能畫圖，同時再搭配區域性顯示，這樣就能實現自己想要的效果。話不多說，今天把大致的實現思路寫一下，就不直接把專案的原始碼貼出來，寫個簡單的Demo就好了。

二、正文

1、首先新建個專案，然後建立個自定義控制元件，命名為CurveChartDrawingVisual，然後讓它繼承FrameworkElement。因為要使用DrawingVisual物件的話，需要為它建立一個主機容器。關於其他相關DrawingVisual的細節這裡不做過多闡述，不明白的可以去微軟官網看。

2、實現的具體程式碼如下，相關細節有備註標註了。這裡記得要重寫一下VisualChildrenCount屬性和重寫GetVisualChild()方法，不然圖畫不出來

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

    private double offset_x = 0;//滑動條偏移值
    private double
 
 y_scale;//y軸放心縮放比例

    private List<int> list_points;//曲線資料

    private static int Top_Val_Max = 100;//y軸最大值
    private static int Top_Val_Min = 0;//y軸最小值
    private static int X_Sex = 20;//x軸分度值
    private static int Y_Sex = 20;//x軸分度值
    private static int Bottom = 30;//底部x軸座標顯示高度

    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();//凍結筆，提高效能關鍵所在
        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));

        //橫線
        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);
        }

        //豎線與文字
        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軸文字
            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();
                    }
                }
                //深色豎線
                dc.DrawLine(primarygrid_pen, point1, point2);
                continue;
            }
            //淺色豎線
            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、接著測試一下，開啟MainWindow，新增我們的自定義控制元件，這裡區域性顯示需要搭配一個ScrollViewer來實現，程式碼如下

<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、接著就是後臺程式碼，比較簡單，就是自動生成一個List，然後傳給自定義控制元件，Canvas的寬度直接設定為List的長度，因為這裡是水平方向一個畫素點畫一個點，然後滑動條滑動時再將對應的偏移值傳遞到控制元件，再通過偏移值更新檢視

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、執行效果如下，滑動條拖到哪裡就顯示哪裡，這樣就算資料量再大也沒問題