How to speed up rendering of vertical scrollbar markers

Question

I have a customized vertical scrollbar which displays markers for selected items in a DataGrid.

Answer 1

Here you go, I tried to attempt it for you.

• Created a class MarkerCanvas deriving Canvas with a property to bind with the data grid
• Attached SelectionChanged to listen to any change and requested the canvas to redraw itself by InvalidateVisual
• overrided the method OnRender to take control of drawing and did the necessary check and calculation
• finally rendered the rectangle on the calculated coordinates using the given brush

MarkerCanvas class

class MarkerCanvas : Canvas
{
    public DataGrid Grid
    {
        get { return (DataGrid)GetValue(GridProperty); }
        set { SetValue(GridProperty, value); }
    }

    // Using a DependencyProperty as the backing store for Grid.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty GridProperty =
        DependencyProperty.Register("Grid", typeof(DataGrid), typeof(MarkerCanvas), new PropertyMetadata(null, OnGridChanged));

    private static void OnGridChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        MarkerCanvas canvas = d as MarkerCanvas;

        if (e.NewValue != null)
        {
            (e.NewValue as DataGrid).SelectionChanged += canvas.MarkerCanvas_SelectionChanged;
        }
        if (e.OldValue != null)
        {
            (e.NewValue as DataGrid).SelectionChanged -= canvas.MarkerCanvas_SelectionChanged;
        }
    }

    void MarkerCanvas_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        InvalidateVisual();
    }

    public Brush MarkerBrush
    {
        get { return (Brush)GetValue(MarkerBrushProperty); }
        set { SetValue(MarkerBrushProperty, value); }
    }

    // Using a DependencyProperty as the backing store for MarkerBrush.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty MarkerBrushProperty =
        DependencyProperty.Register("MarkerBrush", typeof(Brush), typeof(MarkerCanvas), new PropertyMetadata(Brushes.SlateGray));


    protected override void OnRender(System.Windows.Media.DrawingContext dc)
    {
        base.OnRender(dc);

        if (Grid==null || Grid.SelectedItems == null)
            return;

        object[] markers = Grid.SelectedItems.OfType