Setting the Scrollbar Thumb size

Question

I am attempting to work out the algorithm associated with sizing of the WPF Scrollbar thumb element.

The thumb element can be sized using the Scrollbar.ViewportSi

Answer 1

On my side, I preserved a minimum thumb length because touch inputs require a thumb of a minimum size to be touch optimized.

You can define a ScrollViewer ControlTemplate that will use the TouchScrollBar as its horisontal and vertical ScrollBar.

See UpdateViewPort method for the math.

Sorry, I don't see the use case for explicitly setting the scrollbar thumb to cover a percentage of the track length

public class TouchScrollBar : System.Windows.Controls.Primitives.ScrollBar
{
    #region Fields

    #region Dependency properties

    public static readonly DependencyProperty MinThumbLengthProperty =
        DependencyProperty.Register
        ("MinThumbLength", typeof(double), typeof(TouchScrollBar), new UIPropertyMetadata((double)0, OnMinThumbLengthPropertyChanged));

    #endregion

    private double? m_originalViewportSize;

    #endregion

    #region Properties

    public double MinThumbLength
    {
        get { return (double)GetValue(MinThumbLengthProperty); }
        set { SetValue(MinThumbLengthProperty, value); }
    }

    #endregion

    #region Constructors

    public TouchScrollBar()
    {
        SizeChanged += OnSizeChanged;
    }

    private bool m_trackSubscribed;
    void OnSizeChanged(object sender, SizeChangedEventArgs e)
    {
        SubscribeTrack();
    }

    private void SubscribeTrack()
    {
        if (!m_trackSubscribed && Track != null)
        {
            Track.SizeChanged += OnTrackSizeChanged;
            m_trackSubscribed = true;
        }

    }

    #endregion

    #region Protected and private methods

    #region Event handlers

    #region Dependency properties event handlers

    private void OnMinThumbLengthPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        TouchScrollBar instance = d as TouchScrollBar;
        if(instance != null)
        {
            instance.OnMinThumbLengthChanged(e);

        }
    }

    #endregion

    protected void OnTrackSizeChanged(object sender, SizeChangedEventArgs e)
    {
        SubscribeTrack();
        UpdateViewPort();
    }

    protected override void OnMaximumChanged(double oldMaximum, double newMaximum)
    {
        base.OnMaximumChanged(oldMaximum, newMaximum);

        SubscribeTrack();
        UpdateViewPort();
    }

    protected override void OnMinimumChanged(double oldMinimum, double newMinimum)
    {
        base.OnMinimumChanged(oldMinimum, newMinimum);

        SubscribeTrack();
        UpdateViewPort();
    }

    protected void OnMinThumbLengthChanged(DependencyPropertyChangedEventArgs e)
    {
        SubscribeTrack();
        UpdateViewPort();
    }

    #endregion

    private void UpdateViewPort()
    {
        if(Track != null)
        {
            if(m_originalViewportSize == null)
            {
                m_originalViewportSize = ViewportSize;
            }

            double trackLength = Orientation == Orientation.Vertical ? Track.ActualHeight : Track.ActualWidth;
            double thumbHeight = m_originalViewportSize.Value / (Maximum - Minimum + m_originalViewportSize.Value) * trackLength;
            if (thumbHeight < MinThumbLength && !double.IsNaN(thumbHeight))
            {
                ViewportSize = (MinThumbLength * (Maximum - Minimum)) / (trackLength + MinThumbLength);
            }
        }
    }


    #endregion
}

}