WPF simple zoom and drag support in a ScrollViewer

• Download demo - 24.3 KB

Introduction

I've been looking for a while for Open Source solutions that show a simple way of zooming and dragging arbitrary content which is hosted and managed by a ScrollViewer.

As I did not find a free one, I decided to write my own one and share it with you. It supports zooming by a slider as well as by the mouse wheel.

Using the code

If you have any questions regarding the usage, please feel free to post and I'll try to get back to you ASAP. 

The main view is defined by the XAML below. The content that is to be zoomed and dragged is part of the "grid" control.

<Window x:Class="ZoomExample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
        Title="MainWindow" Height="500" Width="500">
    <Window.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Resources.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Window.Resources>
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <Slider Grid.Column="0" Orientation="Vertical" 
           HorizontalAlignment="Left" Minimum="1" x:Name="slider"/>
        <ScrollViewer Name="scrollViewer" Grid.Column="1" 
              VerticalScrollBarVisibility="Visible" 
              HorizontalScrollBarVisibility="Visible">
            
            <Grid Name="grid" Width="400" 
              Height="400" RenderTransformOrigin="0.5,0.5">
                <Grid.LayoutTransform>
                    <TransformGroup>
                        <ScaleTransform x:Name="scaleTransform"/>
                    </TransformGroup>
                </Grid.LayoutTransform>
                <Viewbox Grid.Column="0" Grid.Row="0">
                    <ContentPresenter Content="{StaticResource Kompass}"/>
                </Viewbox>
            </Grid>
            
        </ScrollViewer>
    </Grid>
</Window>

Zooming and dragging is managed by the code-behind:

public partial class MainWindow : Window
{
    Point? lastCenterPositionOnTarget;
    Point? lastMousePositionOnTarget;
    Point? lastDragPoint;

    public MainWindow()
    {
        InitializeComponent();

        scrollViewer.ScrollChanged += OnScrollViewerScrollChanged;
        scrollViewer.MouseLeftButtonUp += OnMouseLeftButtonUp;
        scrollViewer.PreviewMouseLeftButtonUp += OnMouseLeftButtonUp;
        scrollViewer.PreviewMouseWheel += OnPreviewMouseWheel;

        scrollViewer.PreviewMouseLeftButtonDown += OnMouseLeftButtonDown;
        scrollViewer.MouseMove += OnMouseMove;

        slider.ValueChanged += OnSliderValueChanged;
    }

    void OnMouseMove(object sender, MouseEventArgs e)
    {
        if (lastDragPoint.HasValue)
        {
            Point posNow = e.GetPosition(scrollViewer);

            double dX = posNow.X - lastDragPoint.Value.X;
            double dY = posNow.Y - lastDragPoint.Value.Y;

            lastDragPoint = posNow;

            scrollViewer.ScrollToHorizontalOffset(scrollViewer.HorizontalOffset - dX);
            scrollViewer.ScrollToVerticalOffset(scrollViewer.VerticalOffset - dY);
        }
    }

    void OnMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        var mousePos = e.GetPosition(scrollViewer);
        if (mousePos.X <= scrollViewer.ViewportWidth && mousePos.Y < 
            scrollViewer.ViewportHeight) //make sure we still can use the scrollbars
        {
            scrollViewer.Cursor = Cursors.SizeAll;
            lastDragPoint = mousePos;
            Mouse.Capture(scrollViewer);
        }
    }

    void OnPreviewMouseWheel(object sender, MouseWheelEventArgs e)
    {
        lastMousePositionOnTarget = Mouse.GetPosition(grid);

        if (e.Delta > 0)
        {
            slider.Value += 1;
        }
        if (e.Delta < 0)
        {
            slider.Value -= 1;
        }

        e.Handled = true;
    }

    void OnMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
    {
        scrollViewer.Cursor = Cursors.Arrow;
        scrollViewer.ReleaseMouseCapture();
        lastDragPoint = null;
    }

    void OnSliderValueChanged(object sender, 
         RoutedPropertyChangedEventArgs<double> e)
    {
        scaleTransform.ScaleX = e.NewValue;
        scaleTransform.ScaleY = e.NewValue;

        var centerOfViewport = new Point(scrollViewer.ViewportWidth/2, 
                                         scrollViewer.ViewportHeight/2);
        lastCenterPositionOnTarget = scrollViewer.TranslatePoint(centerOfViewport, grid);
    }

    void OnScrollViewerScrollChanged(object sender, ScrollChangedEventArgs e)
    {
        if (e.ExtentHeightChange != 0 || e.ExtentWidthChange != 0)
        {
            Point? targetBefore = null;
            Point? targetNow = null;

            if (!lastMousePositionOnTarget.HasValue)
            {
                if (lastCenterPositionOnTarget.HasValue)
                {
                    var centerOfViewport = new Point(scrollViewer.ViewportWidth/2, 
                                                     scrollViewer.ViewportHeight/2);
                    Point centerOfTargetNow = 
                          scrollViewer.TranslatePoint(centerOfViewport, grid);

                    targetBefore = lastCenterPositionOnTarget;
                    targetNow = centerOfTargetNow;
                }
            }
            else
            {
                targetBefore = lastMousePositionOnTarget;
                targetNow = Mouse.GetPosition(grid);

                lastMousePositionOnTarget = null;
            }

            if (targetBefore.HasValue)
            {
                double dXInTargetPixels = targetNow.Value.X - targetBefore.Value.X;
                double dYInTargetPixels = targetNow.Value.Y - targetBefore.Value.Y;

                double multiplicatorX = e.ExtentWidth/grid.Width;
                double multiplicatorY = e.ExtentHeight/grid.Height;

                double newOffsetX = scrollViewer.HorizontalOffset - 
                                    dXInTargetPixels*multiplicatorX;
                double newOffsetY = scrollViewer.VerticalOffset - 
                                    dYInTargetPixels*multiplicatorY;

                if (double.IsNaN(newOffsetX) || double.IsNaN(newOffsetY))
                {
                    return;
                }

                scrollViewer.ScrollToHorizontalOffset(newOffsetX);
                scrollViewer.ScrollToVerticalOffset(newOffsetY);
            }
        }
    }
}