drag to pan on an UserControl - winforms

Question

I'm trying to build my own "PictureBox like" control adding some functionalities. For example, I want to be able to pan over a big image by simply clicking and dragging with the mouse.
The problem seems to be on my OnMouseMove method. If I use the following code I get the drag speed and precision I want, but of course, when I release the mouse button and try to drag again the image is restored to its original position.
using System.Drawing;
using System.Windows.Forms;
namespace Testing
{
public partial class ScrollablePictureBox : UserControl
{
private Image image;
private bool centerImage;
public Image Image
{
get { return image; }
set { image = value; Invalidate(); }
}
public bool CenterImage
{
get { return centerImage; }
set { centerImage = value; Invalidate(); }
}
public ScrollablePictureBox()
{
InitializeComponent();
SetStyle(ControlStyles.AllPaintingInWmPaint | ControlStyles.OptimizedDoubleBuffer, true);
Image = null;
AutoScroll = true;
AutoScrollMinSize = new Size(0, 0);
}
private Point clickPosition;
private Point scrollPosition;
protected override void OnMouseDown(MouseEventArgs e)
{
base.OnMouseDown(e);
clickPosition.X = e.X;
clickPosition.Y = e.Y;
}
protected override void OnMouseMove(MouseEventArgs e)
{
base.OnMouseMove(e);
if (e.Button == MouseButtons.Left)
{
scrollPosition.X = clickPosition.X - e.X;
scrollPosition.Y = clickPosition.Y - e.Y;
AutoScrollPosition = scrollPosition;
}
}
protected override void OnPaint(PaintEventArgs e)
{
base.OnPaint(e);
e.Graphics.FillRectangle(new Pen(BackColor).Brush, 0, 0, e.ClipRectangle.Width, e.ClipRectangle.Height);
if (Image == null)
return;
int centeredX = AutoScrollPosition.X;
int centeredY = AutoScrollPosition.Y;
if (CenterImage)
{
//Something not relevant
}
AutoScrollMinSize = new Size(Image.Width, Image.Height);
e.Graphics.DrawImage(Image, new RectangleF(centeredX, centeredY, Image.Width, Image.Height));
}
}
}
But if I modify my OnMouseMove method to look like this:
protected override void OnMouseMove(MouseEventArgs e)
{
base.OnMouseMove(e);
if (e.Button == MouseButtons.Left)
{
scrollPosition.X += clickPosition.X - e.X;
scrollPosition.Y += clickPosition.Y - e.Y;
AutoScrollPosition = scrollPosition;
}
}
... you will see that the dragging is not smooth as before, and sometimes behaves weird (like with lag or something).
What am I doing wrong?
I've also tried removing all "base" calls on a desperate movement to solve this issue, haha, but again, it didn't work.
Thanks for your time.

Answer 1

Finally, I managed to find a solution:
protected Point clickPosition;
protected Point scrollPosition;
protected Point lastPosition;
protected override void OnMouseDown(MouseEventArgs e)
{
clickPosition.X = e.X;
clickPosition.Y = e.Y;
}
protected override void OnMouseUp(MouseEventArgs e)
{
Cursor = Cursors.Default;
lastPosition.X = AutoScrollPosition.X;
lastPosition.Y = AutoScrollPosition.Y;
}
protected override void OnMouseMove(MouseEventArgs e)
{
if (e.Button == MouseButtons.Left)
{
Cursor = Cursors.Hand;
scrollPosition.X = clickPosition.X - e.X - lastPosition.X;
scrollPosition.Y = clickPosition.Y - e.Y - lastPosition.Y;
AutoScrollPosition = scrollPosition;
}
}

Answer 2

This is always confusing every time I have to do it.
For the benefit of those newly arriving to this thread I came up with a slightly simpler solution that gives smooth panning.
Private GrabPoint As Point
Private Sub OnMouseDown(MouseEventArgs e)
GrabPoint = e.Location
End Sub
Private Sub OnMouseMove(MouseEventArgs e)
If e.Button = System.Windows.Forms.MouseButtons.Left Then
AutoScrollPosition = GrabPoint - e.Location - AutoScrollPosition
End If
End Sub
Private Sub OnMouseUp(MouseEventArgs e)
GrabPoint = Point.Empty
End Sub
By the way, the grab and grabbing hand cursors can be downloaded from:
http://theburningmonk.com/2010/03/wpf-loading-grab-and-grabbing-cursors-from-resource/
You can add them to your project as embedded resources and set them with:
Cursor = New Cursor(System.Reflection.Assembly.GetExecutingAssembly.GetManifestResourceStream(String.Format("{0}.{1}.cur", Me.GetType.Namespace, "grabbing")))