Difference between revisions of "Dragging windows"

Revision as of 13:03, 24 April 2006

So, you want to drag a window without using the real title bar?

I have seen lots of people using their own code in the mouse move event to do this, and I've even done it myself.

Until I found the following code:

Declare Function ReleaseCapture Lib "user32" () As Long
Declare Function SendMessage Lib "user32" Alias "SendMessageA" _
  (ByVal hWnd As Long, ByVal wMsg As Long, ByVal wParam As Long, _
  lParam As Any) As Long

Const WM_NCLBUTTONDOWN = &HA1
Const HTCAPTION = 2

And when you want to do the dragging itself:

ReleaseCapture
SendMessage Form.hWnd, WM_NCLBUTTONDOWN, HTCAPTION, ByVal 0&

This should go in the mousedown event of the control/object you want to drag the window by.

It has various advantages over home grown code:

1. It's faster and rarely suffers from flickering.
2. It's easier. Two lines once compared to a huge routine (or two?)
3. It obeys the windows rules and settings on drawing windows while moving.

The only problems I have found with this are:

1. It doesn't work when you use the right mouse button (although this may be just Windows XP)
2. You cant limit it to only moving in one direction

If you want to use this to drag a control, just replace Form.hWnd with Control.hWnd. This will NOT work for dragging windowless controls like Labels or ImageBoxes but you can still drag a windowed control by them.

Resizing windows

A small variation of this code can be used to resize forms and controls by just replacing HTCAPTION with HTBOTTOMRIGHT:

Const HTBOTTOMRIGHT = 17

ReleaseCapture
SendMessage Form.hWnd, WM_NCLBUTTONDOWN, HTBOTTOMRIGHT, ByVal 0&

See also

• Earlsoft ReleaseCapture API page