 WM_GETMINMAXINFO: Restrict Form Resizing
showed how to use subclassing to intercept the form's WM_GETMINMAXINFO
message and how to change the message's RECT data into coordinates that
would
prohibit form resizing. The key to the WM_GETMINMAXINFO technique is the
fact that this message is sent to a window when the size or position of
the window is about to change, providing the opportunity
to set the minimum or maximum allowable size of the window.
Along with the WM_GETMINMAXINFO, the WM_SIZING message is also sent to
the WindowProc during a resize event. And although this message also
passes a RECT structure as part of the message parameters, it differs
from WM_GETMINMAXINFO in that it is sent to a window that the user is
currently resizing. By processing this message we can not only monitor
the size and position of the window's drag rectangle, we can also change
the drag rectangle's size or position by adjusting the values in the
RECT structure passed. By calculating the size of the
form in one direction as the window is sized we can set this same size into the RECT
structure's other dimension, thereby causing the window to resize while retaining its aspect ratio.
To keep things simple, the demo shows maintaining a square aspect ratio. The demo requires only
a command button - the other labels shown in the illustration are informative and explained below.
In addition to the RECT passed as lParam to the WindowProc with the
WM_SIZING message, wParam contains a value (the grey labels) that indicate which of the
eight possible resizing areas is being used to perform the resizing. With this
information we can code to have the aspect-maintaining equation resize
so as to nail the form at a particular corner of interest. The large red
numbers indicate the four possible 'nail' points during a resize, while
the green labels show which corner will be nailed during the resizing in
this demo. |
Option Explicit
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' Copyright ©1996-2011 VBnet/Randy Birch, All Rights Reserved.
' Some pages may also contain other copyrights by the author.
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' Distribution: You can freely use this code in your own
' applications, but you may not reproduce
' or publish this code on any web site,
' online service, or distribute as source
' on any media without express permission.
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Public defWindowProc As Long
Private Const GWL_WNDPROC As Long = (-4)
Private Const WM_DESTROY = &H2
Private Const WM_SIZING = &H214
'wParam for WM_SIZING message
Private Const WMSZ_LEFT = 1
Private Const WMSZ_RIGHT = 2
Private Const WMSZ_TOP = 3
Private Const WMSZ_TOPLEFT = 4
Private Const WMSZ_TOPRIGHT = 5
Private Const WMSZ_BOTTOM = 6
Private Const WMSZ_BOTTOMLEFT = 7
Private Const WMSZ_BOTTOMRIGHT = 8
Private Type RECT
Left As Long
Top As Long
Right As Long
Bottom As Long
End Type
Private Declare Function GetWindowLong Lib "user32" _
Alias "GetWindowLongA" _
(ByVal hwnd As Long, _
ByVal nIndex As Long) As Long
Private Declare Function SetWindowLong Lib "user32" _
Alias "SetWindowLongA" _
(ByVal hwnd As Long, _
ByVal nIndex As Long, _
ByVal dwNewLong As Long) As Long
Private Declare Function CallWindowProc Lib "user32" _
Alias "CallWindowProcA" _
(ByVal lpPrevWndFunc As Long, _
ByVal hwnd As Long, _
ByVal uMsg As Long, _
ByVal wParam As Long, _
ByVal lParam As Long) As Long
Private Declare Sub CopyMemory Lib "kernel32" _
Alias "RtlMoveMemory" _
(hpvDest As Any, _
hpvSource As Any, _
ByVal cbCopy As Long)
Public Sub Unhook(fhwnd As Long)
If defWindowProc Then
Call SetWindowLong(fhwnd, GWL_WNDPROC, defWindowProc)
defWindowProc = 0
End If
End Sub
Public Sub Hook(fhwnd As Long)
defWindowProc = SetWindowLong(fhwnd, _
GWL_WNDPROC, _
AddressOf WindowProc)
End Sub
Function WindowProc(ByVal hwnd As Long, _
ByVal uMsg As Long, _
ByVal wParam As Long, _
ByVal lParam As Long) As Long
Dim rc As RECT
Select Case uMsg
Case WM_SIZING
'copy the RECT pointed to in
'lParam into a RECT structure
CopyMemory rc, ByVal lParam, LenB(rc)
'wParam tells which one of the eight
'possible resizing handles is being used.
'Set the appropriate RECT member to the
'size required to maintain aspect ratio,
'and copy back into the RECT struct for
'processing by Windows.
Select Case wParam
Case WMSZ_LEFT
rc.Bottom = (rc.Right - rc.Left) + rc.Top
CopyMemory ByVal lParam, rc, LenB(rc)
WindowProc = 1
Case WMSZ_RIGHT
rc.Bottom = (rc.Right - rc.Left) + rc.Top
CopyMemory ByVal lParam, rc, LenB(rc)
WindowProc = 1
Case WMSZ_TOP
rc.Right = (rc.Bottom - rc.Top) + rc.Left
CopyMemory ByVal lParam, rc, LenB(rc)
WindowProc = 1
Case WMSZ_BOTTOM
rc.Right = (rc.Bottom - rc.Top) + rc.Left
CopyMemory ByVal lParam, rc, LenB(rc)
WindowProc = 1
Case WMSZ_TOPLEFT
rc.Left = (rc.Top - rc.Bottom) + (rc.Right)
CopyMemory ByVal lParam, rc, LenB(rc)
WindowProc = 1
Case WMSZ_TOPRIGHT
rc.Right = (rc.Bottom - rc.Top) + rc.Left
CopyMemory ByVal lParam, rc, LenB(rc)
WindowProc = 1
Case WMSZ_BOTTOMLEFT
rc.Bottom = (rc.Right - rc.Left) + (rc.Top)
CopyMemory ByVal lParam, rc, LenB(rc)
WindowProc = 1
Case WMSZ_BOTTOMRIGHT
rc.Bottom = (rc.Right - rc.Left) + rc.Top
CopyMemory ByVal lParam, rc, LenB(rc)
WindowProc = 1
End Select
Case WM_DESTROY:
'kill subclassing if active
If defWindowProc <> 0 Then
Call Unhook(form1.hwnd)
End If
End Select
'process windows messages
WindowProc = CallWindowProc(defWindowProc, _
hwnd, _
uMsg, _
wParam, _
lParam)
End Function |
