ACC2000: How to Detect and Prevent the Mouse Wheel from Scrolling Through Records in a Form (Q278379)
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The information in this article applies to:
Microsoft Access 2000
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Advanced: Requires expert coding, interoperability, and multiuser skills.
This article applies to a Microsoft Access database (.mdb) and a Microsoft Access project (.adp).
SUMMARY
Microsoft Access does not provide a method for preventing users from using the mouse wheel to scroll through records on a form. This article shows you how to programmatically prevent users from using the mouse wheel to scroll through records on a form.
MORE INFORMATION
Microsoft provides programming examples for illustration only, without warranty either expressed or implied, including, but not limited to, the implied warranties of merchantability and/or fitness for a particular purpose. This article assumes that you are familiar with the programming language being demonstrated and the tools used to create and debug procedures. Microsoft support professionals can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific needs. If you have limited programming experience, you may want to contact a Microsoft Certified Partner or the Microsoft fee-based consulting line at (800) 936-5200. For more information about Microsoft Certified Partners, please see the following page on the World Wide Web:
http://support.microsoft.com/default.aspx?scid=http://www.microsoft.com/partner/refer ral/
For more information about the support options available from Microsoft, please see the following page on the World Wide Web:
http://support.microsoft.com/default.aspx?scid=http://support.microsoft.com/ directory/overview.asp
By default, users can roll the mouse wheel to scroll through a series of records in a Microsoft Access form. If you want to prevent this, you can use the Win32 API to subclass your forms and to ignore mouse wheel messages sent to the form. There are two approaches for doing this. The first approach is to use Microsoft Visual Basic or Microsoft Visual C++ to create an ActiveX DLL that subclasses your Microsoft Access forms, and then to reference that DLL from your Microsoft Access application. A second method is to write all the code within Microsoft Access itself without using an ActiveX DLL. Because of problems with subclassing windows after loading the Microsoft Office Visual Basic Editor, Microsoft highly recommends that you use Microsoft Visual Basic or Microsoft Visual C++ to create an ActiveX DLL, and that you then reference the DLL from your Microsoft Access application.
Creating the MouseWheel Event by Using a Visual Basic ActiveX DLL
Start Microsoft Visual Basic 6.0.
Create a new ActiveX DLL project, and then open it.
Add the following code to the class module window that appears:
Option Compare Text
Option Explicit
Private frm As Object
Private intCancel As Integer
Public Event MouseWheel(Cancel As Integer)
Public Property Set Form(frmIn As Object)
Set frm = frmIn
End Property
Public Property Get MouseWheelCancel() As Integer
MouseWheelCancel = intCancel
End Property
Public Sub SubClassHookForm()
lpPrevWndProc = SetWindowLong(frm.hwnd, GWL_WNDPROC, _
AddressOf WindowProc)
Set CMouse = Me
End Sub
Public Sub SubClassUnHookForm()
Call SetWindowLong(frm.hwnd, GWL_WNDPROC, lpPrevWndProc)
End Sub
Public Sub FireMouseWheel()
RaiseEvent MouseWheel(intCancel)
End Sub
Set the class module properties as follows:
Class Module: CMouseWheel
-------------------------
Name: CMouseWheel
Instancing: 5 - MultiUse
Add a standard module to the project, and then add the following code:
Option Compare Text
Option Explicit
Public CMouse As CMouseWheel
Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" _
(ByVal hwnd As Long, _
ByVal nIndex As Long, _
ByVal dwNewLong As Long) As Long
Public Declare Function CallWindowProc Lib "user32" Alias "CallWindowProcA" _
(ByVal lpPrevWndFunc As Long, _
ByVal hwnd As Long, _
ByVal msg As Long, _
ByVal wParam As Long, _
ByVal lParam As Long) As Long
Public Const GWL_WNDPROC = -4
Public Const WM_MouseWheel = &H20A
Public lpPrevWndProc As Long
Public Function WindowProc(ByVal hwnd As Long, _
ByVal uMsg As Long, _
ByVal wParam As Long, _
ByVal lParam As Long) As Long
Select Case uMsg
Case WM_MouseWheel
CMouse.FireMouseWheel
If CMouse.MouseWheelCancel = False Then
WindowProc = CallWindowProc(lpPrevWndProc, hwnd, uMsg, wParam, lParam)
End If
Case Else
WindowProc = CallWindowProc(lpPrevWndProc, hwnd, uMsg, wParam, lParam)
End Select
End Function
On the View menu, click Project Explorer to view the Project Explorer.
Select the project node at the very top of the Project Explorer.
On the View menu, click Properties to view the property sheet for the project.
Set the project's Name property to MouseWheel .
On the File menu, click Save Project .
Save the project files as basSubClassWindow.bas , CMouseWheel.cls , and MouseWheel.vbp respectively.
On the File menu, click Make MouseWheel.dll , and then click OK to make the DLL.
Quit Microsoft Visual Basic.
Start Microsoft Access, and then open the sample database Northwind.mdb.
Open the Customers form in Design view.
On the View menu, click Code to display the module of the form in the Visual Basic Editor.
On the Tools menu, click References .
Click to select the check box next to MouseWheel to select the reference. If you do not see this reference listed, click the Browse button, click MouseWheel.DLL in the folder that you saved it to in step 12, and then click Open .
Click OK to close the References dialog box.
Add the following code to the module of the form:
Option Compare Database
Option Explicit
Private WithEvents clsMouseWheel As MouseWheel.CMouseWheel
Private Sub Form_Load()
Set clsMouseWheel = New MouseWheel.CMouseWheel
Set clsMouseWheel.Form = Me
clsMouseWheel.SubClassHookForm
End Sub
Private Sub Form_Close()
clsMouseWheel.SubClassUnHookForm
Set clsMouseWheel.Form = Nothing
Set clsMouseWheel = Nothing
End Sub
Private Sub clsMouseWheel_MouseWheel(Cancel As Integer)
MsgBox "You cannot use the mouse wheel to scroll records."
Cancel = True
End Sub
On the File menu, click Close and Return to Microsoft Access .
Save the form, and then close it.
Open the Customers form in Form view.
Roll the mouse wheel.
Note that you receive the message:
You cannot use the mouse wheel to scroll records.
Also note that the current record has not changed, indicating that the mouse wheel message was not processed by Microsoft Access.
Creating the MouseWheel Event Completely Within Microsoft Access
WARNING: If possible, you should use the method listed in the "Creating the MouseWheel Event by Using a Visual Basic ActiveX DLL" section earlier in this article. You can use the method discussed in this section only in a situation where the users of your application will not be loading the Visual Basic Editor within Microsoft Access, such as in a Microsoft Access run-time application. If you implement this solution and your users open the Visual Basic Editor, the code in this section will cause Microsoft Access to stop responding. Additionally, you must quit and restart Microsoft Access before testing this code if you have loaded the Visual Basic Editor at least once during the Microsoft Access session. Microsoft highly recommends that you save your work often and that you keep current backups of your database if you use this approach.
The approach demonstrated in this section shows how to use a custom class module to create a custom event named MouseWheel , which you can use in your forms to detect when the user has rolled the mouse wheel. This custom event exposes a Cancel argument that you can use to prevent the mouse wheel roll message from being intercepted by Microsoft Access, thereby preventing record scrolling in the form.
To create the custom procedures, follow these steps:
CAUTION : Following the steps in this example will modify the sample database Northwind.mdb. You may want to back up the Northwind.mdb file and perform these steps on a copy of the database.
Start Microsoft Access.
Open the sample database Northwind.mdb.
On the Insert menu, click Module to create a new module in the Visual Basic Editor.
Add the following code to the module:
Option Compare Database
Option Explicit
Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" _
(ByVal hwnd As Long, _
ByVal nIndex As Long, _
ByVal dwNewLong As Long) As Long
Public Declare Function CallWindowProc Lib "user32" Alias "CallWindowProcA" _
(ByVal lpPrevWndFunc As Long, _
ByVal hwnd As Long, _
ByVal msg As Long, _
ByVal wParam As Long, _
ByVal lParam As Long) As Long
Public Const GWL_WNDPROC = -4
Public Const WM_MouseWheel = &H20A
Public lpPrevWndProc As Long
Public CMouse As CMouseWheel
Public Function WindowProc(ByVal hwnd As Long, _
ByVal uMsg As Long, _
ByVal wParam As Long, _
ByVal lParam As Long) As Long
'Look at the message passed to the window. If it is
'a mouse wheel message, call the FireMouseWheel procedure
'in the CMouseWheel class, which in turn raises the MouseWheel
'event. If the Cancel argument in the form event procedure is
'set to False, then we process the message normally, otherwise
'we ignore it. If the message is something other than the mouse
'wheel, then process it normally
Select Case uMsg
Case WM_MouseWheel
CMouse.FireMouseWheel
If CMouse.MouseWheelCancel = False Then
WindowProc = CallWindowProc(lpPrevWndProc, hwnd, uMsg, wParam, lParam)
End If
Case Else
WindowProc = CallWindowProc(lpPrevWndProc, hwnd, uMsg, wParam, lParam)
End Select
End Function
On the File menu, click Save . Save the module as basSubClassWindow .
On the Insert menu, click Class Module .
Add the following code to the class module:
Option Compare Database
Option Explicit
Private frm As Access.Form
Private intCancel As Integer
Public Event MouseWheel(Cancel As Integer)
Public Property Set Form(frmIn As Access.Form)
'Define Property procedure for the class which
'allows us to set the Form object we are
'using with it. This property is set from the
'form class module.
Set frm = frmIn
End Property
Public Property Get MouseWheelCancel() As Integer
'Define Property procedure for the class which
'allows us to retrieve whether or not the Form
'event procedure canceled the MouseWheel event.
'This property is retrieved by the WindowProc
'function in the standard basSubClassWindow
'module.
MouseWheelCancel = intCancel
End Property
Public Sub SubClassHookForm()
'Called from the form's OnOpen or OnLoad
'event. This procedure is what "hooks" or
'subclasses the form window. If you hook the
'the form window, you must unhook it when completed
'or Access will crash.
lpPrevWndProc = SetWindowLong(frm.hwnd, GWL_WNDPROC, _
AddressOf WindowProc)
Set CMouse = Me
End Sub
Public Sub SubClassUnHookForm()
'Called from the form's OnClose event.
'This procedure must be called to unhook the
'form window if the SubClassHookForm procedure
'has previously been called. Otherwise, Access will
'crash.
Call SetWindowLong(frm.hwnd, GWL_WNDPROC, lpPrevWndProc)
End Sub
Public Sub FireMouseWheel()
'Called from the WindowProc function in the
'basSubClassWindow module. Used to raise the
'MouseWheel event when the WindowProc function
'intercepts a mouse wheel message.
RaiseEvent MouseWheel(intCancel)
End Sub
On the File menu, click Save . Save the class module as CMouseWheel .
Open the Customers form in Design view.
On the View menu, click Code to view the class module of the form.
Add the following code to the class module of the form:
Option Compare Database
Option Explicit
'Declare a module level variable as the custom class
'and give us access to the class's events
Private WithEvents clsMouseWheel As CMouseWheel
Private Sub Form_Load()
'Create a new instance of the class,
'and set the class's Form property to
'the current form
Set clsMouseWheel = New CMouseWheel
Set clsMouseWheel.Form = Me
'Subclass the current form by calling
'the SubClassHookForm method in the class
clsMouseWheel.SubClassHookForm
End Sub
Private Sub Form_Close()
'Unhook the form by calling the
'SubClassUnhook form method in the
'class, and then destroy the object
'variable
clsMouseWheel.SubClassUnHookForm
Set clsMouseWheel.Form = Nothing
Set clsMouseWheel = Nothing
End Sub
Private Sub clsMouseWheel_MouseWheel(Cancel As Integer)
'This is the event procedure where you can
'decide what to do when the user rolls the mouse.
'If setting Cancel = True, we disable the mouse wheel
'in this form.
MsgBox "You cannot use the mouse wheel to scroll through records."
Cancel = True
End S
ub
On the File menu, click Close and Return to Microsoft Access .
Save the form, and then close it.
NOTE: Do not open the form in Form view at this time. If you do, Microsoft Access will stop responding because the Visual Basic Editor has been loaded.
Quit Microsoft Access.
Restart Microsoft Access, and open the sample database Northwind.mdb.
Open the Customers form in Form view.
Roll the mouse wheel.
Note that you receive the message:
You cannot use the mouse wheel to scroll through records.
Also note that the current record has not changed, indicating that the mouse wheel message was not processed by Microsoft Access.
