Access Cookbook, 2nd Edition [Electronic resources] نسخه متنی

Recipe 11.4 Restrict Mouse Movement to a Specific Region

11.4.1 Problem

You'd like to be able to
restrict mouse-cursor movement to certain areas of the current form.
You think it would help users of your application if the mouse stays
where it needs to be until they're done with it. How
can you limit mouse movement in Access?

11.4.2 Solution

The Windows API's

ClipCursor subroutine will limit the movement of
the mouse to a single form or region on a form, as
you'll see in this solution.

To try out this technique, load and run the frmClip form from

11-04.MDB . This form, shown in Figure 11-5, limits the mouse movement to the area of the
form once you click the large button. If you click the button again
or close the form, code attached to either event frees the mouse
cursor to move anywhere on the screen. If you move the form, Windows
frees the mouse cursor for you.

Figure 11-5. frmClip limits mouse movement to the area of the form

To use this technique in your own applications, follow these steps:

1. Import the module basClipCursor from
   

   11-04.MDB . This module contains the function
   declarations and user-defined types that you'll
   need.

2. To limit the mouse to a single form, you'll need to
   get the form coordinates and tell Windows to use those coordinates as
   limits for the mouse. To do this, you'll need code
   something like the following (because this code fragment uses
   Me, it must be in a form's
   module, not a global module):

   Dim typRect as acb_tagRect
   Call acb_apiGetWindowRect (Me.Hwnd, typRect)
   Call acb_apiClipCursor(typRect)

3. To free the mouse cursor, use code like this:

   Call acb_apiClipCursor(ByVal vbNullString)

See Recipe 11.4.3 for an example.

11.4.3 Discussion

The
ClipCursor API routine (aliased as
acb_apiClipCursor in the code) expects as its only
parameter a user-defined data type consisting of four long integers
representing coordinates of a rectangle. This data type has been
declared for you in basClipCursor as
acb_tagRect. This is a common data structure, used
often with API routines that interact with the screen or printer. It
is defined as follows:

Type acb_tagRect
lngLeft As Long
lngTop As Long
lngRight As Long
lngBottom As Long
End Type

When
you want to restrict mouse movement, you'll need to
retrieve the coordinates of the current form. You can accomplish this
by calling the GetWindowRect API function (aliased as
acb_apiGetWindowRect in the code), which will fill
in an acb_tagRect structure with the left, top,
right, and bottom coordinates of the window whose handle you pass it.
Therefore, by calling acb_apiGetWindowRect with
the handle of the current form, you'll retrieve the
coordinates of that form in pixels:

Dim typRect as acb_tagRect
Call acb_apiGetWindowRect (Me.hWnd, typRect)

Once you've got a structure containing the
coordinates of the current form, you can call
ClipCursor and pass that filled-in structure to
it. The sample form combines these API calls, as shown here:

Private Sub cmdClip_Click( )
Dim typRect As acb_tagRect
Static sstrCaption As String
' Static variable to keep track of clipping
Static blnClip As Boolean
If blnClip Then
Me.cmdClip.Caption = sstrCaption
Call acb_apiClipCursor(ByVal vbNullString)
Else
sstrCaption = Me.cmdClip.Caption
Me.cmdClip.Caption = "Free the Mouse!"
Call acb_apiGetWindowRect(Me.hWnd, typRect)
Call acb_apiClipCursor(typRect)
End If
blnClip = Not blnClip
End Sub

In the sample routine, which is executed each time you click the
large button on frmClip, blnClip alternates
between True and False, keeping
track of whether mouse clipping is currently in effect. If it is, the
routine calls acb_apiClipCursor to disable
clipping and resets the button's caption. If
clipping is not in effect, the routine stores away the original
caption, sets a new one ("Free the
Mouse!"), retrieves the form's
coordinates, and finally calls acb_apiClipCursor
to restrict the cursor's movement.

To end
the mouse-cursor restrictions, send a null value to
acb_apiClipCursor. To do that, pass the
vbNullString intrinsic constant by value. Because
the acb_apiClipCursor procedure has been declared
to accept any type of parameter, you can send it a structure in one
call and a null value in another.

The method presented in this solution is not foolproof in Access.
You're taking control of a feature that Access
normally controls itself, and sometimes the interaction may be
unpredictable. In this case, if you restrict the mouse movement to a
single form, but then use the mouse to move or resize the form,
Access will free the mouse for you. Therefore, if you want to

force users to stay on a single form,
you're better off using a modal form instead. If, on
the other hand, you're just trying to ensure that
the mouse remains in the area of the form where the users need it to
be, the method described here is appropriate. Restricting the mouse
movement is not meant for every application, but if you want to help
your users out a little, try it.