How To Implement the 'SnapTo' Feature in Visual Basic

MORE INFORMATION

This article uses a class module to make the necessary Windows API calls. One possible extension to this article would be to compile the class code into an ActiveX DLL, and that is why a class module was used. This is a good approach if the 'Snap To' functionality is needed in a number of different projects. The mouse cursor will not be adjusted unless the 'Snap To' functionality is enabled on the target machine.

NOTE: Under Windows 95, Windows 98, and Windows Me, the 'Snap To' functionality does not interact properly with Visual Basic 5.0 applications under some circumstances. Please see the REFERENCES section of this article for more information on this topic.

Step-by-Step Example

1. Start a new Standard EXE project in Visual Basic. Form1 is created by default. Add a second Form to the Project.
2. Place two CommandButtons on Form1. Set the Default property of Command2 to True.
3. Add a Class Module to the Project. Change the property name of the Class Module to Snap.
4. Add the following code to the Snap Class module:

         Option Explicit
   
         Private Declare Function SetCursorPos Lib "user32" _
                       (ByVal x As Long, ByVal y As Long) As Long
   
         Private Declare Function GetWindowRect Lib "user32" _
                       (ByVal hwnd As Long, lpRect As RECT) As Long
   
         Private Declare Function RegOpenKeyEx Lib "advapi32.dll" Alias _
         "RegOpenKeyExA" (ByVal hKey As Long, ByVal lpSubKey As String, _
         ByVal ulOptions As Long, ByVal samDesired As Long, phkResult _
         As Long) As Long
   
         Private Declare Function RegQueryValueEx Lib "advapi32.dll" Alias _
         "RegQueryValueExA" (ByVal hKey As Long, ByVal lpValueName _
         As String, ByVal lpReserved As Long, lpType As Long, lpData As Any, _
         lpcbData As Long) As Long
   
         Private Declare Function RegCloseKey Lib "advapi32.dll" _
         (ByVal hKey As Long) As Long
   
         Private Type RECT
            left As Long
            top As Long
            right As Long
            bottom As Long
         End Type
   
         Private Const HKEY_C_U = &H80000001  ' HKEY_CURRENT_USER
         Private Const subkey = "Control Panel\Microsoft Input Devices\Mouse"
   
         Private buttonHandle As Long
   
         Public Sub setDefaultButton(colControls As Object)
         Dim iIterate As Integer
         For iIterate = 0 To colControls.Count - 1
            If TypeOf colControls(iIterate) Is CommandButton Then
               If colControls(iIterate).Default = True Then
                  buttonHandle = colControls(iIterate).hwnd
                  Exit For
               End If
            End If
         Next iIterate
         End Sub
   
         Public Function snapTo()
            Dim buttonRect As RECT
            Dim RetVal As Long
            Dim x As Long
            Dim y As Long
   
            If buttonHandle And _
             RegGetString$(HKEY_C_U, subkey, "SnapTo") = "ON" Then
               RetVal = GetWindowRect(buttonHandle, buttonRect)
               With buttonRect
                  x = .left + ((.right - .left) / 2)
                  y = .top + ((.bottom - .top) / 2)
               End With
               DoEvents
               RetVal = SetCursorPos(x, y)
               snapTo = True
            Else
               snapTo = False
            End If
         End Function
   
         Function RegGetString$(hInKey As Long, ByVal subkey$, ByVal valname$)
            Dim RetVal$, hSubKey As Long, dwType As Long, vSZ As Long
            Dim R As Long, v$
   
            RetVal$ = ""
            Const ERROR_SUCCESS& = 0
            Const REG_SZ& = 1
            Const KEY_READ = &H20019
   
            R = RegOpenKeyEx(hInKey, subkey$, 0, KEY_READ, hSubKey)
            If R <> ERROR_SUCCESS Then GoTo Quit_Now
   
            vSZ = 256: v$ = String$(vSZ, 0)
            R = RegQueryValueEx(hSubKey, valname$, 0, dwType, ByVal v$, vSZ)
            If R = ERROR_SUCCESS And dwType = REG_SZ Then
               RetVal$ = left$(v$, vSZ - 1)
            Else
               RetVal$ = "--Not String--"
            End If
            If hInKey = 0 Then R = RegCloseKey(hSubKey)
         Quit_Now:
            RegGetString$ = RetVal$
         End Function
   
   					

5. Insert the following into the General Declarations section of Form1:

         Dim objSnap As Snap
   
         Private Sub Command1_Click()
            Form2.Show
         End Sub
   
         Private Sub Form_Activate()
            objSnap.snapTo
         End Sub
   
         Private Sub Form_Load()
            Command1.Caption = "Show Form2"
            Command2.Caption = "Default Button"
            Set objSnap = New Snap
            ' determine the default button and save it in the Snap class
            Call objSnap.setDefaultButton(Me.Controls)
         End Sub
   
         Private Sub Form_Unload(Cancel As Integer)
            Set objSnap = Nothing
         End Sub
   
   					

6. Run the project. When Form1 opens, the mouse pointer should be centered over the button with the caption "Default Button."
7. Click on Command2 to open the other form.
8. Click back and forth between Form1 or Form2, either by clicking on the forms themselves, or on the form Icons in the Windows TaskBar. When focus returns to Form1, the mouse pointer should be positioned on the default button.

NOTE: This technique only works when shifting between forms of the same application. To make it work properly when your application gains focus would require hooking the WM_ACTIVATEAPP message, because Visual Basic does not have an App_Activate event. Please refer to the REFERENCES section of this article for more information on this topic.