HOWTO: Limit Cursor Movement Inside a Form

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, version 5.0

SUMMARY

The ClipCursor API function allows you to limit cursor movement to a predefined area, such as a form. This article includes instructions and sample code to create a project that uses the ClipCursor API function.

MORE INFORMATION

You might want to limit the cursor movement area if you have drag-and-drop functionality within a form and you only want the user to drag an item inside a form.

The ClipCursor function confines the cursor to a rectangular area on the screen. If a subsequent cursor position (set by the SetCursorPos function or the mouse) lies outside the rectangle, Windows automatically adjusts the position to keep the cursor inside the rectangular area.

The form borders serve as the rectangular area for the ClipCursor function. To get the size of the form, use the GetClientRect function. This function retrieves the coordinates of a window's client area. The coordinates are relative to the upper-left corner of a window's client area; the coordinates of the upper-left corner are (0,0). This means you will have to convert the client coordinates to screen coordinates.

The ClientToScreen function converts the client coordinates of a specified point to screen coordinates. The last function used is the OffsetRect function. The OffsetRect function moves the specified rectangle by the specified offsets.

NOTE: If you unload the form after executing the ClipCursor function, the cursor will still be limited to the last coordinates of the now-unloaded form. To prevent this behavior, calling the ClipCursor function with a null argument in the Form_Unload event is recommended.

The next section shows you how to create a sample project that uses these functions.

Steps to Create Project

1. Start a new Standard EXE project in Visual Basic. Form1 is created by default.

2. Add two CommandButtons to Form1.

3. Copy the following code to the Code window of Form1:

         Option Explicit
   

         Private Type RECT
            left As Integer
            top As Integer
            right As Integer
            bottom As Integer
         End Type
   
         Private Type POINT
            x As Long
            y As Long
         End Type
   
         Private Declare Sub ClipCursor Lib "user32" (lpRect As Any)
         Private Declare Sub GetClientRect Lib "user32" _
                                               (ByVal hWnd As Long, _
                                                lpRect As RECT)
         Private Declare Sub ClientToScreen Lib "user32" _
                                               (ByVal hWnd As Long, _
                                                lpPoint As POINT)
         Private Declare Sub OffsetRect Lib "user32" (lpRect As RECT, _
                                             ByVal x As Long, _
                                             ByVal y As Long)
   
         Private Sub Form_Load()
            Command1.Caption = "Limit Cursor Movement"
            Command2.Caption = "Release Limit"
         End Sub
   
         Private Sub Command1_Click()
         'Limits the Cursor movement to within the form.
            Dim client As RECT
            Dim upperleft As POINT
   
            GetClientRect Me.hWnd, client
            upperleft.x = client.left
            upperleft.y = client.top
            ClientToScreen Me.hWnd, upperleft
            OffsetRect client, upperleft.x, upperleft.y
            ClipCursor client
         End Sub
   
         Private Sub Command2_Click()
         'Releases the cursor limits
            ClipCursor ByVal 0&
         End Sub
   
         Private Sub Form_Unload(Cancel As Integer)
         'Releases the cursor limits
            ClipCursor ByVal 0&
         End Sub
   
   

4. On the Run menu, click Start or press the F5 key to start the program. Click "Limit Cursor Movement" and note that the cursor is limited to the borders of the form. Click "Release Limit" and the cursor can move anywhere on the screen.

Keywords          : vb5all vb5howto VBKBGraphics VBKBWinAPI
Version           : WINDOWS:5.0
Platform          : WINDOWS
Issue type        : kbhowto