How to Set Automatic Scrolling On In a List Box In VB 3.0

- Microsoft Visual Basic programming system for Windows,

  versions 1.0, 2.0, and 3.0

SUMMARY

This article demonstrates two techniques for turning on automatic scrolling inside a Visual Basic list box by using the SendMessage Windows API function. This could be useful with database applications or any type of application in which the user needs to be able to scroll a list box without the use of the arrow keys.

MORE INFORMATION

Two different techniques are shown below.

Technique One

Behavior:

Once automatic scrolling is turned on, any vertical movement of the mouse within the entire horizontal region of the desktop containing the list box will cause the visual selection of an item in the list box. This includes the capability of driving the scroll bar thumb to its upper and lower limits.

Drawbacks:

With technique one, as long as the automatic scrolling is on, all mouse movement on the desktop is mapped to the list box. Task switching, by using the ALT-ENTER key combination, is disabled. Windows is basically waiting for the completion of a click sequence. Any click anywhere on the desktop will be directed to the Click event of the list box. The user will need to double-click any other control to get the expected response from the control. Double-clicking any other control also turns off the automatic scrolling.

NOTE: For those programmers looking for a way to defeat the ability of the user to task switch away from their application, this technique offers a method.

Technique two

Behavior:

Once automatic scrolling is turned on, only movement of the mouse within the list box will cause the visual selection of an item. The scroll bar thumb will be driven downward once, but will not be driven back up. Mouse clicks elsewhere on the desktop are processed normally and the automatic scrolling remains on until the mouse is clicked in the list box.

Drawbacks:

The scroll bar of the list box is not driven in both directions based on mouse movement because mouse movements outside the list box are not processed by the list box.

Example of Setting Automatic scrolling for List box

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

2. Add two Labels (Label1 and Label2), a Command button (Command1), and a List box (List1) to Form1.

3. Using the following table as a guide, set the properties of the controls you added in step 2:

      Control Name   Property     New Value
      ----------------------------------------------------------
      Label1         Caption      Turn off Auto-Scrolling by
                                  clicking in the list box
      Label1         Visible      False
      Label2         Visible      False
      Command1       Caption      Turn on Auto-Scroll
   
   

4. Place the following code in the (general) (declarations) section of Form1:

      ' Enter the following Declare statement on one, single line:
      Declare Function SendMessage Lib "User" (ByVal hWnd As Integer,
         ByVal wMsg As Integer, ByVal wparam As Integer,
         ByVal lparam As Any) As Long
      Dim tmp$
      Dim autoselect_on As Integer
   
   

5. Place the following code in the Form Load event procedure of Form1:

      Sub Form_Load ()
         list1.AddItem "test 1"
         list1.AddItem "test 2"
         list1.AddItem "test 3"
         list1.AddItem "test 4"
         list1.AddItem "test 5"
         list1.AddItem "test 6"
         list1.AddItem "test 7"
         list1.AddItem "test 8"
         list1.AddItem "test 9"
      End Sub
   
   

Additional Steps for Technique One

1. Place the following code in the List1 Click event procedure of Form1:

      Sub List1_Click ()
         tmp$ = list1.List(list1.ListIndex)
         label2.Caption = "You selected: " & list1.List(list1.ListIndex)
         label2.Visible = True
      End Sub
   
   

2. Place the following code in the Command1 Click event procedure of Form1:

      Sub Command1_Click ()
         Const WM_LBUTTONDOWN = &H201
         ' Turn on auto-scrolling:
         r% = SendMessage(list1.hWnd, WM_LBUTTONDOWN, 0&, 0&)
         label1.Visible = True
      End Sub
   
   

3. From the Run menu, choose Start (ALT, R, S), or press the F5 key to run the program. Click the Command1 button. Move the mouse up and down inside or outside the List1 box control. Double-click a selection from the List1 box control to stop the automatic scrolling.

Additional Steps for Technique Two

1. Place the following code in the List1 Click event procedure of Form1, replacing the code referred to in step 6 above. The code in the command button is no longer needed.

      Sub List1_Click ()
         Const WM_LBUTTONDOWN = &H201
         tmp$ = list1.List(list1.ListIndex)
         label2.Caption = "You selected: " & list1.List(list1.ListIndex)
         label2.Visible = True
   
         If Not autoselect_on Then
            autoselect_on = True
            r% = SendMessage(list1.hWnd, WM_LBUTTONDOWN, 0&, 0&)
         Else
            autoselect_on = False
         End If
      End Sub
   
   

2. From the Run menu, choose Start (ALT, R, S), or press the F5 key to run

       the program. Click the List1 box. Move the mouse up and down inside the
       List1 box. Click again on a selection from the List1 box to stop the
       automatic scrolling.