HOWTO: Prevent the ENTER Key From Firing in TreeView Control

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0

SUMMARY

You can intercept keyboard input before it reaches a control by creating a keyboard hook. The example below illustrates how to intercept the ENTER key before it reaches a TreeView control.

WARNING: Failure to unhook the keyboard hook before exiting your application will result in errors, Invalid Page Faults, and data loss. This is caused by the system still pointing to your KeyboardProc() function, which no longer exists because it has not been notified of the change.

Always unhook the keyboard upon exiting the application. This is especially important while debugging a program that uses this technique within the Microsoft Visual Basic Development Environment. Clicking the END button on the toolbar or selecting END from the Run menu without unhooking first will cause an Invalid Page Fault and close Microsoft Visual Basic without giving you a chance to save your code. For this reason it is best to test hooks in the EXE rather than from the IDE.

MORE INFORMATION

This method is necessary because the TreeView control processes certain navigational keyboard messages before it actually raises the corresponding keyboard events for those respective messages. The only way to prevent the control from acting on the keyboard messages is to trap and intercept them.

The following sample renders the ENTER, RIGHT ARROW, and LEFT ARROW keys useless when used on a TreeView control. It still allows the UP and DOWN ARROWS to be used to navigate the tree as well as collapse and expand selected nodes.

1. Create a Visual Basic 6.0 Standard EXE project. Form1 is create by default.



2. On the Project menu, click Components. Select Microsoft Windows Common Controls 6.0 (5.0 for Visual Basic 5.0) and click OK.



3. Add a TreeView control to Form1.



4. Paste the following code in the General Declarations section of Form1:
   

   
   Option Explicit
   Private nodx As Node
   
   Private Sub Form_Load()
   ' Set keyboard hook
      hHook = SetWindowsHookEx(WH_KEYBOARD, _
                               AddressOf KeyboardProc, _
                               0&, _
                               App.ThreadID)
   ' Create Tree
      TreeView1.Style = tvwTreelinesPlusMinusPictureText ' Style 7
      TreeView1.LineStyle = tvwRootLines  ' Linestyle 1
          Set nodx = TreeView1.Nodes.Add(, , "r", "Root")
      Set nodx = TreeView1.Nodes.Add("r", tvwChild, "Child1", "Child1")
      Set nodx = TreeView1.Nodes.Add("r", tvwChild, "Child2", "Child2")
      Set nodx = TreeView1.Nodes.Add("r", tvwChild, "Child3", "Child3")
   End Sub
   
   Private Sub Form_Unload(Cancel As Integer)
      Call UnhookWindowsHookEx(hHook)
   End Sub
         
   Private Sub TreeView1_KeyDown(KeyCode As Integer, Shift As Integer)
      Select Case KeyCode
         Case vbKeyDown
            ' Expand tree on down arrow
            TreeView1.SelectedItem.Expanded = True
         Case vbKeyUp
            ' Collapse tree on up arrow
            TreeView1.SelectedItem.Expanded = False
      End Select
   End Sub 




5. On the Project menu, click Add Module. Module1 is created by default.



6. Paste the following code in the General Declaration section of Module1:
   

   
   Option Explicit
   
   Public Declare Function CallNextHookEx Lib "user32" _
      (ByVal hHook As Long, _
      ByVal nCode As Long, _
      ByVal wParam As Long, _
      ByVal lParam As Long) As Long
   
   Public Declare Function UnhookWindowsHookEx Lib "user32" _
      (ByVal hHook As Long) As Long
   
   Public Declare Function SetWindowsHookEx Lib "user32" _
      Alias "SetWindowsHookExA" _
      (ByVal idHook As Long, _
      ByVal lpfn As Long, _
      ByVal hmod As Long, _
      ByVal dwThreadId As Long) As Long
   
   Public Const WH_KEYBOARD = 2
   Public hHook As Long
   
   Public Function KeyboardProc(ByVal nCode As Long, _
                                ByVal wParam As Long, _
                                ByVal lParam As Long) As Long
      If nCode >= 0 Then
      ' Process keys you want to filter in TreeView1 control.
         If Form1.ActiveControl.Name = "TreeView1" Then
            Select Case wParam
               Case vbKeyReturn
                  Debug.Print "    Enter Key"
                  KeyboardProc = 1
                  Exit Function
               Case vbKeyRight
                  Debug.Print "    Right Arrow"
                  KeyboardProc = 1
                  Exit Function
               Case vbKeyLeft
                  Debug.Print "    Left Arrow"
                  KeyboardProc = 1
                  Exit Function
            End Select
         End If
      End If
      KeyboardProc = CallNextHookEx(hHook, nCode, wParam, lParam)
   End Function 




7. Press the F5 key to run the application. When the TreeView control has focus, try the ENTER key along with the LEFT and RIGHT ARROW keys to verify that they are inoperative (as intended). Try the UP and DOWN ARROW keys, confirming that they can be used to navigate the tree, including collapse and expand selected nodes.

REFERENCES

For additional information about intercepting keyboard input, please see the following article in the Microsoft Knowledge Base:

Q177992 HOWTO: Intercept Keyboard Input from Visual Basic

Additional query words:

Keywords          : kbAPI kbCtrl kbTreeView kbGrpUser kbVBp kbVBp500 kbVBp600 kbGrpVB 
Version           : WINDOWS:5.0,6.0
Platform          : WINDOWS 
Issue type        : kbhowto 

Last Reviewed: March 13, 1999