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Tip of the Day
Language: VB5,VB6
Expertise: Intermediate
Jul 10, 1999



Application Security Testing: An Integral Part of DevOps

OverwriteHandler - A class for handling overwrite mode

' OverwriteHandler class
' you can associate this class to a form, using the
' Form property, and all the textbox controls in the
' form will support insert/overwrite mode switching
' Usage:  Dim ov As New OverwriteHandler
'         Private Sub Form_Load()
'            Set ov.Form = Me
'            ' if you want block-shaped caret in ov mode
'            ov.BlockCaret = True
'         End Sub
' The class responds as expected to the INS key to
' switch among insert and overwrite mode. You can do
' the same by acting over the OverwriteMode property
' The OverwriteMode property is automatically reset to
' false whenever the focus enters a new field, but is
' preserved when the focus shifts to another form and back
' The caret shape is not correctly preserved when the input
' focus shifts to another application and then back, but the
' overwrite mode is.
' Notes: this class forces the form's KeyPreview property to True
'        There are a few controls (most notably, the ComboBox
'        control) for which the block caret doesn't display correctly.
'        Other controls manage the caret in a way that this class
'        can display a custom caret. In such cases you should leave
'        the ShowBlock property to False.

Option Explicit

Private Declare Function CreateCaret Lib "user32" (ByVal hWnd As Long, _
    ByVal hBitmap As Long, ByVal nWidth As Long, ByVal nHeight As Long) As Long
Private Declare Function ShowCaret Lib "user32" (ByVal hWnd As Long) As Long

' member variable for OverwriteMode property
Private m_OverwriteMode As Boolean
' member variable for Form property
Private WithEvents HookedForm As Form
' this keeps track of the last active field
Private ActiveControl As Control
' member variable for BlockCaret property
Private m_BlockCaret As Boolean

' The Hooked Form

Public Property Get Form() As Form
    Set Form = HookedForm
End Property

Public Property Set Form(ByVal newValue As Form)
    Set HookedForm = newValue
    ' enforce KeyPreview = True, so that this class
    ' can trap keys pressed in the form
    If Not (HookedForm Is Nothing) Then
        HookedForm.KeyPreview = True
    End If
End Property

' The current Overwrite flag

Public Property Get OverwriteMode() As Boolean
    OverwriteMode = m_OverwriteMode
End Property

Public Property Let OverwriteMode(ByVal newValue As Boolean)
    m_OverwriteMode = newValue
End Property

' True if the overwrite caret should be displayed as a block

Property Get BlockCaret() As Boolean
    BlockCaret = m_BlockCaret
End Property

Property Let BlockCaret(ByVal newValue As Boolean)
    m_BlockCaret = newValue
End Property

' rebuild the caret block
' at times you may need to call this method directly, for example
' when the focus shift to another application and then back.

Public Sub UpdateCaret()
    On Error Resume Next
    If Not CheckActiveControl Then
        ' active control doesn't support carets
    ElseIf OverwriteMode And BlockCaret Then
        ' show the caret as a block
        CreateCaret ActiveControl.hWnd, 0, 6, 14
        ShowCaret ActiveControl.hWnd
        ' show the caret as a vertical line
        CreateCaret ActiveControl.hWnd, 0, 1, 14
        ShowCaret ActiveControl.hWnd
    End If
End Sub

' reset the OverwriteMode property if the focus
' has moved to another field
' the function returns True if the active control
' supports a caret

Private Function CheckActiveControl() As Boolean
    If Not Form.ActiveControl Is ActiveControl Then
        Set ActiveControl = Form.ActiveControl
        OverwriteMode = False
    End If
    On Error Resume Next
    ' the following expression is always True, unless the
    ' active control doesn't refer to a textbox
    CheckActiveControl = (ActiveControl.SelLength >= 0)
End Function

' trap user's key presses

Private Sub HookedForm_KeyPress(KeyAscii As Integer)
    On Error Resume Next
    If Not CheckActiveControl Then
        ' active control is not of type textbox
        ' do nothing
    ElseIf m_OverwriteMode And KeyAscii >= 32 And ActiveControl.SelLength = 0 _
        ' we are in overwrite mode, the user hasn't pressed a
        ' control key and there's no text currently highlighted
        If Mid$(ActiveControl.Text, ActiveControl.SelStart + 1, 1) <> vbCr Then
            ' extend the selection if not at the end of the line
            ActiveControl.SelLength = 1
        End If
    End If
End Sub

' check if INS key has been pressed

Private Sub HookedForm_KeyDown(KeyCode As Integer, Shift As Integer)
    If CheckActiveControl Then
        If KeyCode = 45 And Shift = 0 Then
            ' Insert key has been pressed
            OverwriteMode = Not OverwriteMode
        End If
    End If
End Sub

' rebuild the correct caret when the forms regains the focus

Private Sub HookedForm_Activate()
    If CheckActiveControl Then UpdateCaret
End Sub

Francesco Balena
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