* VB-CODE (4)
Tip 130: Using the Undo Feature with a Text Box Control

July 1, 1995

Abstract
Almost all Microsoft=AE Windows=AE-based applications provide an Edit =
menu on which
you can select the Undo command to reverse the most recently made =
changes to an
edit control. This article explains how you can add this functionality =
to your own
Microsoft Visual Basic=AE application.

Using SendMessage to Undo Edit Control Changes
When you modify the contents of an edit control in Microsoft=AE Visual =
Basic=AE, such
as a Text Box control, the data you add or delete is saved in an =
internal buffer
by the Microsoft Windows=AE operating system. You can use the Windows =
application
programming interface (API) SendMessage function to allow your user to =
retrieve
the modified text.

Let's assume that you have typed some text into a Text Box control. You =
now want
to delete some of that text. To do this, you select the text and press =
the DEL
key. The text you selected is removed from the Text Box control. You can =
retrieve
this text within a Visual Basic application by sending an EM_UNDO =
message to
Windows. The EM_UNDO message tells the operating system that you want to =
undo the
last change you made to the edit control. In this case, the edit control =
is the
Text Box.

After the EM_UNDO message is sent, the original contents of the Text Box =
control
are restored. The modified text is still stored in the internal Windows =
buffer.
Therefore, in your application, you need to send an EM_EMPTYUNDOBUFFER =
message to
clear or delete the contents of this internal buffer. The =
EM_EMPTYUNDOBUFFER
message clears the undo flag, which means that you can no longer undo =
your last
change to the edit control.
As shown in the example program below, you can also determine if an undo =
operation
can be performed on the edit control. The EM_CANUNDO message returns an =
integer
value set to True if there is text in the undo buffer, or zero if no =
text is
available. You can perform an undo operation only if the contents of an =
edit
control have been previously modified and the data is stored in the undo =
buffer.

Example Program
This program shows how to add the Undo and Redo editing features to your =
Visual
Basic application.

 1. Create a new project in Visual Basic. Form1 is created by default.
 2. Add the following Constant and Declare statements to the General =
Declarations
    section of Form1 (note that the Declare statement must be typed as a =
single
    line of code):

Private Declare Function SendMessage Lib "User" (ByVal hWnd As Integer,
   ByVal wMsg As Integer, ByVal wParam As Integer, lParam As Long) As =
Long
Const WM_USER =3D &H400
Const EM_CANUNDO =3D WM_USER + 22
Const EM_EMPTYUNDOBUFFER =3D WM_USER + 29
Const EM_UNDO =3D WM_USER + 23

 3. Add a Text Box control to Form1. Text1 is created by default. Set =
its
    MultiLine property to True.
 4. Add a Command Button control to Form1. Command1 is created by =
default. Set
    its Caption property to "Undo".
 5. Add the following code to the Click event for Command1:

Private Sub Command1_Click()
    Dim OK As Long
    OK =3D SendMessage(Text1.hWnd, EM_UNDO, 0, 0&)
    OK =3D SendMessage(Text1.hWnd, EM_EMPTYUNDOBUFFER, 0, 0&)
End Sub

 6. Add a second Command Button control to Form1. Command2 is created by =
default.
    Set its Caption property to "Redo".
 7. Add the following code to the Click event for Command2:

Private Sub Command2_Click()
    Dim OK As Long
    OK =3D SendMessage(Text1.hWnd, EM_CANUNDO, 0, 0&)
    If OK =3D 0 Then
        MsgBox "Cannot undo the changes you made", 16, "Error"
    End If
    OK =3D SendMessage(Text1.hWnd, EM_UNDO, 0, 0&)
End Sub

Run the example program by pressing F5. Type some text into the Text Box =
control.
Assume that you typed the line, "We will go shopping on Monday and =
Tuesday."
Select the words "Monday and". Press DEL to delete the text. Click the =
Redo
command button. The original sentence is restored. Click Redo a second =
time. The
modified sentence is restored. The Redo function is similar to the =
cut-and-paste
functions you see in word-processing programs.

Select the words "Monday and" a second time and again delete the text. =
Click Undo
to restore the original text. Notice that clicking the Undo command =
button a
second time does nothing. This is because the Undo routine clears the =
undo flag
and the edit buffer. You can only undo one change at a time.

Additional References
"EM_CANUNDO." (Development Library, Product Documentation, SDKs,
   Windows 3.1 SDK, Programmer's Reference Volume 3: Messages, =
Structures,
   and Macros)
"EM_EMPTYUNDOBUFFER." (Development Library, Product Documentation, SDKs,
   Windows 3.1 SDK, Programmer's Reference Volume 3: Messages, =
Structures,
   and Macros)
"EM_UNDO." (Development Library, Product Documentation, SDKs, Windows =
3.1 SDK,
   Programmer's Reference Volume 3: Messages, Structures, and Macros)
"SendMessage." (Development Library, Product Documentation, Windows 3.1 =
SDK,
   Programmer's Reference Volume 2: Functions)