Creating An Undo Handler To Undo Changes Done By Excel VBA

Class modules (2)

clsExecAndUndo

OK, another big piece of code (explanation below the code)...

'===================================
' Module    : clsExecAndUndo
' Company   : JKP Application Development Services (c) 2005
' Author    : Jan Karel Pieterse
' Created   : 31-8-2005
' Purpose   : Class module, stores the objects processed and
'             handles the exection of the commands
' Copyright : This code is free for you to use for applications
'             for personal use.
'             It is not allowed to use this for a commercial program,
'             unless you have my consent.
'             If you want to include this code in freeware, make sure you add :

'-------------------------------------------------------------------------
' This code originates from    : Jan Karel Pieterse
' Company                      : JKP Application Development Services (c) 2005
'                                www.jkp-ads.com
'-------------------------------------------------------------------------
'=====================================
Option Explicit

Private mcolUndoObjects As Collection
Private mUndoObject As clsUndoObject

Public Function AddAndProcessObject(oObj As Object, sProperty As String, vValue As Variant) As Boolean
    Set mUndoObject = New clsUndoObject
    With mUndoObject
        Set .ObjectToChange = oObj
        .NewValue = vValue
        .PropertyToChange = sProperty
        mcolUndoObjects.Add mUndoObject
        If .ExecuteCommand = True Then
            AddAndProcessObject = True
        Else
            AddAndProcessObject = False
        End If
    End With
End Function

Private Sub Class_Initialize()
    Set mcolUndoObjects = New Collection
End Sub

Private Sub Class_Terminate()
    ResetUndo
End Sub

Public Sub ResetUndo()
    While mcolUndoObjects.Count > 0
        mcolUndoObjects.Remove (1)
    Wend
    Set mUndoObject = Nothing
End Sub

Public Sub UndoAll()
    Dim lCount As Long
    '    On Error Resume Next
    For lCount = mcolUndoObjects.Count To 1 Step -1
        Set mUndoObject = mcolUndoObjects(lCount)
        mUndoObject.UndoChange
        Set mUndoObject = Nothing
    Next
    ResetUndo
End Sub

Public Sub UndoLast()
    Dim lCount As Long
    '    On Error Resume Next
    If mcolUndoObjects.Count >= 1 Then
        Set mUndoObject = mcolUndoObjects(mcolUndoObjects.Count)
        mUndoObject.UndoChange
        mcolUndoObjects.Remove mcolUndoObjects.Count
        Set mUndoObject = Nothing
    Else
        ResetUndo
    End If
End Sub

Public Function UndoCount() As Long
    UndoCount = mcolUndoObjects.Count
End Function

Short explanation of the subs and functions shown above

Sub/Function	Goal
AddAndProcessObject	This is the entry routine of this class. It receives the object and its properties to be modified and adds a member to the collection of changed objects (members of clsUndoObject), so changes can be undone later on. It calls the routines needed to store previous settings and execute the change.
ResetUndo	Clears the undo stack of this utility (run when instances of the class are destroyed)
UndoAll	Method to call to undo all operations stored in the collection of changed objects. This procedure is called when control-z is pressed or Edit, undo is selected.
UndoLast	Only undoes the last change. This needs to be tied to a button or a shortcut-key programmatically in your application.
UndoCount	Returns the number of objects stored in the collection

That sums up the two central (class-) modules of this example. The next page will describe implementation of the technique.