Using the RAII design pattern in VBA to automatically restore settings

Question

Using a technique described in this StackOverflow answer by @David Murdoch I created a custom class with methods to speed up Excel (Activate turns off screen updates and set the calculation mode to manual) and to restore it to normal settings (Shutdown). When the object reaches the end of its life and Class_Terminate is triggered it runs the Shutdown procedure.

The purpose of all this is to avoid problems that occur if (when) I forget to reset these settings at the end of a script or if my code (contains a bug that) skips the section resetting these settings.

I am looking for a broad critique of the nit-picky variety. In other words, how you would code this.

This code intentionally ignores the initial state of the settings and always turns on screen updating and sets the calculation mode to automatic when the object is destroyed.

The following is in a class module called ExcelTurboBooster.

Option Explicit
'
'  Use the code below to turn off screen updates and automatic
'   calculations.  Screen updates and automatic calculations will be
'   restored when the calling procedure terminates.
'
' EXAMPLE USAGE
' ======================================================================
'   Dim objExcelTurboBooster As New ExcelTurboBooster
'   objExcelTurboBooster.Activate   ' Activates booster
'   objExcelTurboBooster.Shutdown   ' Use to deactivate before procedure
'                                   '  terminates
' ======================================================================
'
'
Public Sub Activate()
    Application.Calculation = xlCalculationManual
    Application.ScreenUpdating = False
End Sub

Public Sub Shutdown()
    On Error Resume Next
    Application.Calculation = xlCalculationAutomatic
    Application.ScreenUpdating = True
End Sub

Private Sub Class_Terminate()
    Shutdown
End Sub

Answer 1

I think you have a potential usage issue with wrapping these methods in a class. While it's convenient that the Class_Terminate method will automatically re-enable updating, it makes keeping track of the usage of such an object the responsibility of the caller (or calling module or of the entire project). Where you can easily get into trouble is when you start making calls to other objects or functions that might need to make the same disable/enable actions for speed. This is especially apparent if you create a personal library of routines that you can reuse.

My preferred approach to this was to create a standalone module that I can easily import into any VBA project and use immediately. The public methods are DisableUpdating and EnableUpdating. In every day use I can now create:

Sub MainProgram()
    DisableUpdating
    ...
    LibrarySub1 <some vars>
    ...
    EnableUpdating
End Sub

Sub LibrarySub1(<some vars>)
    DisableUpdating
    ...
    EnableUpdating
End Sub

But you can see the potential problem in nesting these calls.

My solution in the standalone module is:

Option Explicit

Private updateNestCount As Long
Private calculationState As XlCalculation

Public Sub DisableUpdating(Optional msg As String = vbNullString)
    If updateNestCount = 0 Then
        Application.ScreenUpdating = False
        '--- save the current calculation state for later restoration
        calculationState = Application.Calculation
        Application.Calculation = xlCalculationManual
    End If
    updateNestCount = updateNestCount + 1

    '--- print a debug message if the caller wants one
    If Len(msg) > 0 Then
        Debug.Print "DisableUpdating(" & updateNestCount & "): " & msg
    End If
End Sub

Public Sub EnableUpdating(Optional msg As String = vbNullString)
    If updateNestCount = 1 Then
        Application.ScreenUpdating = False
        '--- restore the calculation back to its original state
        Application.Calculation = calculationState
    End If
    updateNestCount = updateNestCount - 1

    '--- print a debug message if the caller wants one
    If Len(msg) > 0 Then
        Debug.Print "EnableUpdating (" & updateNestCount & "): " & msg
    End If
End Sub

Now I can liberally sprinkle these Disable/Enable calls in my libraries and other routines and the private variable takes care counting how deeply nested it might be. I've run into the need to track some debug on occasion, so the optional msg comes in handy.

I think your class/object approach leaves too much of the burden on the calling program to keep track of the object lifetime and you might find this a simpler approach. Just make sure to always pair the calls in a single routine AND if you have an error handler, that the enable method is accessed within the error handling.

UPDATE: In response to a comment below, I'm posting my updated code module in its entirety (which includes a bonus high-performance timer, just for fun). Copy and paste this code into a file outside of the VBA Editor, then import it into your code and you'll get Intellisense to help with the functions.

Attribute VB_Name = "Lib_PerformanceSupport"
Attribute VB_Description = "Methods to control disabling/enabling of the Application level screen updates. Supports call nesting and debug messaging, plus high precision timer calls."
'@Folder("Libraries")
Option Explicit

'------------------------------------------------------------------------------
' For Update methods
'
Private Type SavedState
    screenUpdate As Boolean
    calculationType As XlCalculation
    eventsFlag As Boolean
    callCounter As Long
End Type

Private previousState As SavedState

Private Const DEBUG_MODE As Boolean = False 'COMPILE TIME ONLY!!

'------------------------------------------------------------------------------
' For Precision Counter methods
'
Private Type LargeInteger
    lowpart As Long
    highpart As Long
End Type

#If VBA7 Then
    Private Declare PtrSafe Function QueryPerformanceCounter Lib _
                         "kernel32" (ByRef lpPerformanceCount As LargeInteger) As Long
    Private Declare PtrSafe Function QueryPerformanceFrequency Lib _
                         "kernel32" (ByRef lpFrequency As LargeInteger) As Long
#Else
    Private Declare Function QueryPerformanceCounter Lib _
                         "kernel32" (ByRef lpPerformanceCount As LargeInteger) As Long
    Private Declare Function QueryPerformanceFrequency Lib _
                         "kernel32" (ByRef lpFrequency As LargeInteger) As Long
#End If

Private counterStart As LargeInteger
Private crFrequency As Double

Private Const TWO_32 As Double = 4294967296#               ' = 256# * 256# * 256# * 256#

'==============================================================================
' Screen and Event Update Controls
'
Public Sub ReportUpdateState()
Attribute ReportUpdateState.VB_Description = "Prints to the immediate window the current state and values of the Application update controls."
    Debug.Print ":::::::::::::::::::::::::::::::::::::::::::::::::::::"
    Debug.Print "Application.ScreenUpdating      = " & Application.ScreenUpdating
    Debug.Print "Application.Calculation         = " & Application.Calculation
    Debug.Print "Application.EnableEvents        = " & Application.EnableEvents
    Debug.Print "--previousState.screenUpdate    = " & previousState.screenUpdate
    Debug.Print "--previousState.calculationType = " & previousState.calculationType
    Debug.Print "--previousState.eventsFlag      = " & previousState.eventsFlag
    Debug.Print "--previousState.callCounter     = " & previousState.callCounter
    Debug.Print "--DEBUG_MODE is currently " & DEBUG_MODE
End Sub

Public Sub DisableUpdates(Optional ByVal debugMsg As String = vbNullString, _
                          Optional ByVal forceZero As Boolean = False)
Attribute DisableUpdates.VB_Description = "Disables Application level updates and events and saves their initial state to be restored later. Supports nested calls. Displays debug messages according to the module-global DEBUG_MODE flag."
    With Application
        '--- capture previous state if this is the first time
        If forceZero Or (previousState.callCounter = 0) Then
            previousState.screenUpdate = .ScreenUpdating
            previousState.calculationType = .Calculation
            previousState.eventsFlag = .EnableEvents
            previousState.callCounter = 0
        End If

        '--- now turn it all off and count
        .ScreenUpdating = False
        .Calculation = xlCalculationManual
        .EnableEvents = False
        previousState.callCounter = previousState.callCounter + 1

        '--- optional stuff
        If DEBUG_MODE Then
            Debug.Print "Updates disabled (" & previousState.callCounter & ")";
            If Len(debugMsg) > 0 Then
                Debug.Print debugMsg
            Else
                Debug.Print vbCrLf
            End If
        End If
    End With
End Sub

Public Sub EnableUpdates(Optional ByVal debugMsg As String = vbNullString, _
                         Optional ByVal forceZero As Boolean = False)
Attribute EnableUpdates.VB_Description = "Restores Application level updates and events to their state, prior to the *first* DisableUpdates call. Supports nested calls. Displays debug messages according to the module-global DEBUG_MODE flag."
    With Application
        '--- countdown!
        If previousState.callCounter >= 1 Then
            previousState.callCounter = previousState.callCounter - 1
        ElseIf forceZero = False Then
            '--- shouldn't get here
            Debug.Print "EnableUpdates ERROR: reached callCounter = 0"
        End If

        '--- only re-enable updates if the counter gets to zero
        '    or we're forcing it
        If forceZero Or (previousState.callCounter = 0) Then
            .ScreenUpdating = True
            .Calculation = xlCalculationAutomatic
            .EnableEvents = True
        End If

        '--- optional stuff
        If DEBUG_MODE Then
            Debug.Print "Updates enabled (" & previousState.callCounter & ")";
            If Len(debugMsg) > 0 Then
                Debug.Print debugMsg
            Else
                Debug.Print vbCrLf
            End If
        End If
    End With
End Sub

'==============================================================================
' Precision Timer Controls
' based on https://stackoverflow.com/a/31387007/4717755
'
Private Function LI2Double(ByRef lgInt As LargeInteger) As Double
Attribute LI2Double.VB_Description = "Converts LARGE_INTEGER to Double"
    '--- converts LARGE_INTEGER to Double
    Dim low As Double
    low = lgInt.lowpart
    If low < 0 Then
        low = low + TWO_32
    End If
    LI2Double = lgInt.highpart * TWO_32 + low
End Function

Public Sub StartCounter()
Attribute StartCounter.VB_Description = "Captures the high precision counter value to use as a starting reference time."
    '--- Captures the high precision counter value to use as a starting
    '    reference time.
    Dim perfFrequency As LargeInteger
    QueryPerformanceFrequency perfFrequency
    crFrequency = LI2Double(perfFrequency)
    QueryPerformanceCounter counterStart
End Sub

Public Function TimeElapsed() As Double
Attribute TimeElapsed.VB_Description = "Returns the time elapsed since the call to StartCounter in microseconds."
    '--- Returns the time elapsed since the call to StartCounter in microseconds
    If crFrequency = 0# Then
        Err.Raise Number:=11, _
                  Description:="Must call 'StartCounter' in order to avoid " & _
                                "divide by zero errors."
    End If
    Dim crStart As Double
    Dim crStop As Double
    Static counterEnd As LargeInteger
    QueryPerformanceCounter counterEnd
    crStart = LI2Double(counterStart)
    crStop = LI2Double(counterEnd)
    TimeElapsed = 1000# * (crStop - crStart) / crFrequency
End Function

Answer 2

I would point out that the naming convention is inconsistent -- we have Activate and a Shutdown --- they don't say the same thing to me. I'd expect corresponding actions to have names like Activate -> Deactivate, Enable -> Disable, Startup -> Shutdown. Given the nature of the class, Enable/Disable pair seems the best suited for what your class is going to do, since it's basically toggling an application level state.

Your Shutdown has an On Error Resume Next but says nothing if it was successful. I would at least expect the procedure to exit with something like ShutDown = (Err.Number = 0) so that the calling client can at least know there was a clean exit or not. However, I'm more inclined to encourage to rethink whether you really want a Resume Next there, because you are dealing with altering the application's state, so it is crucial to know that the alterations were successful so that the clients can proceed with the assumption that they are in the correct state.

The other thing that occurs to me is that you probably don't actually need the methods at all. You only need the Class_Initialize and Class_Terminate. Your calling code then becomes...

Set booster = New ExcelTurboBooster
'Do lot of complicated stuff....
Set booster = Nothing 

Look, ma, no methods! In fact, technically you only need the first line. The last line is superficial because when your procedure that instantiate the object exits (even if it exited badly), the instance will go poof and the application state should be reset. The biggest downside of that method, though, is that error handling becomes somehow strange inside the Initialize/Terminate events, so it could be difficult to communicate to your clients there has been a problem.

The downside with the no-method approach is that it totally can't go into a public field of any standard module or maybe long-lived classes either because then they will persist too much longer. One'd have to take discipline to use it only as a local variable inside procedure so that it will be destroyed at the end of the procedure. Otherwise, you've built up a new layer of complication for nothing.

Answer 3

It's dangerous to not save the state of Application.Calculation and Application.ScreenUpdating.

Also, what's the purpose of the On Error Resume Next? There's no way for those two calls to fail, and the error handling gets turned back on when Shutdown exits.