Adding a layer of abstraction to UserFrom - am I doing it right? - excel

Question

I'd like to create a UserForm showing a progress of some operation (let's call it ProgressForm). I'd also like to make this form simple to reuse in multiple Workbooks by me and other coworkers. Finally, I'd like to make my ProgressForm as fool-proof as possible.
To make my form simple to use I've decided to create 3 "methods":
P_Begin(Goal) - to set up a goal and prepare form
P_Step() - to record a progress and update form
P_End() - to dispose a form
Now, to make it fool-proof I need some "system" that prevents using P_Step or P_End before P_Begin is called (as we need to set our goal FIRST before we try to make any progress).
My idea is to use a flag (let's call it "IsCreated") that will tell whether or not P_Begin was called. Here's what I've got so far:
Private IsCreated As Boolean
Private Goal As UInteger
Private Progress As UInteger
Function P_Begin(pGoal As UInteger)
If IsCreated Then
Err.Raise 5
End If
Goal = pGoal
Progress = 0
IsCreated = True
' Prepare ProgressForm elements here
Me.Show vbModeless
End Function
Function P_Step()
If Not IsCreated Then
Err.Raise 5
End If
Progress = Progress + 1
' Update ProgressForm elements here
End Function
Function P_End()
If Not IsCreated Then
Err.Raise 5
End If
IsCreated = False
Me.Hide
End Function
This is how I imagine sample use:
Sub DoingSomething()
ProgressForm.P_Begin pGoal:=100
For i = 1 To 100
' Doing Something
ProgressForm.P_Step
Next i
ProgressForm.P_End
ProgressForm.P_Begin pGoal:=200
For i = 1 To 200
'Doing Something Else
ProgressForm.P_Step
Next i
ProgressForm.P_End
' and so on...
End Sub
Looks pretty nice, right? Well, there's a "little" problem: IsCreated variable is not initialized when P_Begin is first called so my code is unreliable. I have some ideas how to deal with it, but none of them satisfies me:
Making IsCreated Public and setting it to False in Workbook_Open Sub - I don't like it as it makes ProgressForm less simple to use - Form user would have to remember to set IsCreated to False in every workbook that uses ProgressForm. Also: abstraction.
Dropping fool-proof requirement by not using IsCreated flag at all.
Rely on a fact that uninitialized Boolean is by default set to False, so IsCreated is conveniently set to False by default - it's just wrong, very, very wrong.
Somehow pushing IsCreated initialization to ProgressForm_Initialize() method. However, in order to show my form I first need to call P_Begin to set a goal. But P_Begin relies on IsCreated variable... Oops, circular dependency.
Somehow wrapping ProgressForm in class? It's just my guess, I must admit: I don't know how OOP works in VBA.
Somehow counting ProgressForms and setting IsCreated to false in the first one? Sounds a bit messy. Once again: it's just my wild guess based on this post, which I don't understand entirely...
Usually when I encounter a problem with apparently no solution, it usually means that my idea is fundamentally incorrect. Maybe there is better, VBA-way to achieve desired results?

Answer 1

Change as follows (possible solution):
Private IsCreated As Integer
...
IsCreated = 99 ' in P_Begin
...
If (IsCreated <> 99) then ' raise error
...
IsCreated = 0 ' in P_End
This relies on the fact that VB will [probably] never initialize an integer variable to 99.