Cat On A Spreadsheet

Cat On A Spreadsheet

Designing Fully Event-Driven Excel Models

In the previous post, we explored using event handlers like Worksheet_Change and Workbook_SheetChange to trigger calculations in response to user actions. That’s powerful, but you can go even further: you can replace traditional Excel formulas with a fully event-driven system where VBA controls all calculations.

 

This approach is especially useful in large financial models, simulations, or dashboards where volatile formulas bog down performance.

 

What Is an Event-Driven Model?

 

In a standard workbook:

  • Cells contain formulas.

  • When any precedent cell changes, Excel recalculates dependent cells.

 

In an event-driven model:

  • Cells store only inputs and outputs.

  • VBA event handlers detect changes to inputs and run custom calculation logic that updates outputs.

 

This way, calculations occur only when and where they’re needed, not across the entire dependency chain.

 

Core Components

  • Defined Input Ranges: Where the user enters data.

  • Defined Output Ranges: Where results appear.

  • Calculation Procedures: VBA functions that process inputs and write outputs.

  • Event Handlers: Code that ties everything together, ensuring calculations run only when inputs change.

 

Example: A Mini Event-Driven Model

 

Let’s build a simple profit calculator.

 

  • Inputs:

    • Quantity (B2)

    • Unit Price (B3)

    • Cost per Unit (B4)

  • Output:

    • Profit (B6)

 

Instead of formulas, we’ll make VBA handle everything.

Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("B2:B4")) Is Nothing Then
     Application.EnableEvents = False
     RunProfitCalc
     Application.EnableEvents = True
End If
End Sub
Private Sub RunProfitCalc()
Dim qty As Double
Dim price As Double
Dim cost As Double
Dim profit As Double

qty = Range("B2").Value
price = Range("B3").Value
cost = Range("B4").Value

profit = (qty * price) - (qty * cost)
Range("B6").Value = profit
End Sub

 

Now, whenever the user changes any input (B2:B4), the profit is recalculated instantly.

 

Example: Modularizing with Multiple Procedures

 

For larger models, separate calculations into different procedures. For example:

Public Sub Greet()
     MsgBox "Hello, " & pName & " (" & pEmail & ")"
End Sub

 

This way, each metric has its own calculation block. That makes the model easier to debug and expand.

 

Example: Multi-Sheet Event-Driven Logic

 

In more complex workbooks, you might centralize calculations in one module and call them from any sheet’s change event.

Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
Select Case Sh.Name
     Case "Sales"
          If Not Intersect(Target, Sh.Range("B2:B4")) Is Nothing Then RunProfitCalc Sh
     Case "Inventory"
          If Not Intersect(Target, Sh.Range("C2:C10")) Is Nothing Then RunStockCheck Sh
End Select
End Sub

 

Each sheet is treated as a module of the system, with its own input triggers and calculation routines.

 

Advantages of Event-Driven Models

 

  • Performance: Only essential calculations are executed.

  • Clarity: Inputs and outputs are explicitly defined.

  • Flexibility: You can implement custom business logic far beyond what formulas allow.

  • Scalability: Easy to extend with new procedures.

 

Pitfalls and Safeguards

 

  • Complex Debugging: Tracing logic is harder since calculations happen in VBA, not visible formulas.

  • Risk of Broken Links: If input/output references are wrong, results may silently fail.

  • Maintenance Burden: Future users must understand the VBA layer.

 

Best practices:

 

  • Always use Application.EnableEvents = False before making programmatic changes.

  • Document inputs/outputs clearly in the workbook.

  • Consider logging calculation runs (e.g., timestamp + procedure name).

  • Test thoroughly for edge cases (clearing cells, pasting ranges, undo).

 

Final Thoughts

 

Event-driven models transform Excel into a controlled application platform rather than just a formula-driven calculator. This approach is not always necessary—but in performance-critical scenarios, or when creating Excel-based apps for teams, it can dramatically improve efficiency and user experience.

With ThisWorkbook.Sheets(1)
     .Range("A1").Value = "Timestamp"
     .Range("B1").Value = "Log Level"
     .Range("C1").Value = "Message"
End With
End Sub

 

You can call this sub before running ParseLogFile.

 

⚠️ Bonus: Handling Irregular Lines

 

Some logs may have missing or malformed entries. Use error handling:

On Error Resume Next
' ... parsing logic ...
If Err.Number <> 0 Then
     Message = "PARSE ERROR: " & LineText
     Err.Clear
End If
On Error GoTo 0

 

🧠 Why Use VBA for This?

 

While tools like Python are more common for text processing, VBA offers:

  • Native integration with Excel

  • Immediate visualization and manipulation of parsed data

  • A great option for non-developers working in Excel-driven environments

 

🔚 Final Thoughts

 

VBA is more than just forms and macros—it’s a lightweight data processing engine built right into Excel. With a bit of string manipulation and file handling, you can turn chaotic log files into structured, actionable data in minutes.

 

Want to go further? Try:

  • Parsing multi-line log entries

  • Highlighting rows with ERROR or WARN

  • Exporting results as a CSV or Excel table

18 August 2025

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Cat On A Spreadsheet

Cat On A Spreadsheet