How do you cause a custom function to execute when changes are made to worksheets / cells?

So, if I have a custom function in a cell, that uses data from multiple worksheets, I need for the custom function to run based on those changes.  Obviously, it may or may not impact the value returned into the cell by the function, but I need to make sure it runs.  In other words, I want it to behave like any other excel function.

My custom function is currently working, but If I change values in cells within the worksheet, it does not re-execute.

Any help on this would be greatly appreciated.
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byundtConnect With a Mentor Commented:
Application.Volatile will make your function update even if you change a value in a completely different workbook. I believe it will handle your situation exactly as you describe.

You must evaluate whether Application.Volatile makes your workbook recalc time too sluggish. Remember that it recalculates every time anything changes in any open workbook. If you manipulate a lot of data, then there could be an issue.

Application.Volatile will not update in response to format changes. I don't believe you are wanting that feature, but some people do--and they are disappointed by this gap between feature and requirement.
I like to put this in "ThisWorkbook"

Sub auto_open()

   ' Run the macro any time a entry is made

    For Each myWorksheet In ThisWorkbook.Worksheets
        myWorksheet.OnEntry = "MyFunction"
End Sub

Public Sub MyFunction()
    MsgBox ("changes made")
End Sub

Be careful with recursive calls

By the way - You should make it a habit to close and re-open the workbook whenever you create/modify a "Sub auto_open"

This is VERY important - otherwise, it won't function as desired, or not at all.

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While I personally prefer to pass the arguments and therefore force recalculation only when required, you might consider making your function volatile as discussed in http://www.decisionmodels.com/calcsecretsj.htm
Volatile functions (even UDFs) recalculate whenever anything changes in any open workbook.

Function MyFunc()
Dim MyVar As Variant
'Do Stuff
MyFunc = SomeCalculation
End Function
jamunseyAuthor Commented:
Thanks for the input.  I am thinking the Application.Volatile(TRUE) may work, but I want to clarify that for the worksheet that contains the function, it exists in many cells.  It may be somewhere in the neighborhood of a hundred cells.  Each function call has different inputs.  For example:

                       C1 = 102345
A2:  rtuo         C2  = Analyze_data($C$1, A2)
A3:  wejo        C3  = Analyze_data($C$1, A3)
A4:  bmnr       C4  = Analyze_data($C$1, A4)

So, the Function Analyze_data will run on the 2 pieces of data that are passed to it, but it also calls several support functions that access other worksheet cells to complete the analysis and return the final result in the given cell (like Cell C2).  So, I could change data on one of the other worksheets, and want to make sure that Analyze_data creates the updated result.  

With this additional explanation, Do you still think the Application.Volatile(TRUE) will work?  

Thanks so much.
jamunseyAuthor Commented:
I performed some preliminary testing and the Application.Volatile worked like a champ.  I will do some additional testing, but I believe I will be good to go with this as the approach.  'slycode' ... thanks for your input as well ... much appreciated.
jamunseyAuthor Commented:
Worked Perfectly.
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