Automatically execute VBA macro when cell value changes - excel

Question

I have a worksheet (sheet1) which contains a cell A1 with formula ='sheet2'!D10. I would like to run a macro each time cell A1 in sheet1 changes (as a result of a change in D10 in sheet2). sheet2 is streaming financial data.
Because it is a change in value, Worksheet_Change does not trigger an event. I also can't seem to find a solution with Worksheet_Calculate.
In my research, the closest solution I could find was offered here, but I have not been able to successfully implement it.

Answer 1

You are going to have to use Worksheet_Calculate. It's unclear on whether the 'streaming' will trigger a Worksheet_Calculate in Sheet2 but the linked cell in Sheet1 will definitely trigger a Worksheet_Calculate in that worksheet's private code sheet providing you have calculation set to automatic.
You need a variable that will hold previous values of Sheet1!A1 that can be compared to the current value of Sheet1!A1. Some prefer to use a public var declared in a public module's declaration area; I prefer to use a static var within Sheet1's Worksheet_Calculate itself.
From Microsoft Docs,
Normally, a local variable in a procedure ceases to exist as soon as the procedure stops. A static variable continues to exist and retains its most recent value. The next time your code calls the procedure, the variable is not reinitialized, and it still holds the latest value that you assigned to it. A static variable continues to exist for the lifetime of the class or module that it is defined in.
The first issue is seeding the static variable for its first use. A variant-type variable that has never been given a value report True when tested with IsEmpty so when the workbook is first opened, the first calculation cycle will simply record the value of Sheet1!A1 into the static var. Any future calculation cycle will compare the value in Sheet1!A1 to the value held in the static var and if they are different, the external sub procedure ('... run a macro ...' according to your question's narrative) will be run and the new value of Sheet1!A1 will be stored in the static var. In this way, any change in the value returned by the formula in Sheet1!A1 will force a calculation cycle, hence the worksheet's Worksheet_Calculate event sub procedure which will in turn run your external sub procedure.
In Sheet1's private code sheet
Option Explicit
Private Sub Worksheet_Calculate()
Static s2d10 As Variant
If IsEmpty(s2d10) Then
'load static var with expected value
s2d10 = Cells(1, "A").Value2
ElseIf s2d10 <> Cells(1, "A").Value2 Then
'run sub procedure here
'... run a macro ...'
'load A1's current value into the static var
s2d10 = Cells(1, "A").Value2
End If
End Sub

Answer 2

Selection_Change & Change
I went into a different direction and got lost. I think there might be some useful stuff in here, so here's the code anyway. It could be working in most conditions, just lose the 'str1' lines.
The 'str1' lines are for debugging purposes and show the behavior of the cells at different conditions.
Not sure if the sub ChangeD10 is emulating your conditions.
Throwing in the towel, but would appreciate any pinpointing of errors in the code.
Option Explicit
Private TargetValue As Variant
Private TargetAddress As String
Private Sub Worksheet_Change(ByVal Target As Range)
'The Playground
Const cStrWs1 As String = "Sheet1"
Const cStrWs2 As String = "Sheet2"
Const cStrCell1 As String = "A1"
Const cStrCell2 As String = "D10"
'Other Variables
Dim oWs1 As Worksheet
Dim oWs2 As Worksheet
Dim oRng As Range
Dim varA1_Before As Variant
Dim varA1_Now As Variant
'Debug
Const r1 As String = vbCr
Dim str1 As String
'Initialize
Set oWs1 = ThisWorkbook.Worksheets(cStrWs1)
Set oWs2 = ThisWorkbook.Worksheets(cStrWs2)
Set oRng = oWs2.Range(cStrCell2)
varA1_Before = oWs1.Range(cStrCell1).Value
str1 = "Worksheet_Change"
'Play
If Target.Address = oRng.Address Then
If Target.Value <> TargetValue Then
varA1_Now = oWs2.Range(cStrCell2).Value
oWs1.Range(cStrCell1).Value = varA1_Now
str1 = str1 & r1 & Space(1) & "Cell '" & cStrCell2 & "' changed " _
& "(Target.Value <> TargetValue)" & r1 & Space(2) _
& "Before: TargetValue (" & TargetAddress & ") = '" _
& TargetValue & "'," & r1 _
& " varA1_Before (" & Range(cStrCell1).Address _
& ") = " & varA1_Before & "'," & r1 & Space(2) _
& "Now: Target.Value (" & Target.Address & ") = '" _
& Target.Value & "'," & r1 _
& " varA1_Now (" & Range(cStrCell1).Address _
& ") = " & varA1_Now & "'."
Else
str1 = str1 & r1 & Space(1) & "Cell '" & cStrCell2 _
& "' didn't change. TargetValue = '" & TargetValue _
& "' and Target.Value = '" & Target.Value & "'."
End If
Else
str1 = str1 & r1 & Space(1) & "Cell '" & cStrCell2 _
& "' not changed. The Target.Address is '" _
& Target.Address & "', TargetValue is '" & TargetValue _
& "' and Target.Value is '" & Target.Value & "'."
End If
Debug.Print str1
End Sub
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Const r1 As String = vbCr
Dim str1 As String
str1 = "Worksheet_SelectionChange"
If Target.Cells.Count = 1 Then
str1 = str1 & r1 & Space(1) & "Cell '" & Target.Address _
& "' selected " & r1 & Space(2) _
& "Before: TargetValue (" & TargetAddress & ") = '" _
& TargetValue & "'," & r1 & Space(2) _
& "Now: Target.Value (" & Target.Address & ") = '" _
& Target.Value & "'."
TargetValue = Target.Value
TargetAddress = Target.Address
Else
str1 = str1 & r1 & Space(1) & "Multiple cells in range '" _
& Target.Address & "'."
End If
Debug.Print str1
End Sub
Sub ChangeD10()
ThisWorkbook.Worksheets("Sheet2").Cells(10, 4) = 22
End Sub