Execute code when file is modified in Mathematica 12.0

Question

I have a text file called file.txt that is being modified every few seconds by an external program (my Stenography machine's software).

myCode is a program which takes as input a List called data which is Imported from file.txt.

How can I execute myCode everytime this file is modified?

The following code does not work but looks like what I need.

file = NotebookDirectory[] <> "file.txt";
Dynamic[
    Refresh[
        fileSize := FileByteCount[file];
        data = Import[file, "Table"];
        myCode[data]
        ,TrackedSymbols :> {fileSize}
    ]
]

• to help de-abstract the problem, here is are two example cases of myCode and file.txt

  myCode[data_] := Style[ListLinePlot[Flatten[data], PlotTheme -> "Marketing"], Magnification -> 2];
  Export[NotebookDirectory[]<>"file.txt", {1, 2, 3}]
  

This question has been asked 8 years ago for Mathematica 9.0. Sadly the answer provided then does not work in Mathematica 12.0

Note: I am not interested in repeatedly executing myCode every $n$ seconds. I only want to execute myCode IF file.txt is modified.

---

I'm afraid I oversimplified my question above. I wanted to try and present a minimal example. My true problem is below:

myCode is a program which takes the last element of data (a list of integers Imported from file.txt) and computes the ratio with the last element of another list (imported from another file which is sometimes modified (when this second file is modified it is modified at the same time $t$ as when file.txt is modified)). myCode then AppendsTo this ratio to a third list (which is then plotted). I only want it to do this operation if file.txt is modified.

Answer 1

Look at your code. You request that the code inside Request is executed every 5 seconds. AT the same time you specify TrackedSymbols :> {fileSize} what means, that the code should be executed when fileSizechanges. But this is not necessary because the code is executed every 5 seconds anyway.

Here is a code example that runs on version 12.1:

 myCode[d_] = ListLinePlot[d];
Dynamic[
 data = Import[file];
 myCode[data], UpdateInterval -> 2]

Answer 2

update: A superior solution can be found here

---

Here is a simple solution using FinishDynamic inside a While loop that checks every $\Delta t$ seconds for changes and runs myCode once if a change is detected:

step 1

Execute:

Dynamic[x]

step 2

Run a While loop that calculates one FileHash of file.txt before and another FileHash after a small interval $\Delta t$. In each iteration: use an If statement to check if the hashes are equal or not. If they are not equal then myCode is executed. FinishDynamic pauses the While loop during each iteration until the output of myCode is rendered in the Dynamic cell above the cell containing the While loop.

Δt = 0.1;
file = NotebookDirectory[] <> "file.txt";
myCode[data_] := Style[ListLinePlot[Flatten[data], PlotTheme -> "Marketing"], Magnification -> 2];

While[True,
      FinishDynamic[];
      fileHashT = FileHash[file];
      Pause[Δt];
      fileHashTplusΔt = FileHash[file];
      
      If[ fileHashT != fileHashTplusΔt,
      
          data = Import[file, "Table"];
          x = myCode[data]
       ]   
]

---

note:

in this example, file.txt was created with:

Export[NotebookDirectory[]<>"file.txt",{1,2,3}]//SystemOpen