SystemOpen inside a button waits until the end for being evaluated?

Question

I am writting with the hope of getting some help when using "SystemOpen".

Here is a simple example:

dir = "C:\\test2\\";
batFile  = StringJoin[dir, "test", ".bat"];    
txtFile = StringJoin[dir, "test", ".txt"]


Export[
    batFile
  , StringJoin["echo %date% %time%>", dir, "log.txt"]
  , "Text"
];



Button["Press to Run"
  , Export[txtFile, DateString[]]
  ; SystemOpen[ batFile  ]
  ; Pause[5]
  ; test = Import[txtFile, "Text"]
  ; Export[
        txtFile
      , StringJoin[test, "\n", DateString[]]
    ]
]

First line -> we define the working directory.
Second line -> We create a .bat file in the selected directory. When running it will print the current time in a log.txt file in the same directory.
Third line -> We create a button.

When we press the button:
- it exports the initial time in a "test.txt" file.
- Then it should run the .bat file and save the same time.
- Then it should pause for 5 seconds.
- Finally, it will read the initial time in "test.txt" and
- append a new line with the new current time.

We should see that the log.txt file has the initial time. On the other hand, it has the ending one!!!

Mathematica clearly does all the operations and leaves SystemOpen for the end (I tried other ideas such as loops or similar, always to find that SystemOpen executes at the end!!!).

Is this a bug? can it be solved? I really want to use SystemOpen as I know that it works like charm except for this issue.

Thanks to george2079 (SystemOpen), he realized that this happens when using a Button.

For example:

Do[
    Export[txtFile, DateString[]]
  ; SystemOpen[batFile]
  ; Pause[5]
  ; test = Import[txtFile, "Text"]
  ; Export[
        txtFile
      , StringJoin[test, "\n", DateString[]]
    ]
  , {i, 1, 1}
]

will work fine. My program has several menus and buttons and I need that button. Does someone know how to bypass this problem using "systemopen"?

Thanks in advance.

You don't have the probleme if you add the option Method-> "Queued" to the Button. (the default is "Preemptive"). — andre314, May 25 '17 at 17:44
You seem to think Export will append content to an already existing file. It doesn't. It replaces the content of a preexisting file with the new content. — m_goldberg, May 25 '17 at 17:50
@andre that does fix things, but does it actually make sense? — george2079, May 25 '17 at 18:56
@george2079 I can' t say if that really make sense. I have just tested Method -> "Queued" to see if that had an effect, and it turned out that the problem disappeared. The ideas I had in mind were A) Method ->"Preemptive" is thought to execute a piece code quickly (Timeout= 5 second) so it don't care of what's happening when it launches a system-process (though even with Method-> ""Queued" it is done asynchronously ). — andre314, May 25 '17 at 20:44
As I supposed that theses processes timing are unpredictable, it is possible that the system process start with a delay (maybe when Mathmetica has ended a piece of code) B) Many questions comes from that the OPs are not aware of the option Method->"Queued" Maybe pure speculation.I'm not a expert at all. — andre314, May 25 '17 at 20:45
Thanks a lot. That fixed the issue :) Still I don't understand why other functions are done in the correct order without the need of "Queued" method...
@m_goldberg, there is a StringJoin which joins test (contains the imported value of the file) with the new value and exports it to the same file again, so I am joining previous value with the new one. I know that you can write stuff at the end of the file (>>>) but, in reality, I wanted to prepend the new values to the old ones (the new ones in the beginning of the file). I did not know how to do it so I simply StringJoined {new,old}. — neuron, May 26 '17 at 08:44

Kuba · Answer 1 · 2017-05-30T21:05:09.557

You need Method -> "Queued" for a Button, the reason is the following:

communication through a preemptive link blocks the FrontEnd

No dynamic updates, not any UI interaction is possible till it is finished.
SystemOpen needs FrontEnd to finish

At the end it is the FrontEnd which does the final step, take a look:
```
<< GeneralUtilities`

PrintDefinitions @ SystemOpen
```
It boils down to the line with:
```
MathLink`CalFrontEnd @ FrontEnd`SystemOpen
```
And this step is delayed till your code finishes.

SystemOpen inside a button waits until the end for being evaluated?

1 Answers1

Further reading: