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Thread: Problem with closeEvent handler implementation.

  1. #1
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    Default Problem with closeEvent handler implementation.

    I'm trying to reimplement a closeEvent handler but is not working as i want. I hope you can help me. I need start a python program as a last "task" of my main program. Here my first attempt.
    Qt Code:
    1. void MainWindow::closeEvent(QCloseEvent *wd) {
    2. QString uploaderPath = "python " + QCoreApplication::applicationDirPath( ) + "/Uploader/setter.py";
    3.  
    4. QProcess *up = new QProcess( this );
    5. up->start( uploaderPath );
    6.  
    7. wd->accept();
    8. }
    To copy to clipboard, switch view to plain text mode 
    Error message when i close my main program:
    QProcess: Destroyed while process ("python") is still running.
    So, i tried to create a second thread where i could run this python program. I thought this program would run regardless of my main program. I close my main program, since the second(python) will run in another thread he will do what he should and will die too. But i was wrong. Here the code.
    Second attempt.
    Qt Code:
    1. #ifndef RESETTER_H
    2. #define RESETTER_H
    3.  
    4. #include <QtCore>
    5.  
    6. class Resetter : public QThread {
    7. public:
    8. Resetter( );
    9. void run( );
    10. };
    11.  
    12. #endif // RESETTER_H
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    Qt Code:
    1. #include "resetter.h"
    2.  
    3. Resetter::Resetter( ) { }
    4.  
    5. void Resetter::run( ) {
    6. QString uploaderPath = "python " + QCoreApplication::applicationDirPath( ) + "/Uploader/setter.py";
    7.  
    8. QProcess *up = new QProcess( this );
    9. up->start( uploaderPath );
    10. }
    To copy to clipboard, switch view to plain text mode 

    Qt Code:
    1. void MainWindow::closeEvent(QCloseEvent *wd) {
    2. Resetter rs;
    3.  
    4. rs.start( );
    5.  
    6. wd->accept();
    7. }
    To copy to clipboard, switch view to plain text mode 

    Error message when i close my main program:
    QThread: Destroyed while thread is still running
    Segmentation fault (core dumped)

    Or create that object on the heap:
    Qt Code:
    1. void MainWindow::closeEvent(QCloseEvent *wd) {
    2. Resetter *rs = new Resetter( );
    3.  
    4. rs->start( );
    5.  
    6. wd->accept();
    7. }
    To copy to clipboard, switch view to plain text mode 
    Error message when i close my main program:
    QCoreApplication::applicationDirPath: Please instantiate the QApplication object first
    QObject: Cannot create children for a parent that is in a different thread.
    (Parent is QThread(0x112e700), parent's thread is QThread(0x1098e60), current thread is QThread(0x112e700)

  2. #2
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    Default Re: Problem with closeEvent handler implementation.

    You can either make the program wait for the process or use QProcess::startDetached().

    Cheers,
    _

  3. The following user says thank you to anda_skoa for this useful post:

    robgeek (18th February 2016)

  4. #3
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    Default Re: Problem with closeEvent handler implementation.

    What is likely causing your problem (well, several things, actually) are the facts that you are creating your QProcess instances with the MainWindow instance as parent. This means the QProcess instance will be destroyed when the main window is destroyed. Secondly, QApplication's default setting is to quit when the last top-level window closed. In your case, quitting will ensure that the MainWindow instance is destroyed along with your QProcess. Creating your Resetter instance on the stack is useless, since it will be destroyed as soon as the closeEvent() exits.

    You can try starting your process detached, as anda_skoa suggests, or you can set the quit on last window closed flag to false. Or, maybe you can create a second, non-visible top-level widget that can be the parent of your QProcess. You can connect the QProcess::finished() signal to that widget's close() slot. Other possibilities as well - turn the quit flag off, keep the QProcess as a child of MainWindow, but connect the finished() signal to the app's quit() slot.

  5. #4
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    Default Re: Problem with closeEvent handler implementation.

    Quote Originally Posted by anda_skoa View Post
    You can either make the program wait for the process or use QProcess::startDetached().

    Cheers,
    _
    Its working as i want. Thanks!
    But what's the difference between use QProcess::startDetached() and QThreads as i tried?

  6. #5
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    Default Re: Problem with closeEvent handler implementation.

    When using QProcess::startDetached(), an external process that is then detached from the parent process, so that it can persist after the parent process has ended.
    I write the best type of code possible, code that I want to write, not code that someone tells me to write!

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