Qt debug pointer not following expected flow in 2nd and successive iterations

Question

I want to display a "generating image..." kind of modal dialog, other than the main GUI. This "generating image..." dialog should be temporary, and be displayed and disappear without user intervention.

For displaying this dialog, the Qt code should check for existence of a .txt file in a specific location in the PC's hard disk. If the .txt file exists, then the dialog should pop-up.

For making this dialog disappear, the Qt code should check whether that .txt file contains the string "OK" in the first line. The dialog should disappear only when this "OK" is found, until then it should continue to display "generating image..."

A good way to do this is to use signal slot mechanism. I would like to know, what functions should be used as SIGNALS in both the cases, of displaying and removing the dialog.

So far, I could manage a simple code, illustrating a "generating image..." using signal slot mechanism, but with setValue() and pressing a push button(i.e. involving user intervention), and not with the checking of .txt file or the "OK" string inside that .txt file(user non-intervention).

Please advise me, whether my logic can be implemented? If yes, how? Also, what SIGNALs should be used?

************************UPDATED SECTION(as of Feb 24th '14):****************************************************

I have revised the code according to Dmitry Sazonov's suggestions. I am able to display the loading GIF whenever a new file is created/deleted in a designated directory. Now I want to close this loading qDialog, when the usbResponse.txt file has "ok" inside it. I tried using signal slot, to implement hide(), but could not get it.

I do not get errors, but the qDialog window does not close as expected. I tried both, secDialog.close() and secDialog.hide(), but the window didn't close. Perhaps because the secDialog object is not the same in both the SLOTs. So I also made secDialog, a global object, but I received an error as follows:-

   QWidget: Must construct a QApplication before a QWidget

I looked it up: https://qt-project.org/forums/viewthread/12838

Changed the build modes, accordingly, but that didn't help either. Please tell me how do I close my qDialogs, when I find the "ok" in usbResponse.txt file.

************************UPDATED SECTION(as of Mar 14th '14):****************************************************

I could close the qDialog containing the GIF using hide(). I have done a total overhaul of the code. As mentioned above, the qDialog containing the GIF should appear whenever a text file called usbResponse.txt exists at a designated location. Also taking @Dmitry Sazonov's advice, I am able to close the GIF whenever that txt file i.e. usbResponse.txt is modified, using FileSystemWatcher.

I'm continuously scanning for the existence of the .txt using threads. When I find the file, I display the loading GIF. When the .txt is modified the GIF should disappear. This works fine for the first iteration, i.e. when (the following are observations after debugging)

1. the usbResponse.txt exists => GIF is displayed

2. when usbResponse.txt is modified => GIF is hidden & the .txt is deleted.

THe problem, in next iteraiton,(i.e. all iterations after the first)

1. the usbResponse.txt is created => the GIF is displayed.

2. when usbResponse.txt is modified, the debug pointer continues to remain in

   afterFileHasBeenFound()

whereas it should have gone in

closeModified(const QString &str)

What is my mistake here? Here is my code:

mainwindow.h

    #ifndef MAINWINDOW_H
    #define MAINWINDOW_H

    #include <QMainWindow>
    #include <QFile>
    #include <QDebug>
    #include <QFileSystemWatcher>
    #include "dialog.h"
    #include "mythread.h"

    namespace Ui {
    class MainWindow;
    }

    class MainWindow : public QMainWindow
    {
        Q_OBJECT

    public:
        explicit MainWindow(QWidget *parent = 0);
        ~MainWindow();


    public slots:
        void afterFileHasBeenFound();
        void closeModified(const QString &str);

    private slots:

    private:
        Ui::MainWindow *ui;
        Dialog *pDialog;
        MyThread *mThread;
    };

#endif // MAINWINDOW_H

dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include <QMovie>
#include <QLabel>

#define GIF_PATH "E:\\QT1\\timeStampPopUp\\timeStampPopUp\\loading.gif"
namespace Ui {
class Dialog;
}

class Dialog : public QDialog
{
    Q_OBJECT

public:
    explicit Dialog(QWidget *parent = 0);
    ~Dialog();
    void displayLoadingGif();

private:
    Ui::Dialog *ui;
};

#endif // DIALOG_H

mythread.h

#ifndef MYTHREAD_H
#define MYTHREAD_H

#include <QThread>
#include <QtCore>
#include <QDebug>

#define FILE_PATH "E:\\QT1\\dialogClose2\\dialogClose2\\usbResponse.txt"

class MyThread : public QThread
{
    Q_OBJECT
public:
    explicit MyThread(QObject *parent = 0);
    void run();
    QString name;
    int exec();
    void checkFile();

signals:
    void testSignal(QString message);
    void fileFoundDisplayGif();

public slots:

};

#endif // MYTHREAD_H

dialog.cpp

#include "dialog.h"
#include "ui_dialog.h"

Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);
    displayLoadingGif();
}

Dialog::~Dialog()
{
    delete ui;
}

void Dialog::displayLoadingGif()
{
    QMovie *pMovie = new QMovie(GIF_PATH);
    ui->loadingGifLabel->setMovie(pMovie);
    pMovie->start();
}

mythread.cpp

#include "mythread.h"

MyThread::MyThread(QObject *parent) :
    QThread(parent)
{
}

void MyThread::run()
{
    exec();
}

int MyThread::exec()
{
    while(1)
    {
        checkFile();
        emit(testSignal("hello world!!"));
        sleep(1);
    }
}

void MyThread::checkFile()
{
    QFile file(FILE_PATH);
    if(file.exists())
    {
        qDebug()<<"exists";
        emit(fileFoundDisplayGif());
    }
    else
        qDebug()<<"doesn't exist";
}

mainwindow.cpp

    #include "mainwindow.h"
    #include "ui_mainwindow.h"

    MainWindow::MainWindow(QWidget *parent) :
        QMainWindow(parent),
        ui(new Ui::MainWindow)
    {
        ui->setupUi(this);
        mThread = new MyThread(this);
        mThread->name = "mThread";
        connect(mThread, SIGNAL(fileFoundDisplayGif()), this, SLOT(afterFileHasBeenFound()), Qt::QueuedConnection);
        mThread->start();
    }

    MainWindow::~MainWindow()
    {
        delete ui;
    }

    void MainWindow::afterFileHasBeenFound()
{
    if(pDialog != NULL)
        return;
    pDialog = new Dialog();
    pDialog->setModal(true);
    pDialog->show();
}

void MainWindow::closeModified(const QString &str)
{
    Q_UNUSED(str)
    if(pDialog != NULL)
    {
        pDialog->hide();
    }
    QFile file(FILE_PATH);
    file.remove();
    pDialog = NULL;
}

main.cpp

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QFileSystemWatcher fileWatcher;
    fileWatcher.addPath(FILE_PATH);
    QStringList fileList = fileWatcher.files();
    Q_FOREACH(QString file, fileList)
        qDebug() << "File name " << file;
    MainWindow* mc = new MainWindow;
    QObject::connect(&fileWatcher, SIGNAL(fileChanged(QString)), mc, SLOT(closeModified(QString)));
    mc->show();


    return a.exec();
}

Answer 1

Do not use timers for checking file. Use QFileSystemWatcher for traking file modifications.

Implement a slot that will check file content on modification. And call hide() method, when your "OK" text appeared.

IMHO: your solution is to messy. There are a lot of other syncronization mechanisms between processed. Can you modify code of tool that generates image? Should it really work as another process?