How to connect signal-slog in second thread?

[arcull]

Hi everyone.

I'm working on a little desktop gui app, which starts some other thread which would watch for changes in a certain file. I know I don't need a second thread for this, but would really like to, since I would do some time consuming processing on that file change event and would like to keep my main thread where gui app resides, still responsive to user comands.

Ok, so I have my second thread, but the problem is, that I can not connect signal-slot in it. The message I get at run time is "QObject::connect: No such slot QThread::HandleFileChange(const QString)". The relevant part of the code is bellow. Much thanks for help.

worker.h

#ifndef WORKER_H
#define WORKER_H
#include <QThread>
#include <QFileSystemWatcher>
 
class Worker: public QThread
{
public:
    Worker(QString SrcFileParam,QString WorkerFolderParam);
 
    void run();
 
private:
    QString SrcFilename,WorkerFolder;
    int Cntr;
    QFileSystemWatcher * watcher;
 
public slots:
    void HandleFileChange(const QString fileName);
 
};
 
#endif // WORKER_H

#ifndef WORKER_H
#define WORKER_H
#include <QThread>
#include <QFileSystemWatcher>

class Worker: public QThread
{
public:
    Worker(QString SrcFileParam,QString WorkerFolderParam);

    void run();

private:
    QString SrcFilename,WorkerFolder;
    int Cntr;
    QFileSystemWatcher * watcher;

public slots:
    void HandleFileChange(const QString fileName);

};

#endif // WORKER_H

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worker.cpp

#include "Worker.h"
#include <iostream>
#include <QMessageBox>
#include <QFile>
#include <QFileInfo>
 
Worker::Worker(QString SrcFileParam,QString WorkerFolderParam)
{    
    SrcFilename = SrcFileParam;
    WorkerFolder = WorkerFolderParam;
 
    watcher = new QFileSystemWatcher(this);
    watcher->addPath(this->SrcFilename);
 
}
 
void Worker::run() {
    connect(watcher,SIGNAL(fileChanged(QString)),this,SLOT(HandleFileChange(const QString)));  //problematical line
    for (int i=1;i<100;i++) {
        //std::cout << qPrintable(i);
        //std::cout << "printed from Worker class \nl";
        //QMessageBox::information(0,tr("Info"),QString::number(i));
    }
 
}
 
void Worker::HandleFileChange(const QString fileName) {
 // some code
}

#include "Worker.h"
#include <iostream>
#include <QMessageBox>
#include <QFile>
#include <QFileInfo>

Worker::Worker(QString SrcFileParam,QString WorkerFolderParam)
{    
    SrcFilename = SrcFileParam;
    WorkerFolder = WorkerFolderParam;

    watcher = new QFileSystemWatcher(this);
    watcher->addPath(this->SrcFilename);

}

void Worker::run() {
    connect(watcher,SIGNAL(fileChanged(QString)),this,SLOT(HandleFileChange(const QString)));  //problematical line
    for (int i=1;i<100;i++) {
        //std::cout << qPrintable(i);
        //std::cout << "printed from Worker class \nl";
        //QMessageBox::information(0,tr("Info"),QString::number(i));
    }

}

void Worker::HandleFileChange(const QString fileName) {
 // some code
}

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mainwindow.cpp

Worker wkr1(ui->lineEdit->text(),ui->lineEdit_2->text());
wkr1.start();
wkr1.wait();

Worker wkr1(ui->lineEdit->text(),ui->lineEdit_2->text());
wkr1.start();
wkr1.wait();

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[prasad_N]

signal is fileChanged ( const QString &) not fileChanged (QString)
and you need to add Q_OBJECT marco in worker class.

one more thing is, if we use signal/slot across threads we need to start local event loop with exec() at the end of the run function;

[arcull]

Thanks prasad_N, but

signal is fileChanged ( const QString &) not fileChanged (QString)

according to codecompletion that is not true.

and you need to add Q_OBJECT marco in worker class.

would you please show a short sample how to do it, remember a newbie

one more thing is, if we use signal/slot across threads we need to start local event loop with exec() at the end of the run function;

I'm not sure I understand this one,should I put exec() in main thread or inside Worker::run. If you can add adjustments to my case I would be really gratefull, much thanks again.

[anda_skoa]

according to codecompletion that is not true

Correct, just QString is the simplified/normalized argument type signature.

would you please show a short sample how to do it, remember a newbie

Q_OBJECT is placed right after the opening brace { of the class declaration

class MyClass : public QObject
{
    Q_OBJECT
};

class MyClass : public QObject
{
    Q_OBJECT
};

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I'm not sure I understand this one,should I put exec() in main thread or inside Worker::run. If you can add adjustments to my case I would be really gratefull, much thanks again.

You only need event loop on the thread that receives the signals. In your case that is the main thread.
The instance of Worker is created by the main thread, thus the slot HandleFileChange(QString) will be executed in the main thread.
The signal source, in this case the file system watcher, is also in the main thread, so its events are also handled there.

The only thing the worker thread does in this example is running a for loop and then exit.

Cheers,
_

[arcull]

Correct, just QString is the simplified/normalized argument type signature.


Q_OBJECT is placed right after the opening brace { of the class declaration

class MyClass : public QObject
{
    Q_OBJECT
};

class MyClass : public QObject
{
    Q_OBJECT
};

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You only need event loop on the thread that receives the signals. In your case that is the main thread.
The instance of Worker is created by the main thread, thus the slot HandleFileChange(QString) will be executed in the main thread.
The signal source, in this case the file system watcher, is also in the main thread, so its events are also handled there.

The only thing the worker thread does in this example is running a for loop and then exit.

Cheers,
_

Thanks. You are right, that is how it would work now. But I would like the second thread (Worker class) to receive the signals and connect them to correct slot, ie. I would like the watching and handling file changes to be done by the second thread. How could I accieve this? Besides, what would I put in the run method of Worker class in this case, because I need it to "keep alive" and not exit if that is not requested from the main thread. Would an infinite loop be ok? Please suggest, a short snippet of code would be great. Thanks for your help.

[anda_skoa]

There are three options:

1) treat your run() method like you would treat the main() function, i.e. forget about the encapsulating object.
Just create an instance of the actual worker object and call exec() on the thread.

2) Use the moveToThread approach: create an instance of a QObject derived worker class and an instance of plain QThread and call worker->moveToThread(threadInstance)

3) Create the file system watcher in run() instead of your thread class' constructor and use Qt:irectConnection when connecting to the slot in your thread class.
Call exec() at the end of run()

Cheers,
_

[arcull]

Thanks anda_skoa.

1) treat your run() method like you would treat the main() function, i.e. forget about the encapsulating object.
Just create an instance of the actual worker object and call exec() on the thread.

This one sounds the easiest So in my case, run() method of Worker class would have just one line of code to connect signal slot? After calling exec() on the thread, will the thread stay alive, that is, would it handle signal slot events? Am I still able to terminate this thread from the first one? Thanks again.

[d_stranz]

Maybe reading some of the Qt threading examples and tutorials would help you.

[prasad_N]

may help you this : http://blog.debao.me/2013/08/how-to-...ht-way-part-1/

[arcull]

Ok, I've solved it with a bit of trial and error. Here is a working sample:
mainwindow.cpp

Worker *wrkr1;
wrkr1 = new Worker(ui->lineEdit->text(),ui->lineEdit_2->text());
wrkr1->start();

Worker *wrkr1;
wrkr1 = new Worker(ui->lineEdit->text(),ui->lineEdit_2->text());
wrkr1->start();

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workder.h

#ifndef WORKER_H
#define WORKER_H
#include <QThread>
#include <QFileSystemWatcher>
#include <QObject>
 
class Worker: public QThread
{
Q_OBJECT
 
public:
    Worker(QString SrcFileParam,QString WorkerFolderParam);
 
    void run();
 
private:
    QString SrcFilename,WorkerFolder;
    int Cntr;
    QFileSystemWatcher * watcher;
 
public slots:
    void HandleFileChange(const QString fileName);
 
};
 
#endif // WORKER_H

#ifndef WORKER_H
#define WORKER_H
#include <QThread>
#include <QFileSystemWatcher>
#include <QObject>

class Worker: public QThread
{
Q_OBJECT

public:
    Worker(QString SrcFileParam,QString WorkerFolderParam);

    void run();

private:
    QString SrcFilename,WorkerFolder;
    int Cntr;
    QFileSystemWatcher * watcher;

public slots:
    void HandleFileChange(const QString fileName);

};

#endif // WORKER_H

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worker.cpp

#include "Worker.h"
#include <iostream>
#include <QMessageBox>
#include <QFile>
#include <QFileInfo>
 
Worker::Worker(QString SrcFileParam,QString WorkerFolderParam)
{
    std::cout << "Worker constructor \n";
    SrcFilename = SrcFileParam;
    WorkerFolder = WorkerFolderParam;
 
    watcher = new QFileSystemWatcher();
    watcher->addPath(this->SrcFilename);
 
}
 
void Worker::run() {
    connect(watcher,SIGNAL(fileChanged(QString)),this,SLOT(HandleFileChange(const QString)));
    qDebug("method run in Worker thread");
    exec();
}
 
void Worker::HandleFileChange(const QString fileName) {
	qDebug("HandleFileChange triggered...");
	//some code
}

#include "Worker.h"
#include <iostream>
#include <QMessageBox>
#include <QFile>
#include <QFileInfo>

Worker::Worker(QString SrcFileParam,QString WorkerFolderParam)
{
    std::cout << "Worker constructor \n";
    SrcFilename = SrcFileParam;
    WorkerFolder = WorkerFolderParam;

    watcher = new QFileSystemWatcher();
    watcher->addPath(this->SrcFilename);

}

void Worker::run() {
    connect(watcher,SIGNAL(fileChanged(QString)),this,SLOT(HandleFileChange(const QString)));
    qDebug("method run in Worker thread");
    exec();
}

void Worker::HandleFileChange(const QString fileName) {
	qDebug("HandleFileChange triggered...");
	//some code
}

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To sum up, I've made 3 crucial mistakes:
1)forgetting Q_OBJECT macro, without it the Worker thread wont have signal/slot functionality
2)starting the second thread and then waiting.
3)missing exec() in my Worker thread

Now it works,I'm happy. Thank you all for help.

[anda_skoa]

So in my case, run() method of Worker class would have just one line of code to connect signal slot?

No.
As I wrote, this approach treats run() like main().
There is no slot to connect to unless you create an object that has such a slot.

After calling exec() on the thread, will the thread stay alive, that is, would it handle signal slot events?

Yes.

Am I still able to terminate this thread from the first one?

Yes, simply by calling the thread's quit() method.

Ok, I've solved it with a bit of trial and error. Here is a working sample:

No.
That still processes everything in the main thread.
Your worker thread does nothing, it jus sits there waiting for events that never come.

Cheers,
_