Background thread won't react to signal until it is finished

[kshegunov]

@vlada said:

In the worker class there is a function

That'll do it. You're blocking the event loop, as @Chris-Kawa correctly guessed. :)
You must have a running event loop (unblocked) so the slot invocation events are processed. My advice is to run this particular for loop through the event queue, for example:

class MyClass : public QObject
{
    // ...
    Q_INVOKABLE void slowFunction(int);
}

void MyClass::start()
{
    const int startIndex = 0;
    QMetaObject::invokeMethod(this, "slowFunction", Qt::QueuedConnection, Q_ARG(int, startIndex));
}

void MyClass::slowFunction(int i)
{
    if (i >= something)
        return;

    // Do the slow thing

    // Queue the function for the next iteration
    i++;
    QMetaObject::invokeMethod(this, "slowFunction", Qt::QueuedConnection, Q_ARG(int, i));
}

[Chris Kawa]

What @kshegunov said or use a 0-timeout timer as a loop:

void MyClass::start()  {
   startTimer(0);
}

void MyClass::timerEvent(QTimerEvent* e) {
   if (something--)
      slowFunction();
   else
      killTimer(e->timerId());
}

[vlada]

Thanks a lot for your suggestions. I fixed my code and now it works correctly. Unfortunately it is a little slower then it used to be. But that is not a big problem.

[vlada]

Sorry to reopen this old thread but I still have some unsolved issues.

I found one case where it would be very difficult and slow to implement the above suggestions. Would it be possible to tell the thread to pause a "for" or a "while" loop and check if didn't receive a cancel signal?

Another problem I just discovered is that old threads are not deleted. Here is my code:

    QThread* searchThread = new QThread;
    FileSearch* search = new FileSearch(directories, 2); // 2 = get all files from subdirectories
    search->moveToThread(searchThread);
    connect(searchThread, SIGNAL(started()), search, SLOT(start()));
    connect(search, SIGNAL(fileCountUpdate(int)), this, SLOT(onFileCountUpdate(int)));
    connect(search, SIGNAL(filesFound(QStringList)), this, SLOT(onFilesFound(QStringList)));
    connect(this, SIGNAL(¨), search, SLOT(cancelScanning()));
    connect(search, SIGNAL(finished()), search, SLOT(deleteLater()));
    connect(searchThread, SIGNAL(finished()), searchThread, SLOT(deleteLater()));
    searchThread->start();

If I run this thread a couple times and then emit the cancelScanning() signal, I can see it was called on all the previously run (and finished) threads except of the one which is currently running.

My thread should find all files in a given path and all subdirectories. It works but it is sometimes difficult to cancel or takes a long time if there are many files or directories in one location.

What I can not successfully solve is the situation when there is a search running (first of all I don't know how to detect this at all). What happens now is that if I start a new thread which finishes before the current one, I get response (return value emitted as a signal) from both threads. Instead of it, I would like to get response only from last one called. How could I achieve this? Thank you.

[karlheinzreichel]

You can use a QWaitCondition to break the thread when the thread is in a loop.
See example : http://doc.qt.io/qt-5/qtcore-threads-waitconditions-example.html

regards
Karl-Heinz

[kshegunov]

@vlada said in Background thread won't react to signal until it is finished:

Would it be possible to tell the thread to pause a "for" or a "while" loop and check if didn't receive a cancel signal?

Nope. There are other options though - one is to batch up the data processing (as recommended above) - split the chunks of work through the event loop. Another option is to use an (atomic) flag or a volatile variable for that. E.g:

class FileSearch : public QObject
{
    // ...
private:
    volatile bool scanning;
};

void FileSearch::start()
{
    // ...
    scanning = true;
    while ( scanning )  {
         // ... do scanning or w/e
    }
    // ...
}

void FileSearch::cancelScanning()
{
    scanning = false;
}

and you connect that slot directly:

connect(this, SIGNAL(...), search, SLOT(cancelScanning()), Qt::DirectConnection);

What happens now is that if I start a new thread which finishes before the current one, I get response (return value emitted as a signal) from both threads. Instead of it, I would like to get response only from last one called. How could I achieve this?

Cancel the current operation before starting another one (if I get the question correctly).

PS.
Unless you're stuck to Qt4 then you should migrate the signal-slot connections to the Qt5 syntax.

[Chris Kawa]

@kshegunov

From the creator of C++:

Do not use volatile except in low-level code that deals directly with hardware.
Do not assume volatile has special meaning in the memory model. It does not. It is not -- as in some later languages -- a synchronization mechanism. To get synchronization, use atomic, a mutex, or a condition_variable.

[kshegunov]

Yeah, I know Chris, in this particular case the race is benign.

PS. And it's last on the list ... ;D

[dheerendra]

Signals/Slots communication across thread works through event loop. Inside the start() try to call processEvents. You start() function is busy & may not get time to process the slot().

:) So many responses. I did not see all them b4 posting my response.

[vlada]

I tried to call the processEvents() by a timer every 200ms or after processing 1000 items and that seems to do the trick.

Also I found out I was missing one more connection:

connect(search, SIGNAL(finished()), searchThread, SLOT(quit()));

That was causing that my old threads were still alive even though the start() function already finished.

@kshegunov Thank you for the hint regarding new SLOT/SIGNAL syntax. I somehow didn't realize it has changed since the old one still works.