QInputDialog & Threading ( worker, show dialog, wait for input, continue)

Question

I have a question about a QInputDialog. I have spent the last 3 days on Google and thought it is time to ask here, since I cannot find any answer;

My application has a main thread (also known as the GUI-thread in Qt). This GUI-thread creates a worker. Which is ran in a different thread. This worker scans folders. It also sends the GUI-thread information about progress. Working fine.

Now here is the problem. The worker thread can come across a situation in which it must ask the user for input. A QString. It must wait for an answer before continuing scanning the remaining folders. However, a worker thread cannot show a QInputDialog it turns out. Only the GUI-thread.

I cannot use slots and signals either because they cannot return a value in Qt. I tried using a slot and a referenced QString, but it crashes sometimes. Not thread-safe I suppose.

I tried QMetaObject::invokeMethod but couldn't get it to work either. Also, is that even thread safe?

Anyone here that has a solution for this?

Below is my code if that helps, it has been "compacted" so I don't waste your valuable time on getting familiar with my variable names and actual stuff.

INITIAL CODE (QInputDialog in worker thread) MainWindow.cpp

void MainWindow::worker_create(){
    Worker* worker = new Worker;
    QThread* thread = new QThread;
    worker->moveToThread(thread);
    connect(thread, SIGNAL(started()), worker, SLOT (start_work()));
    connect(worker, SIGNAL(worker_status_changed(QByteArray)), ChanComObject, SLOT(worker_update(QByteArray)));
    connect(worker, SIGNAL(finished(QString)), this, SLOT(worker_destroy(QString)));
    connect(worker, SIGNAL(finished(QString)), worker, SLOT(deleteLater()));
    connect(worker, SIGNAL(finished(QString)), thread, SLOT (quit()));
    connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
    thread->start();
}

Worker.cpp

Worker::ask(){
    QStringList listToChooseFrom;
    listToChooseFrom.append("A");
    listToChooseFrom.append("B");
    QString answer = QInputDialog::getItem(this, "Title", "Message", listToChooseFrom, 0, false, &ok);
    return answer;
}

SNEAKY REFERENCE CODE (Referenced QString, works sometimes, other times it crashes) Sneaky reference trick I tried was like this.. first in MainWindow.cpp:

void MainWindow::worker_create(){
    // all other stuff from above, but an extra connect:
    connect(worker, SIGNAL(worker_asks(Qstring*)), SLOT(gui_thread_dialog(QString*)));
}
void MainWindow::gui_thread_dialog(*sneakyReturnValue){
     QString answer = QInputDialog::getItem(this, "Title", "Message", listToChooseFrom, 0, false, &ok);
    *sneakyReturnValue = answer;
}

Then in the worker..

Worker::ask(){
    QString sneakyReturnValue;
    emit worker_asks(*sneakyReturnValue);
    return sneakyReturnValue;
}

INVOKEMETHOD CODE (Can't get it working due to parent(), not sure if thread-safe either) The invokeMethod I tried I never got working, but it went like.. in MainWindow.cpp

Q_INVOKABLE QString askUser();

..and in MainWindow.cpp

QString MainWindow::askUser(){
    QStringList listToChooseFrom;
    listToChooseFrom.append("A");
    listToChooseFrom.append("B");
    return QInputDialog::getItem(this, "Title", "Message", listToChooseFrom, 0, false, &ok);
}

and finally in Worker.cpp

Worker::ask(){
    QString dialogReturn;
    QStringList listToChooseFrom;
    listToChooseFrom.append("A");
    listToChooseFrom.append("B");
    bool u = QMetaObject::invokeMethod(parent,
                              "askUser",
                              Qt::BlockingQueuedConnection,
                              Q_RETURN_ARG(QString, dialogReturn),
                              Q_ARG(QStringList, listToChooseFrom));
    if(!u){ qDebug() << "invokeMethod failed"; }
    result = dialogReturn;
}

But I couldn't get a reference to the GUI-thread working.. the first argument of invokeMethod. parent isn't working. I think my worker is not a child of my GUI-thread automatically. Well, parent() isn't working at least.

Answer 1

The closest solution you have proposed is to use QMetaObject::invokeMethod(), but you should not use parent() but pass it an object of the class that has the method to invoke, in this example I will pass the GUI:

worker.h

#ifndef WORKER_H
#define WORKER_H

#include <QDebug>
#include <QStringList>
#include <QInputDialog>
#include <QThread>

class Worker : public QObject
{
    Q_OBJECT
    QObject *mGui;

public:
    explicit Worker(QObject *gui, QObject *parent = nullptr):QObject(parent){
        mGui = gui;
    }
    virtual ~Worker(){}

public slots:
    void start_work(){
        ask_user();
    }
    void ask_user(){
        QStringList choices;
        choices.append("one");
        choices.append("two");
        QString retVal;
        for(int i=0; i < 10; i++){
            QMetaObject::invokeMethod(mGui, "callGuiMethod", Qt::BlockingQueuedConnection,
                                      Q_RETURN_ARG(QString, retVal),
                                      Q_ARG(QStringList, choices));
            qDebug()<<retVal;
            QThread::sleep(5);//emulate processing
        }
    }

signals:
    void finished();
};

#endif // WORKER_H

widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include "worker.h"

#include <QThread>
#include <QWidget>

class Widget : public QWidget
{
    Q_OBJECT
    QThread workerThread;
public:
    explicit Widget(QWidget *parent = nullptr):QWidget(parent){
        worker_create();
    }

    Q_INVOKABLE QString callGuiMethod(QStringList items){
        return QInputDialog::getItem(0, "title", "label", items , 0, false);
    }

    ~Widget() {
        workerThread.quit();
        workerThread.wait();
    }

public slots:
    void worker_create(){
        Worker *worker = new Worker(this);
        worker->moveToThread(&workerThread);
        connect(&workerThread, &QThread::started, worker, &Worker::start_work);
        connect(worker, &Worker::finished, worker, &QObject::deleteLater);
        connect(worker, &Worker::finished, &workerThread, &QThread::quit);
        connect(&workerThread, &QThread::finished, &workerThread, &QObject::deleteLater);
        workerThread.start();
    }
};

#endif // WIDGET_H

The complete example can be found at the following link