Controller-Worker thread qml signal repeating

Eligijus

Hello,
My application communicates to an USB device by custom protocol. I'm moving blocking methods to another thread. And it looks something like this:

cpp side:

#include <QObject>
#include <QThread>
#include <QDebug>

class Worker : public QObject
{
    Q_OBJECT
public:
    explicit Worker(QObject *parent = nullptr) : QObject(parent) {}

signals:
    void blockingFunction1Finished();

public slots:
    void blockingFunction1() {
        qDebug() << "blocking function1 started";
        QThread::sleep(1);
        emit blockingFunction1Finished();
    }
};

class Controller : public QObject
{
    Q_OBJECT
public:
    explicit Controller(QObject *parent = nullptr) : QObject(parent) {
        Worker *worker = new Worker;
        worker->moveToThread(&mWorkerThread);
        connect(this, &Controller::beginBlockingFunction1, worker, &Worker::blockingFunction1);
        connect(worker, &Worker::blockingFunction1Finished, this, &Controller::blockingFunction1Finished);
        mWorkerThread.start();
    }

    Q_INVOKABLE void startBlockingFunction1() {
        emit beginBlockingFunction1();
    }

signals:
    void beginBlockingFunction1();
    void blockingFunction1Finished();

public slots:

private:
    QThread mWorkerThread;
};

register:
qmlRegisterType<Controller>("Qt.Test", 1, 0, "Controller");

qml side:

import Qt.Test 1.0


Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Controller {
        id: controller
        onBlockingFunction1Finished: console.log("blocking function1 finished")
    }

    Button {
        onClicked: controller.startBlockingFunction1()
    }
}

What bugs me is that I have to? make 2 connections to know that blocking method finished. One connection on cpp side from Worker to Controller and one in qml side.

Is there a way to get around connecting Worker signal to Controller connect(worker, &Worker::blockingFunction1Finished, this, &Controller::blockingFunction1Finished); and just have one connection in qml onBlockingFunction1Finished: console.log("blocking function1 finished").

And lastly I believe that there is no elegant way to call Worker slot from qml without making a connection and writing a wrapper method to emit a signal:

    Q_INVOKABLE void startBlockingFunction1() {
        emit beginBlockingFunction1();
    }

is this correct?

J.Hilk

@Eligijus

hi, there are a couple of things you can do, that I know of, to make your life a bit easier.

first of, no need to wrap your signal in function to call it from qml. The Q_INVOKABLE macro also works directly for signals. Signals are (basically) just functions.

AFAIK you can no simply access a signal in qml, that is emitted from a different Thread in a c++ backend.

But you should be able to chain-connect one signal to another, that reduces the issue to one line of code

Eligijus

@J.Hilk Thanks for your feedback.
Could you give me an example how would chain-connecting one signal to another would look like. Or what should I add to my example code to fix the issue?

J.Hilk

@Eligijus said in Controller-Worker thread qml signal repeating:

@J.Hilk Thanks for your feedback.
Could you give me an example how would chain-connecting one signal to another would look like. Or what should I add to my example code to fix the issue?

looking through your example, I think you're already doing that. this

connect(worker, &Worker::blockingFunction1Finished, this, &Controller::blockingFunction1Finished);

connects the signal from your worker to the signal of your controller.

kkettinger

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