Can an object living in one thread emit a signal from another thread?

rrd0

I have an object a created in the main thread, where its signals are connected to slots of other object, b, that also lives in the main thread.

Then I create an object c and move it to a worker thread, where it will periodically access object a through a pointer, change its state and emit its signals.

Here is the code:

(Note: some lines were purposely ignored to keep the example short)

class A : public QObject
{
    Q_OBJECT
    friend class C;
signals:
    void sigA();
private:
    quint8 _m1;
    quint8 _m2;
};

class B : public QObject
{
    Q_OBJECT
public slots:
    void slotB();
};

class C : public QObject
{
    Q_OBJECT
public:
    C(A* a) : _a{a}, _timer{new QTimer{this}}
    {
        _timer->setInterval(5);
        connect(_timer, &QTimer::timeout, this, &C::doSomeWork);
    }

public slots:
    void start() { _timer->start(); }

private slots:
    void doSomeWork()
    {
        // update members of `a`
        emit _a->sigA();
        // ...
    }

private:
    A* _a;
    QTimer* _timer;
};

int main(int argc, char* argv[])
{
    QApplication app(argc, argv);

    A* a {new A};
    B* b {new B};
    QObject::connect(a, &A::sigA, b, &B::slotB);

    QThread* worker {new QThread};
    C* c {new C{a}};    
    c->moveToThread(worker);

    QObject::connect(worker, &QThread::started, c, &C::start);    
    worker->start();

    return app.exec();
}

I hope a signals, emitted from thread worker, will trigger b slots in main thread. And I hope I'll have no concurrency problems because a members are 8-bit sized.

But c, that lives in thread worker, will be posting events in the main thread event queue. I'm not using any synchronization primitive, so I'm afraid I may run into problems.

May I use the signal/slot mechanism the way I'm using in the code example? Am I doing something wrong?

Kent-Dorfman

@rrd0 said in Can an object living in Thread A emit a signal from Thread B?:

Then I create an object C and move it to a worker thread W, where it will access object A through a pointer, change its state and emit its signals.

I think that is asking for trouble. Use the signal/slot mechanism exclusively. have C emit signals to slots in A. Don't access A members directly via its object pointer.

jsulm

This post is deleted!

J.Hilk

@rrd0
I principle, there should be no reason for problems.

Connecting signals to signals is an established feature of Qt's Signal/Slot mechanism the fact that it in the end, will cross 3 different threads does't matter much.

As long, as you do not force a DirectConnection at least. Also keep in mind, that your original signal will pass through 3 different event loops, so the execution of the final slot may very well be delayed by significant amount of time.

rrd0

@J.Hilk said in Can an object living in Thread A emit a signal from Thread B?:

@rrd0
I principle, there should be no reason for problems.

Connecting signals to signals is an established feature of Qt's Signal/Slot mechanism the fact that it in the end, will cross 3 different threads does't matter much.

But what about concurrent issues? I updated my question and added a code example. Can you please have a look?

KroMignon

@rrd0 I don't understand the meaning of this construct... why not move a in the worker thread?

rrd0

@KroMignon said in Can an object living in one thread emit a signal from another thread?:

@rrd0 I don't understand the meaning of this construct... why not move a in the worker thread?

I'm actually trying to do that, but the problem I have is a little more complex than the example I wrote here. If a could keep a in the main thread it would be a lot easier for me.

SGaist

Hi,

Why exactly are you trying to emit a signal from an instance of class A in C ? That doesn't look clean and it's not the correct way to emit a signal. The purpose of a signal is to notify the outside that something has changed inside an object.