why signal/slot not working with multiple threads? - multithreading

Question

class A : public QObject{
Q_OBJECT
signals:
void a_sig();
public:
A(){ }
public slots:
void begin(){
QObject::connect(&_timer, SIGNAL(timeout()), this, SIGNAL(a_sig()));
_timer.start(1000);
}
private:
QTimer _timer;
};
class B : public QObject{
Q_OBJECT
public:
B(){ value = 0; }
public slots:
void b_slot(){
++value;
QFile file("out.txt");
file.open(QIODevice::WriteOnly);
QTextStream out(&file);
out << value << "\n";
file.close();
}
private:
int value;
};
int main(int argc, char **argv){
QCoreApplication app(argc, argv);
A a;
B b;
QThread aThread;
QThread bThread;
QObject::connect(&aThread, SIGNAL(started()), &a, SLOT(begin()));
QObject::connect(&a, SIGNAL(a_sig()), &b, SLOT(b_slot()));
a.moveToThread(&aThread);
b.moveToThread(&bThread);
aThread.start();
bThread.start();
return app.exec();
}
I'm trying to understand why b_slot() isn't getting called. Can anyone explain what's happening, and why b_slot() isn't getting called?

Answer 1

The problem is the ownership of the _timer member of the A class.
Since you're not explicitly initializing it, it is initialized without a parent object. So the a.moveToThread(&aThread) isn't moving the timer to aThread, and things get confused after that.
Change A's constructor to:
A() : _timer(this) {}
and your b_slot() will get called.

Answer 2

The problem is that while object a is moved to aThread, _timer object still belongs to the original main thread. Try initializing _timer inside begin method like that:
void begin() {
_timer = new QTimer;
QObject::connect(_timer, SIGNAL(timeout()), this, SIGNAL(a_sig()));
_timer->start(1000);
}
private:
QTimer *_timer;