wait until a signal is processed

[mentalmushroom]

Hi all, I am developing a multi-threaded app that emits signals processed in another threads. I need to wait for a signal to be processed. According to the documentation, Qt::BlockingQueuedConnection seems to be right for that case, but the following code doesn't work (connect returns false):

bool con = connect(thread, SIGNAL(mysignal2(MyClass &)), this, SLOT(myslot2(MyClass &)), Qt::BlockingQueuedConnection);

bool con = connect(thread, SIGNAL(mysignal2(MyClass &)), this, SLOT(myslot2(MyClass &)), Qt::BlockingQueuedConnection);

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The same problem is also with Qt::QueuedConnection, but if a don't specify the connection type, it works fine, but of course doesn't wait for a signal processing.

So, this code works:

bool con = connect(thread, SIGNAL(mysignal2(MyClass &)), this, SLOT(myslot2(MyClass &)));

bool con = connect(thread, SIGNAL(mysignal2(MyClass &)), this, SLOT(myslot2(MyClass &)));

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I use Q_DECLARE_METATYPE and call qRegisterMetaType before establishing a connection. Please, give me a tip what can be wrong here.

[wysota]

Do you get a runtime warning about the connections? How do you use qRegisterMetaType? In general what you have will probably not work because you are using non-const references. Either use const references or if you have to modify the objects being passed through the signal, use pointers. And think twice if you really need a signal to handle this situation. Do you expect more than one receiver to connect to it? Do you expect a situation that no slot is connected to the signal? If answer to any of the two last questions is "no" then you probably shouldn't be using signals at all.

[mentalmushroom]

i have MyClass declared as

class MyClass
{
};
 
Q_DECLARE_METATYPE(MyClass)

class MyClass
{
};

Q_DECLARE_METATYPE(MyClass)

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and this is how i establish my connections:

MyClass myclass;
 
qRegisterMetaType<MyClass>();
 
bool con;
con = connect(thread, SIGNAL(mysignal1(int)), this, SLOT(myslot1(int)), Qt::BlockingQueuedConnection);
con = connect(thread, SIGNAL(mysignal2(MyClass &)), this, SLOT(myslot2(MyClass &)), Qt::BlockingQueuedConnection);

MyClass myclass;

qRegisterMetaType<MyClass>();

bool con;
con = connect(thread, SIGNAL(mysignal1(int)), this, SLOT(myslot1(int)), Qt::BlockingQueuedConnection);
con = connect(thread, SIGNAL(mysignal2(MyClass &)), this, SLOT(myslot2(MyClass &)), Qt::BlockingQueuedConnection);

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i see the following in the output window:

QObject::connect: Cannot queue arguments of type 'MyClass&'
(Make sure 'MyClass&' is registered using qRegisterMetaType().)
The program '[13916] testsignals.exe: Native' has exited with code 0 (0x0).

QObject::connect: Cannot queue arguments of type 'MyClass&'
(Make sure 'MyClass&' is registered using qRegisterMetaType().)
The program '[13916] testsignals.exe: Native' has exited with code 0 (0x0).

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maybe the issue is caused by reference?

[wysota]

You can try registering the reference as well but in my opinion this doesn't make much sense as the data will get copied anyway.

qRegisterMetaType<MyClass&>("MyClass&");

qRegisterMetaType<MyClass&>("MyClass&");

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By the way, are you sure about the affinity of your "thread" object? Can you show us how you declare and use it?

[mentalmushroom]

just tried

qRegisterMetaType<MyClass&>("MyClassRef");

qRegisterMetaType<MyClass&>("MyClassRef");

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it can't be compiled.

and i found the following code works:

bool con = connect(thread, SIGNAL(mysignal3(MyClass)), this, SLOT(myslot3(MyClass)), Qt::BlockingQueuedConnection);

bool con = connect(thread, SIGNAL(mysignal3(MyClass)), this, SLOT(myslot3(MyClass)), Qt::BlockingQueuedConnection);

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here is the code where the thread is created and started:

testsignals::testsignals(QWidget *parent, Qt::WFlags flags)
	: QMainWindow(parent, flags)
{
	ui.setupUi(this);
 
	MyThread *thread = new MyThread(this);
 
	qRegisterMetaType<MyClass>();
 
	bool con;
	con = connect(thread, SIGNAL(mysignal1(int)), this, SLOT(myslot1(int)), Qt::BlockingQueuedConnection);
	con = connect(thread, SIGNAL(mysignal2(MyClass &)), this, SLOT(myslot2(MyClass &)), Qt::BlockingQueuedConnection);
 
 
	thread->start();
	thread->wait();
}

testsignals::testsignals(QWidget *parent, Qt::WFlags flags)
	: QMainWindow(parent, flags)
{
	ui.setupUi(this);

	MyThread *thread = new MyThread(this);

	qRegisterMetaType<MyClass>();

	bool con;
	con = connect(thread, SIGNAL(mysignal1(int)), this, SLOT(myslot1(int)), Qt::BlockingQueuedConnection);
	con = connect(thread, SIGNAL(mysignal2(MyClass &)), this, SLOT(myslot2(MyClass &)), Qt::BlockingQueuedConnection);


	thread->start();
	thread->wait();
}

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and here is the thread's run procedure:

void MyThread::run()
{
	emit mysignal1(44);
 
	MyClass m;
	emit mysignal2(m);
}

void MyThread::run()
{
	emit mysignal1(44);

	MyClass m;
	emit mysignal2(m);
}

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i know passing stack allocated data by reference isn't good, but just for test...

[wysota]

It's not about stack or non-stack. It's about mutable and non-mutable. A reference to an object is mutable, a copy of the object is not. Thus the latter works and the former doesn't.

And your thread affinity is ok - the Thread object works in context of the main thread and the "m" object works in context of the worker thread.

[mentalmushroom]

ok, but is there any way to pass a reference or const reference to signal/slot? actually i need to pass a QExplicitlySharedPointer as a signal/slot argument, and it seems if i don't pass it as a reference, i have to implement a copy constructor and possibly assignment operator that is inconvenient when such signal/slots are used in a several different classes with a lot of member values.

[wysota]

A const reference is not a problem, it should work out of the box. Passing a reference doesn't make sense. As I said, before reaching the slot, the data will be copied so you'd operate on a copy anyway. Better use pointers. But before you do that, as I already said, think a couple of times if signals are a good approach here. It might be much easier to do what you want using events, for instance like this:

MyEvent event;
event.setSomeData(data);
QApplication::sendEvent(someObjectInAnotherThread, &event); // this will block
data = event.data(); // modified content

MyEvent event;
event.setSomeData(data);
QApplication::sendEvent(someObjectInAnotherThread, &event); // this will block
data = event.data(); // modified content

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or using a classical approach with a wait condition blocking the sending thread.

[mentalmushroom]

strange, i've just noticed when calling QCoreApplication::sendEvent on some object in another thread the following error appears: "ASSERT failure in QCoreApplication::sendEvent: Cannot send events to objects owned by a different thread. …"assertion_failure.png

[wysota]

Yes, unfortunately that's true. But you can have the same effect using a wait condition. Post the event, wait on the condition until it is signalled by the handling object when the data is ready.