Qt multithreading and let the second thread update the main (UI) thread

[Cupidvogel]

Yeah, that is definitely one way of doing it, but I did not do it because it would introduce cyclic dependency, that is A will include B and B will include A, etc. But I will make it work anyhow. In the mean time, I tried out the entire revised code.

The query results are coming out correctly, I am applying a qDebug() on them when they are generated and they are printed correctly. But the view is not being updated through the signal/slot. I am getting this debugging message:

QObject::connect: Cannot queue arguments of type 'std::shared_ptr<QueryResult>'
(Make sure 'std::shared_ptr< QueryResult >' is registered using qRegisterMetaType().)
 
//this starts coming from the qDebug() statement...
Query result:  context of a process and investigates the 
Query result:  this proved it as because

QObject::connect: Cannot queue arguments of type 'std::shared_ptr<QueryResult>'
(Make sure 'std::shared_ptr< QueryResult >' is registered using qRegisterMetaType().)

//this starts coming from the qDebug() statement...
Query result:  context of a process and investigates the 
Query result:  this proved it as because

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How can I make it work?

[anda_skoa]

Yeah, that is definitely one way of doing it, but I did not do it because it would introduce cyclic dependency, that is A will include B and B will include A, etc.

How does that introduce a dependency of any sort, much less a cyclic one?
Neither receiver nor sender need to know about each other!

QObject::connect: Cannot queue arguments of type 'std::shared_ptr<QueryResult>'
(Make sure 'std::shared_ptr< QueryResult >' is registered using qRegisterMetaType().)

QObject::connect: Cannot queue arguments of type 'std::shared_ptr<QueryResult>'
(Make sure 'std::shared_ptr< QueryResult >' is registered using qRegisterMetaType().)

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Seems like you have an argument type that you did not register with the Qt type system, as explained earlier by jefftee.
Especially the shared_ptr looks strange given that you should have a "const QueryResult&" as the argument type for both signal and slot.

So make sure that

1) your QueryResult class has been marked with Q_DECLARE_METATYPE
2) your QueryResult class has been registered with qRegisterMetaType()
3) your signal has a signature with either "QueryResult" or "const QueryResult&" as its argument type
4) your slot has a matching signature.

Cheers,
_

[Cupidvogel]

How does that introduce a dependency of any sort, much less a cyclic one?
Neither receiver nor sender need to know about each other!
_

I have worked around it.

How does that introduce a dependency of any sort, much less a cyclic one?
1) your QueryResult class has been marked with Q_DECLARE_METATYPE
2) your QueryResult class has been registered with qRegisterMetaType()
_

How do I do it? I found an example here: http://stackoverflow.com/questions/1...eta-type-in-qt, is this how it will work?

class QueryResult
{
 
QString queryString;
QString metadata;
int offset;
}
Q_DECLARE_METATYPE( QueryResult* )

class QueryResult
{

QString queryString;
QString metadata;
int offset;
}
Q_DECLARE_METATYPE( QueryResult* )

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Where do I call the qRegisterMetaType() method on QueryTerm then? And okay, I will change the signature to const QueryAction& instead of std::shared_ptr<QueryAction>.

[jefftee]

Where do I call the qRegisterMetaType() method on QueryTerm then? And okay, I will change the signature to const QueryAction& instead of std::shared_ptr<QueryAction>.

You can register the meta type at any time, but obviously must be done before you attempt to use it... I typically do that early in the constructor for the main widget.

[Cupidvogel]

Okay, I wil try to do it. But in the mean time, performance has not only not improved, it has badly worsened. I am just passing in a dummy QString to the view instead of the QueryAction object to check whether it actually works..

[jefftee]

Post your code again please. I have lost track of where your overall code base has morphed since the many posts to this thread have been made. Also, please elaborate on what performance badly worsened means.

[Cupidvogel]

Sure. It has become slow in the sense that previously, when there was a query term which resulted in a voluminous number of results, say 600~700, the UI used to kind of freeze, scrolling through the list of results (stored as QPushButtons in a QTableWidget) would become slow and dragging. But it worked correctly and more or less well. The loading size also stalled. But now the loading style is freezing completely, the cursor changes to the wheel icon (suggesting that the app has hung), and the results start updating, but soon stop, and finally, after quite some time, the view is updated with the remaining results, and the process completes.

QueryView.hpp:

QThread *t;

QThread *t;

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QueryView.cpp:

QueryView::QueryView()
{
	connect(SignalClass.get(),SIGNAL(updateViewSignal(QueryTerm&)), this, SLOT(updateView(QueryTerm&)));
	connect(queryButton, SIGNAL(clicked()),this, SLOT(doQuery()));
}
 
void QueryView::doQuery()
{
	t = new QThread()
	emit fSignalClass->stopExistingQuery();
	std::shared_ptr<QueryAction> action = std::make_shared<QueryAction>(queryTerm);
	action->moveToThread(t);
	t->start();
	emit fSignalClass->excuteSignal();
 
}
 
QueryView::updateView(QString& term)
{
	...update view with query result, currently a string for testing purpose
}

QueryView::QueryView()
{
	connect(SignalClass.get(),SIGNAL(updateViewSignal(QueryTerm&)), this, SLOT(updateView(QueryTerm&)));
	connect(queryButton, SIGNAL(clicked()),this, SLOT(doQuery()));
}
 
void QueryView::doQuery()
{
	t = new QThread()
	emit fSignalClass->stopExistingQuery();
	std::shared_ptr<QueryAction> action = std::make_shared<QueryAction>(queryTerm);
	action->moveToThread(t);
	t->start();
	emit fSignalClass->excuteSignal();
 
}
 
QueryView::updateView(QString& term)
{
	...update view with query result, currently a string for testing purpose
}

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QueryAction.hpp:

class QueryAction
 
{
	QMutex m_mutex;
}

class QueryAction
 
{
	QMutex m_mutex;
}

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QueryAction.cpp:

QueryAction::QueryAction(QueryTerm& queryTerm): fQueryTerm(queryTerm)
{
	connect(SignalClass.get(), SIGNAL(excuteSignal()), this, SLOT(startSearch()));
	connect(SignalClass.get(), SIGNAL(stopExistingQuery()), this, SLOT(stopQuery()));
	continueQuery = true;
}
 
void QueryAction::startSearch()
{
	//init cursor
	while (db_cursor != empty)
	{
		...get query result
		QMutexLocker locker(&m_mutex);
		if (continueQuery)
			emit fSignalClass->updateViewSignal(aResult.asQString());
		else
			break;
	}
}
 
void QueryAction::stopQuery()
{
	QMutexLocker locker(&m_mutex);
	continueQuery = false;
}

QueryAction::QueryAction(QueryTerm& queryTerm): fQueryTerm(queryTerm)
{
	connect(SignalClass.get(), SIGNAL(excuteSignal()), this, SLOT(startSearch()));
	connect(SignalClass.get(), SIGNAL(stopExistingQuery()), this, SLOT(stopQuery()));
	continueQuery = true;
}
 
void QueryAction::startSearch()
{
	//init cursor
	while (db_cursor != empty)
	{
		...get query result
		QMutexLocker locker(&m_mutex);
		if (continueQuery)
			emit fSignalClass->updateViewSignal(aResult.asQString());
		else
			break;
	}
}
 
void QueryAction::stopQuery()
{
	QMutexLocker locker(&m_mutex);
	continueQuery = false;
}

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[jefftee]

Your use of pointers in the emit'ing of signals for a different class/object seem unnatural to me. Not sure if that's a performance problem or not, but IMHO you should rework your signal/slot usage so that each class emits a signal when it has something that others may be interested in.

So in your example above, updateViewSignal should be a signal defined by QueryAction. All it needs to do when it has new data that anyone may be interested in is:

emit updateViewSignal(aResult.asQString());

emit updateViewSignal(aResult.asQString());

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The way you have your signals setup now, the object that wants to emit the signal has to have a pointer to the object you want to receive the signal, which totally undoes the abstraction that signals/slots provides.

[Cupidvogel]

Okay. So suppose the updateViewSignal signal belongs to QueryAction and it emits it like you said. How do the view class listen to it?

[jefftee]

Same way any other signal/slot connection works. To make any connect you need the address of the sender and receiver objects and the name/signature of the signal and the name/signature of the receiving slot.

[anda_skoa]

How do I do it? I found an example here: http://stackoverflow.com/questions/1...eta-type-in-qt, is this how it will work?

class QueryResult
{
 
QString queryString;
QString metadata;
int offset;
}
Q_DECLARE_METATYPE( QueryResult* )

class QueryResult
{

QString queryString;
QString metadata;
int offset;
}
Q_DECLARE_METATYPE( QueryResult* )

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No, the type itself:

Q_DECLARE_METATYPE(QueryResult)

Q_DECLARE_METATYPE(QueryResult)

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And okay, I will change the signature to const QueryAction& instead of std::shared_ptr<QueryAction>.

You mean QueryResult, right?

Your use of pointers in the emit'ing of signals for a different class/object seem unnatural to me.

Indeed, already pointed that out, only to be told that my advice was worked around :-/

Cheers,
_

[Cupidvogel]

Sorry guys for the late reply. I did as you said, moved all signals and slots to respective classes instead of relying on Signal Class as I was doing. Still the same performance, app hangs badly, the loading symbol completely freezes, but after some time all query results do show up on the UI. Here is my code:

QueryView.hpp:

class QueryView
{
	QThread *t;
	QueryAction *action;
	signals:
	    void excuteSignal();
	    void stopExistingQuery();
}

class QueryView
{
	QThread *t;
	QueryAction *action;
	signals:
	    void excuteSignal();
	    void stopExistingQuery();
}

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QueryView.cpp:

QueryView::QueryView()
{
	connect(SignalClass.get(),SIGNAL(updateViewSignal(QueryTerm&)), this, SLOT(updateView(QueryTerm&)));
	connect(queryButton, SIGNAL(clicked()),this, SLOT(doQuery()));
}
 
void QueryView::doQuery()
{
	t = new QThread()
	emit stopExistingQuery();
	action = new QueryAction(queryTerm);
 
	connect(this,SIGNAL(excuteSignal()), fSearchAction, SLOT(startQuery()));
    connect(this,SIGNAL(stopExistingQuery()), fSearchAction, SLOT(stopQuery()));
    connect(fSearchAction, SIGNAL(updateViewSignal(QString)),this, SLOT(updateView(QString)));
 
	action->moveToThread(t);
	t->start();
	emit excuteSignal();
 
}
 
QueryView::updateView(QString& term)
{
	...update view with query result, currently a string for testing purpose, as a QPushButton, forming a series of buttons in a QTableWidget
}

QueryView::QueryView()
{
	connect(SignalClass.get(),SIGNAL(updateViewSignal(QueryTerm&)), this, SLOT(updateView(QueryTerm&)));
	connect(queryButton, SIGNAL(clicked()),this, SLOT(doQuery()));
}
 
void QueryView::doQuery()
{
	t = new QThread()
	emit stopExistingQuery();
	action = new QueryAction(queryTerm);

	connect(this,SIGNAL(excuteSignal()), fSearchAction, SLOT(startQuery()));
    connect(this,SIGNAL(stopExistingQuery()), fSearchAction, SLOT(stopQuery()));
    connect(fSearchAction, SIGNAL(updateViewSignal(QString)),this, SLOT(updateView(QString)));

	action->moveToThread(t);
	t->start();
	emit excuteSignal();
 
}
 
QueryView::updateView(QString& term)
{
	...update view with query result, currently a string for testing purpose, as a QPushButton, forming a series of buttons in a QTableWidget
}

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QueryAction.hpp:

class QueryAction
 
{
	QMutex m_mutex;
 
	signals:
    	void updateSearchView2(QString);
}

class QueryAction
 
{
	QMutex m_mutex;

	signals:
    	void updateSearchView2(QString);
}

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QueryAction.cpp:

QueryAction::QueryAction(QueryTerm& queryTerm): fQueryTerm(queryTerm)
{
	QMutexLocker locker(&m_mutex);
	continueQuery = true;
}
 
void QueryAction::startSearch()
{
	//init cursor
	while (db_cursor != empty)
	{
		...get query result
		QMutexLocker locker(&m_mutex);
		if (continueQuery)
			emit updateViewSignal(aResult.asQString());
		else
			break;
	}
}
 
void QueryAction::stopQuery()
{
	QMutexLocker locker(&m_mutex);
	continueQuery = false;
}

QueryAction::QueryAction(QueryTerm& queryTerm): fQueryTerm(queryTerm)
{
	QMutexLocker locker(&m_mutex);
	continueQuery = true;
}
 
void QueryAction::startSearch()
{
	//init cursor
	while (db_cursor != empty)
	{
		...get query result
		QMutexLocker locker(&m_mutex);
		if (continueQuery)
			emit updateViewSignal(aResult.asQString());
		else
			break;
	}
}
 
void QueryAction::stopQuery()
{
	QMutexLocker locker(&m_mutex);
	continueQuery = false;
}

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[jefftee]

Post a minimal compilable example please so that I can recreate and/or step through your code and see what's going on.

[anda_skoa]

Here is my code:

No, it is not.
That is the same code snippets with the same problems pointed out several times.

Cheers,
_

[Cupidvogel]

Umm, like what? You mentioned that the signals and slots should not be from a separate class. I did that. What else needs to be done? I am currently passing a simple QString to display in the UI