Connecting QML to a C++ signal - connection doesn't work on the first time

[frankiefrank]

I'm playing around with connections between C++ and QML.

I've found a case in which it seems like a signal connection doesn't work at first, then works on calls from other locations in the code.

A bit of code to clarify:

// Header
#include <QObject>
#include <QDateTime>
 
class MyQmlInterface : public QObject
{
	Q_OBJECT
 
public:	
 
	Q_PROPERTY(QString singleValue READ singleValue WRITE setSingleValue NOTIFY singleValueChanged)
 
	const QString &singleValue() const;
	void setSingleValue(const QString &newSingleValue);
	void emitCppSignal(const QString &passedStr);
 
	Q_INVOKABLE QDateTime getCurrentDateTime() const {
         return QDateTime::currentDateTime();
	}
 
signals:
	void singleValueChanged();
        // THIS is the signal I want to connect to
	void signalFromCpp(const QString &passedStr);
 
public slots:
	void somethingHappened();
 
private:
	QString mSingleValue;
};
 
// Cpp
 
const QString &MyQmlInterface::singleValue() const 
{
	return mSingleValue;
}
 
void MyQmlInterface::setSingleValue(const QString &newSingleValue)
{
	if (newSingleValue == mSingleValue)
		return;
	mSingleValue = newSingleValue;
	emit singleValueChanged();
}
 
void MyQmlInterface::somethingHappened()
{
	setSingleValue("O RLY");
}
 
// Method to emit the signal
void MyQmlInterface::emitCppSignal(const QString &passedStr)
{
	emit signalFromCpp(passedStr);
}
 
// QML
 
import QtQuick 1.0
 
Rectangle {
    width: 360
    height: 360
 
    Text {
        id: myText
        anchors.centerIn: parent
        text: myInterface.singleValue
    }
 
    Connections {
            target: myInterface
            onSignalFromCpp: {
                myText.text += passedStr
                myText.rotation += 90
            }
        }
}

// Header
#include <QObject>
#include <QDateTime>

class MyQmlInterface : public QObject
{
	Q_OBJECT

public:	

	Q_PROPERTY(QString singleValue READ singleValue WRITE setSingleValue NOTIFY singleValueChanged)
	
	const QString &singleValue() const;
	void setSingleValue(const QString &newSingleValue);
	void emitCppSignal(const QString &passedStr);

	Q_INVOKABLE QDateTime getCurrentDateTime() const {
         return QDateTime::currentDateTime();
	}

signals:
	void singleValueChanged();
        // THIS is the signal I want to connect to
	void signalFromCpp(const QString &passedStr);

public slots:
	void somethingHappened();

private:
	QString mSingleValue;
};

// Cpp

const QString &MyQmlInterface::singleValue() const 
{
	return mSingleValue;
}

void MyQmlInterface::setSingleValue(const QString &newSingleValue)
{
	if (newSingleValue == mSingleValue)
		return;
	mSingleValue = newSingleValue;
	emit singleValueChanged();
}

void MyQmlInterface::somethingHappened()
{
	setSingleValue("O RLY");
}

// Method to emit the signal
void MyQmlInterface::emitCppSignal(const QString &passedStr)
{
	emit signalFromCpp(passedStr);
}

// QML

import QtQuick 1.0

Rectangle {
    width: 360
    height: 360

    Text {
        id: myText
        anchors.centerIn: parent
        text: myInterface.singleValue
    }

    Connections {
            target: myInterface
            onSignalFromCpp: {
                myText.text += passedStr
                myText.rotation += 90
            }
        }
}

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Now here is where I initialize my Qml display and expect to see the result of my signal being emitted:

// The types are what you expect :)
        mView = new QDeclarativeView(this);
	mEngine = mView->engine();
	mContext = mEngine->rootContext();
 
	mQmlInterface = new MyQmlInterface;
 
	QString filePath = "D:\\QmlTest\\QmlTest.qml";
	mView->setSource(filePath);
 
	mContext->setContextProperty("myInterface", mQmlInterface);
	mQmlInterface->setSingleValue("Set after context...");
 
	ui.verticalLayout->addWidget(mView);
	mView->show();
 
	//! If needed - this is how to clean up in case Qml wants to quit
	//connect(mEngine, SIGNAL(quit()), this, SLOT(declarativeEngineQuit()));
 
	// This first signal call doesn't work
	mQmlInterface->emitCppSignal("this getting to you?");

// The types are what you expect :)
        mView = new QDeclarativeView(this);
	mEngine = mView->engine();
	mContext = mEngine->rootContext();

	mQmlInterface = new MyQmlInterface;

	QString filePath = "D:\\QmlTest\\QmlTest.qml";
	mView->setSource(filePath);

	mContext->setContextProperty("myInterface", mQmlInterface);
	mQmlInterface->setSingleValue("Set after context...");

	ui.verticalLayout->addWidget(mView);
	mView->show();

	//! If needed - this is how to clean up in case Qml wants to quit
	//connect(mEngine, SIGNAL(quit()), this, SLOT(declarativeEngineQuit()));
	
	// This first signal call doesn't work
	mQmlInterface->emitCppSignal("this getting to you?");

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The above code runs when I click on a button to "start" the qml test. In a different place in my code (index change of a table widget) I call the same method.

mQmlInterface->emitCppSignal("this getting to you?");

mQmlInterface->emitCppSignal("this getting to you?");

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Current behavior is that only the index change calls are getting through the QML. For my implementation I can live with that but why is it happening? Is this a bug?

[A.J.Rimmer]

You have to register your class to qdeclarative engine to be able to connect to C++ object signals which are not notify signals of object properties. Registration can be done using qmlRegisterType template function:

template<typename T>
  int qmlRegisterType(const char *uri, int versionMajor, int versionMinor, const char *qmlName)

template<typename T>
  int qmlRegisterType(const char *uri, int versionMajor, int versionMinor, const char *qmlName)

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For you code it would be for example:

qmlRegisterType<MyQmlInterface>("MyQmlInterfaceNamespace", 1, 0, "MyQmlInterface");

qmlRegisterType<MyQmlInterface>("MyQmlInterfaceNamespace", 1, 0, "MyQmlInterface");

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More info:
http://doc.qt.nokia.com/4.7/qdeclara...mlRegisterType
http://doc.qt.nokia.com/4.7/qml-extending.html

[frankiefrank]

Thanks for your reply.

The code given here should still work. And as matter of fact, it does, for any later call to emitCppSignal. That's why I wrote, it doesn't work only for the first call.
I changed some things in my actual project so I don't have this first call in my working code, but if I manage to find time to repeat it I'll try to post a better explanation of the issue.