Update Q_PROPERTY: how does the notify signal work?

maxwell31

Hi,

I have a c++ class, which exposes a QList<QObject*> as a model and also has a int property:

  Q_PROPERTY(QVariant customModel READ getModel NOTIFY customModelChanged)
  Q_PROPERTY(int currentValue READ getCurrentValue NOTIFY customModelChanged)

customModelChanged gets emitted whenever the model is changed. In qml I have a property set to currentValue, however, I can see (having a breakpoint in getCurrentValue during debugging) that getCurrentValue is not called when customModelChanged is emitted. Do I missunderstand somethinbg?

ODБOï

Hi,

getCurrentValue is not supposed to be called when currentValueChanged is emited

You generally use getCurrentValue from qml side to get the value of a private property (like accessor)

class MachineBackend : public QObject
{
    Q_OBJECT
public:
    Q_PROPERTY(QString message READ getMessage  WRITE setMessage NOTIFY messageChanged)

QString getMessage(){
return m_message;
}
void setMessage(QString newMessage){
m_message=newMessage;
emit messageChanged() // Will notify en ensure value is refreshed
}
private :
 QString m_message;

then in qml :

Text{
~~text : myObj.getMessage // will refresh~~
text : myObj.message
}

dheerendra

in addition to previous post, you should always follow <propertyName> and propertyNameChanged signal.

SGaist

Hi,

Some small yet useful improvements:

void setMessage(const QString& newMessage){ // const reference avoids useless copies
    if (m_message == newMessage) // Don't notify if the value doesn't change
        return;
    m_message=newMessage;
    emit messageChanged(m_message) // Send the changed value further
}

GrecKo

@LeLev said in Update Q_PROPERTY: how does the notify signal work?:

getCurrentValue is not supposed to be called when currentValueChanged is emited

OP doesn't have a currentValueChanged signal.

You generally use getCurrentValue from qml side to get the value of a private property (like accessor)

No, you generally use currentValue from qml, you shouldn't call getters directly. And it won't work in your example since getMessage is not invokable (that's a good thing).

ODБOï

@GrecKo this is just naming confusion,my bad

J.Hilk

@LeLev said in Update Q_PROPERTY: how does the notify signal work?:

getCurrentValue is not supposed to be called when currentValueChanged is emited

as far as I know, that is exactly what is happening, the as READ marked function is called from the QML side, as soon as you emit the currentValueChanged signal.
But that is happening automaticaly in the background, all part of the Q_PROPERTY macro, if you want to call the getter manually, you will have to mark it as Q_INVOKABLE

ODБOï

http://doc.qt.io/qt-5/qtqml-cppintegration-exposecppattributes.html#exposing-properties

maxwell31

For clarification, I wanted to use the same notify signal for several properties

SGaist

Then make it a separate signal and connect explicitly to that one.

A notify signal is associated to one property (single responsibility paradigm). Since you want a catch all signal, it should be a clear separated element. This will have the added values of making your code cleaner, clearer and avoid maintenance hell.

dheerendra

In addition to what @SGaist said, if you use same signal for every properties, property binding for those properties will not work. Also you will breaking declarative flow. It will be confusing and confused code. Better avoid it.

Diracsbracket

@SGaist said in Update Q_PROPERTY: how does the notify signal work?:

emit messageChanged(m_message)

I often see that the notify signal of a Q_PROPERTY is defined as an argument-less void method, i.e.

emit messageChanged(()

Common sense tells me that providing an argument to the method avoids an unnecessary call to the READ getter method. If that is correct, why would one define argument-less notify signals?

SGaist

You may have a property that requires some processing to retrieve (for example slow disk/network access) and you do not want to do that for every signal émission only when there's an actual need for the data.

GrecKo

And the QML engine doesn't event read the parameter of the notify signal if there's one.