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Need QML advice



  • So I've created a GUI based on a QML ApplicationWindow, that has several pages of gauges for monitoring data (pages are loaded with Loaders). On my main page, I've created some space using a Rectangle that hides (zero width) until a condition is set. I want to use the space inside the rectangle for showing warnings, like if a gauge has gone out of a 'normal' range.

    I'm stumbling on how to implement sending the relevant information into the Rectangle. I've been thinking that I could define normal ranges inside each CircularGauge, and if the value of the gauge goes outside the range, I could set a flag, but then how to dynamically send that information to one or more (depending on how many gauges are out of range) TextFields, or other Items in the warning Rectangle?

    I would love to figure out how to show a copy of the gauge that is out of range.

    The data for the gauges will eventually come from a C++ backend, but right now I'm using a QML page with various animations to 'fake' the gauge data. I was wondering if I could use the C++ side to instantiate items inside the rectangle, on the fly?

    As always, I'm willing to do the legwork and studying, if someone can get me started in the right direction.



  • Okay, here is a more specific question:

    How can I create a signal that is emitted when the value goes out of a range I define:

    CircularGauge {
                id: Pressure
    value: valueSource.Pressure
    
                property real normalRangeHi: 200
                property real normalRangeLo: 20
                
    
                Layout.preferredHeight: 150
                Layout.preferredWidth: 150
                Layout.fillHeight: true
                Layout.fillWidth: true
                Layout.alignment: Qt.AlignHCenter | Qt.AlignVCenter
    
                maximumValue: 100
                
                        }
            
    

  • Moderators

    CircularGauge {
      // [...]
    
      signal valueOutOfRange()
    
      onValueChanged: {
        if (value > maximumValue) {
          valueOutOfRange()
        }
      }
    }
    

    Is that what you want to achieve?



  • @sierdzio
    Yes, I think that will be half of it (thank you!), but because the 'receiver' is on another page (separate qml file), not an object of the circular gauge, it isn't aware of the signal. How do I make the signal available like that? And is it possible to also pass a bit of information with the signal? Like this:

    signal valueOutOfRange()
    
      onValueChanged: {
        if (value > maximumValue) {
          outOfRangeValue = value
          valueOutOfRange(outOfRangeValue)
        }
      }
    

  • Moderators

    @MScottM said in Need QML advice:

    @sierdzio
    Yes, I think that will be half of it (thank you!), but because the 'receiver' is on another page (separate qml file), not an object of the circular gauge, it isn't aware of the signal. How do I make the signal available like that?

    There are at least two ways to do it:

    • add same signal to all components above your gauge up until you reach a common root element with your receiver - then add signals down that branch of parent-child relation. In each such "proxy" element you only need to emit the signal when child object emits it:
    // SomeComponent.qml
    Item {
      signal valueOutOfRange()
    
      CircularGauge {
        onValueOutOfRange: parent.valueOutOfRange()
      }
    }
    
    • use some global context property / context object. Preferably some QObject you define in C++ and attach to your QML via QQmlEngine->rootContext()->setContextProperty() (writing names from memory, actual may be different)

    And is it possible to also pass a bit of information with the signal?

    Yes, you can pass any variable through a signal. This is described fully in the documentation.



  • @sierdzio
    First - thank you for your advice so far.

    I'm having a hard time getting it working right now, but with what you've given me above, I know I'll figure it out before too long.

    I wanted to ask about best practice. Sending a signal up and down a chain of parented items seems...inelegant, but in the documentation about interacting between C++ and QML is this statement:

    "Warning: Although it is possible to access QML objects from C++ and manipulate them, it is not the recommended approach, except for testing and prototyping purposes. One of the strengths of QML and C++ integration is the ability to implement UIs in QML separate from the C++ logic and dataset backend, and this fails if the C++ side starts manipulating QML directly. Such an approach also makes changing the QML UI difficult without affecting its C++ counterpart."

    So maybe the first method is considered the best way?


  • Moderators

    The second way I described is not about accessing QML objects from C++, it is about sending a signal from QML to C++, and then connecting to the re-emitted signal in another place. There is no tight integration here and no mixing of logic with UI. At least in my view.



  • @sierdzio

    Okay - still having a hard time (trying to learn!). All of the examples show declaring a QQmlEngine and creating a component

    QQmlEngine engine;
    QQmlComponent component(&engine, "MyItem.qml");
    QObject *object = component.create();
    

    which I've already done in my main.cpp - or they create a QGuiApplication

    int main(int argc, char *argv[]) {
        QGuiApplication app(argc, argv);
    
        QQuickView view(QUrl::fromLocalFile("MyItem.qml"));
        QObject *item = view.rootObject();
    
        MyClass myClass;
        QObject::connect(item, SIGNAL(qmlSignal(QString)),
                         &myClass, SLOT(cppSlot(QString)));
    

    Is it possible to create a class that is aware of the signals without having to create views or add to the main.cpp file?


  • Moderators

    @MScottM said in Need QML advice:

    Is it possible to create a class that is aware of the signals without having to create views or add to the main.cpp file?

    Yes. Consider this:

    // someclass.h
    class SomeClass : public QObject
    {
      Q_OBJECT
      // blah blah blah...
    public signals:
      void someCppSignal() const;
    
    public slots:
      void someCppSlot();
    };
    
    // main.cpp
    SomeClass someclass;
    QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty("SomeClass", &someclass);
    
    // QML code:
    Item {
      id: sender
      signal valueOutOfRange()
    
      onValueOutOfRange: {
        // If you want to call a slot in C++
        // Let's assume that someCppSlot() emitts someCppSignal()
        SomeClass.someCppSlot()
      }
    }
    
    // Other QML file ("receiver")
    Item {
      id: receiver
      Connections {
        target: SomeClass
        onSomeCppSignal: console.log("Oh look! A signal from C++!")
      } 
    }
    

    Does that help? I can explain more if necessary.



  • @sierdzio

    Thank you! I'm pretty sure I understand how your example is working. I'll spend this weekend seeing if I can implement it in my code.



  • @sierdzio
    Okay...my goal is to pass two pieces of information if a value goes out of range - the name of the gauge and its value. Here the code that I've tried so far.
    In the QML 'sender' (snipped to keep it short):

    CircularGauge {
                id: Temp
                objectName: Temp
                value: valueSource.coolantTemp
    
                property real normalRangeHi: 200
                property real normalRangeLo: 20
    
                signal valueOutOfRange(var messageObject)
    
                  onValueChanged: {
                    if (value > normalRangeHi) {
                      valueOutOfRange("Coolant Temp", Temp.value)
                    } else if (value < normalRangeLo) {
                        valueOutOfRange("Coolant Temp", Temp.value)
                    }
                    onValueOutOfRange:
                        MessageRelay.setValueMsg(valueOutOfRange(messageObject))
                  }
    }
    

    and my MessageRelay.h file:

    #ifndef MESSAGERELAY_H
    #define MESSAGERELAY_H
    
    #include <QObject>
    #include <QQmlProperty>
    #include <QVariant>
    #include <QDebug>
    
    class MessageRelay : public QObject
    {
        Q_OBJECT
        Q_PROPERTY(QVariant valueMsg READ valueMsg WRITE setValueMsg NOTIFY valueMsgChanged)
    
    private:
    
        QVariant m_messageToRelay;
    
    public:
    
        QVariant valueMsg() const {
           return m_messageToRelay;
    }
    
    signals:
    
        void valueMsgChanged(const QVariant relayMessage);
    
    public slots:
    
        void setValueMsg(QVariant relayMessage){
            m_messageToRelay = relayMessage;
            emit valueMsgChanged(m_messageToRelay);
          }
    };
    
    #endif // MESSAGERELAY_H
    

    I can't get past this error from moc:

    error: no match for call to '(QVariant) ()'
    case 0: reinterpret_cast< QVariant>(_v) = _t->valueMsg(); break;

    I haven't put any code in the 'receiver' yet.



  • Okay - got a little further (edited above code to what I have now). My QML file with the gauge is saying that "Reference error: MessageRelay is not defined" and then "Reference error: messageObject is not defined".

    Is that the purpose of the 'setContextProperty' statement in main.cpp? Or, I am probably not bundling the variables correctly into the messageObject - looking at that now.


  • Moderators

    @MScottM said in Need QML advice:

    void setValueMsg(QVariant relayMessage){

    Qt classes, especially implicitly shared ones, are best passed by const reference const QVariant &relayMessage.

    id: Temp
    objectName: Temp

    Not sure if using a capital letter at the start of an id is a good idea. If it works - fine. But the convention is that capital letters are for QML components only.

    Object name should be a string.

    Now, the actual reply:

    MessageRelay.setValueMsg(valueOutOfRange(messageObject))

    Here you are calling a method on MessageRelay so there is no need to use valueOutOfRange here.

    Also, this code does not belong inside onValueChanged because it is inside another slot. Here is something that should work:

    onValueChanged: {
      if (value > normalRangeHi) {
        valueOutOfRange("Coolant Temp", Temp.value)
      } else if (value < normalRangeLo) {
        valueOutOfRange("Coolant Temp", Temp.value)
      }
    }
    
    onValueOutOfRange: MessageRelay.setValueMsg(messageObject)
    

    Note: since you declared valueMsg as a Q_PROPERTY, the last line can also be:

    onValueOutOfRange: MessageRelay.valueMsg = messageObject
    

    There might be one more problem ahead: you declare valueOutOfRange signal with one argument (messageObject) but then you pass two in onValueChanged. That is likely going to fail, but I may be wrong.



  • @sierdzio
    Okay - here is updated code from my gauge:

    CircularGauge { //portCoolantTemp
                id: temp
    
                property real normalRangeHi: 200
                property real normalRangeLo: 20
    
                signal valueOutOfRange(var msgGauge, var msgValue)
    
                  onValueChanged: {
                    if (value > normalRangeHi) {
                      valueOutOfRange("Coolant Temp", temp.value)
                    } else if (value < normalRangeLo) {
                        valueOutOfRange("Coolant Temp", temp.value)
                    }
                  }
                  onValueOutOfRange:
                      MessageRelay.setValueMsg(msgGauge, msgValue)
    

    and the change to the MessageRelay.h code:

        void setValueMsg(const QVariant &relayMessage){
            m_messageToRelay = relayMessage;
            emit valueMsgChanged(m_messageToRelay);
            qDebug()<<m_messageToRelay;
          }
    };
    
    #endif // MESSAGERELAY_H
    

    my qDebug is in the slot is printing 'QVariant(invalid)', so you are probably right that I need to separate the values to pass them - trying to figure that out now. This is progress to me! Thanks!


  • Moderators

    OK, good progress. Now you need to decide on the nature of your MessageRelay class:

    • if you want to keep valueMsg property, you need it to have only one argument (a property can only have one value). Here what you can do is create a JavaScript array and use it in your valueOutOfRange signal - on C++ side that will be translated to QVariantList
    • if you don't need the valueMsg property, you can remove it and use the setValueMsg slot (you'll need to declare it as slot or Q_INVOKABLE). Or, if you really want to only relay the messages without remembering them, you should be able to emit the valueMsgChanged signal directly from QML, like this: onValueOutOfRange: MessageRelay.valueMsgChanged(messageObject)


  • @sierdzio
    Okay!! I have messages passing from QML to C++ now!

    Here is a snip from MessageRelay.h:

    public:
    
        qint16 m_valueToRelay = 0;
        QString m_nameToRelay = "";
    
    signals:
    
        void valueMsgChanged(const qint16 relayValue);
        void valueNameChanged(const QString relayName);
    
    public slots:    
    
        Q_INVOKABLE void setValueMsg(const qint16 &relayValue){
            if (m_valueToRelay != relayValue) {
                m_valueToRelay = relayValue;
                emit valueMsgChanged(relayValue);
                qDebug() << "relayMessage: " << relayValue;
            }
        }
    
        Q_INVOKABLE void setNameMessage(const QString &relayName){
            if (m_nameToRelay != relayName){
                m_nameToRelay = relayName;
                emit valueNameChanged(relayName);
                qDebug() << "relayName: " << relayName;
            }
        }
    
    };
    

    And here is the QML code from the CircularGauge:

    CircularGauge { //CoolantTemp
                id: temp
    
            //  SIGNALING
                property real normalRangeHi: 200
                property real normalRangeLo: 20
    
                signal valueOutOfRange(var messageValue)
                signal nameOutOfRange(var messageName)
    
                  onValueChanged: {
                    if (temp.value > normalRangeHi) {
                      valueOutOfRange(temp.value)
                      nameOutOfRange("CoolantTemp")
                    } else if (temp.value < normalRangeLo) {
                        valueOutOfRange(temp.value)
                        nameOutOfRange("CoolantTemp")
                    }
                  }
    
                  onValueOutOfRange:
                      MessageRelay.setValueMsg(messageValue)
                  onNameOutOfRange:
                      MessageRelay.setNameMessage(messageName)
    

    and this code gets this output in the console when the gauge goes low:

    relayMessage: 20
    relayName: "CoolantTemp"
    relayMessage: 19
    relayMessage: 18
    relayMessage: 17
    relayMessage: 16
    relayMessage: 15

    But now I think I have a problem. If more than one gauge goes out of range at the same time, which is very possible when Bad Stuff happens, I won't be able to associate the value being sent with the name.

    I've tried several ways to bundle the two variables into one message, but nothing seems to work. The closest I was able to get was some code that compiled and ran, but the QML side never seemed to trigger the signal.

    If there is a way to bundle the variables into one message so that I can tell where the messages are coming from on the receiver side, I would really appreciate the advice - and by the way, I really appreciate the help so far.



  • Here is the code I finally got working:

    QML Gauge:

    CircularGauge { //temp
                id: temp
    
            //  SIGNALING
                property real normalRangeHi: 180
                property real normalRangeLo: 50
    
                signal messageObject(var messageName, var messageValue)
    
                  onValueChanged: {
                    if (temp.value > normalRangeHi) {
                        messageObject("Coolant Temp HI", temp.value)
                        } else if (portCoolantTemp.value < normalRangeLo) {
                        messageObject("Coolant Temp LO", temp.value)
                        } 
                    }
                  onMessageObject: MessageRelay.setValueMsg(messageName, messageValue)
    
    

    MessageRelay.h:

    class MessageRelay : public QObject
    {
        Q_OBJECT
    
    
    private:
    
    
    public:
    
        qint16 m_valueToRelay = 0;
        QString m_nameToRelay = "";
        bool relayAlarm = false;
    
    signals:
    
        void valueMsgChanged(const QString relayName, const qint16 relayValue, const bool relayAlarm);
    
    public slots:    
    
        Q_INVOKABLE void setValueMsg(const QString &relayName, const qint16 &relayValue){
            if(relayValue != 0){
                //qDebug()<< m_nameToRelay;
                m_nameToRelay = relayName;
                m_valueToRelay = relayValue;
                relayAlarm = true;
                emit valueMsgChanged(m_nameToRelay, m_valueToRelay, relayAlarm);
                }
            else {emit valueMsgChanged("", 0, false);
            }
        }
    
    };
    

    And the receiver QML code:

    Connections {
            target: MessageRelay
            onValueMsgChanged: {cnsl(relayName, relayValue, relayAlarm)}
            function cnsl(relayName, relayValue, relayAlarm) {
                if (relayValue > 0){
                    alarmState = relayAlarm
                    txt.text =  relayName + ": " + relayValue
                } else {
                    alarmState = false                
                }
                //console.log(alarmState)
            }
        }
    

    EDIT
    And the main.cpp code registering the class:

    int main(int argc, char *argv[])
    {
        QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    
        QGuiApplication app(argc, argv);
    
        MessageRelay messageRelay;
        QQmlApplicationEngine engine;    
        engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    
        engine.rootContext()->setContextProperty("MessageRelay", &messageRelay);
        
        if (engine.rootObjects().isEmpty())
            return -1;
    
        return app.exec();
    
    }
    
    

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