Best Practice to Add Dynamic QML from C++ data



  • I am working with a ListView using a QAbstractListModel from C++ to adjust some of the QML data. However for each List Item delegate I need to populate some formatted text I parse from a JSON file on the C++ end, such as a header and body text and in some cases no text at all is used. So in other words a List Item delegate can have variable amount of QML Texts, formatted in different ways, or no Text at all.

    I am thinking of dynamically adding these text fields from C++ to a List Item but I'm not sure if that is the way to go. Perhaps the List Item delegate from QML can look at the model (Which is tied to C++) to see how many Text objects it need and create it from the QML side, if possible.

    I really don't know what is the best practice for this so any input would be appreciated.



  • I can't say if it's a best practice but what I would do is save each text part in a different role of the model (you can use Qt::UserRole onward) and let the delegate read and render each of those roles



  • I've done a few roles that are only simple variables at the moment in my QAbstractListModel. Can I make roles that is a C++ object in itself that returns QML structure for these Texts with its own formatting?

    Where can I read up more on Roles?



  • QML is not great at handling custom C++ objects that are not QObjects (see http://doc.qt.io/qt-5/qtqml-cppintegration-data.html). The easiest route is to save each basic element in its own role.

    struct{
    QString m_text;
    QFont m_textFont;
    }
    

    can be replicated saving text in Qt::UserRole, and textFont in Qt::UserRole+1

    Where can I read up more on Roles?

    http://doc.qt.io/qt-5/model-view-programming.html#basic-concepts scroll to "Item roles" paragraph and http://doc.qt.io/qt-5/qtquick-modelviewsdata-modelview.html#models



  • This is great, thanks!

    But I have problem finding documentation on more complex User Roles. Could I implement a QAbstractListModel's data function to return a struct of data rather than a simple property for QML?

    Example:

    QVariant SomeListModel::data(const QModelIndex &index, int role) const {
        if (index.row() < 0 || index.row() >= mData.count())
            return QVariant();
    
        const SomeData &someData = mData[index.row()];
    
        switch (role) {
        case HeaderText:  //::  Simple Property Role returns a QString
            return someData.headerText();
        break;
    
        case TextObj:   //::  Return a more complex data structure, like a struct or class
            return someData.aDataStructOfData();
        break;
    
        }
    
        return QVariant();
    }
    

    So 'aDataStructOfData' would return a struct or even a class of Qt types, like maybe a QString, a QFont etc I could later use to populate in QML?



  • take a look at http://qmlbook.github.io/en/ch15/index.html#models-in-c

    Personally I had little luck integrating custom structs/classes as values in a model but I'm pretty sure it's just because I'm not that good rather than it being impossible


  • Moderators

    @Placeable

    So 'aDataStructOfData' would return a struct or even a class of Qt types, like maybe a QString, a QFont etc I could later use to populate in QML?

    Yes it is possible. Make sure your struct or class uses Q_OBJECT macro. This class can then contain Q_INVOKABLE functions or Q_PROPERTY's which will return your QString or QFont. These propeties or functions defined as such can then be access from QML.

    Since data require a QVariant you will require to return your class as a QVariant. For that you may require you class or struct to be registered using Q_DECLARE_METATYPE.

    case TextObj: 
         return QVariant::fromValue(someData.aDataStructOfData());
    


  • @p3c0 Now I'm very interested here as on paper I see it happening but when I try and do it in practice I always fail.

    for example:
    TestData.h

    #ifndef TESTDATA_H
    #define TESTDATA_H
    #include <QString>
    #include <QMetaType>
    class TestData
    {
        Q_GADGET
        Q_PROPERTY(QString text READ text WRITE setText)
        Q_PROPERTY(QString desctiption READ desctiption WRITE setText)
    public:
        TestData(){}
        const QString& desctiption() const
        {
            return m_desctiption;
        }
        
        void setDesctiption(const QString &desctiption)
        {
            m_desctiption = desctiption;
        }
        const QString& text() const
        {
            return m_text;
        }
        
        void setText(const QString &text)
        {
            m_text = text;
        }
        
    private:
        QString m_text;
        QString m_desctiption;
    };
    Q_DECLARE_METATYPE(TestData)
    #endif // TESTDATA_H
    

    How would you use this as data in a model?

    What I always did is put text in a role, description in another and use them as QStrings


  • Qt Champions 2016

    With my rudimentary knowledge of QML I'd first register the type:

    qmlRegisterType<TestData>("com.testns", 1, 0, "TestData");
    

    And then you should be able to instantiate this in QML. However, I'm not convinced it'd work with gadgets, QML is pretty heavy on the QObject usage ... and honestly I don't know for sure this'd be enough for the type to be returned from a model.



  • @kshegunov said in Best Practice to Add Dynamic QML from C++ data:

    And then you should be able to instantiate this in QML.

    Exactly, instantiate is easy but if you put that thing in a model in C++ and call it in QML how can you tell QML your QVariant is TestData?


  • Qt Champions 2016

    @VRonin said in Best Practice to Add Dynamic QML from C++ data:

    how can you tell QML your QVariant is TestData?

    I don't follow. QML and Qt already know what is in the QVariant that's the purpose of declaring it as a meta type ... and qmlRegisterType should expose the interface of said class to the QML engine, if I'm correct.



  • ok, let's say our main looks like this:

    #include <QApplication>
    #include <QQmlApplicationEngine>
    #include <QQmlContext>
    #include <QStandardItemModel>
    #include "testdata.h"
    int main(int argc, char *argv[])
    {
        QApplication app(argc, argv);
    
    
        QStandardItemModel testModel;
        testModel.insertRows(0,2);
        testModel.insertColumn(0);
    
        TestData tempTestData;
        tempTestData.setText("Data1Text");
        tempTestData.setDesctiption("Data1Desc");
        testModel.setData(testModel.index(0,0),QVariant::fromValue(tempTestData));
    
        tempTestData.setText("Data2Text");
        tempTestData.setDesctiption("Data2Desc");
        testModel.setData(testModel.index(1,0),QVariant::fromValue(tempTestData));
    
        QQmlApplicationEngine engine;
        qmlRegisterType<TestData>("com.testns", 1, 0, "TestData");
        engine.rootContext()->setContextProperty("testModel", &testModel);
        engine.load(QUrl(QStringLiteral("qrc:///main.qml")));
        return app.exec();
    }
    

    and main.qml is

    import com.testns 1.0
    ApplicationWindow {
        visible: true
        width: 640
        height: 480
        title: "Testing Model"
    
        ListView{
            model: testModel
            delegate: ???
        }
    }
    
    

    What do you put in ??? to simply make text and description appear next to each others?

    The official example http://doc.qt.io/qt-5/qtquick-modelviewsdata-cppmodels.html splits the internals on Animal into two different roles each containing a QString, I never saw an example of a delegate handling custom variant types


  • Qt Champions 2016

    @VRonin

    Haven't tried it, but I'd do something along the lines of:

    ApplicationWindow  {
        visible: true
        width: 640
        height: 480
        title: "Testing Model"
    
        ListView  {
            model: testModel
            delegate: Rectangle  {
                height: 25
                width: 100
                Text  {
                    id: tmText
                    text: modelData.text
                    anchors.left: parent.left
                    anchors.top: parent.top
                    anchors.bottom: parent.bottom
                }
                Text  {
                    text: modelData.desctiption
                    anchors.left: tmText.right
                    anchors.right: parent.right
                    anchors.top: parent.top
                    anchors.bottom: parent.bottom
                }
            }
        }
    }
    


  • needs edit instead of modelData and it works! thanks so much


  • Qt Champions 2016

    @VRonin said in Best Practice to Add Dynamic QML from C++ data:

    needs edit instead of modelData and it works! thanks so much

    I'm glad it does. :D
    I was just putting down the code to test it ... thanks for sparing me the trouble :P



  • Okay I got it to work, here's what i got now, and it is a long post so bare with me, - I defined a class in the following way and mind you the QObject *parent in the constructor I always define as 0, I never send in anything there. Dunno if it is intended to do so.

    class ComplexData : public QObject {
        Q_OBJECT
    
        Q_PROPERTY(QString text READ testString WRITE setTestString NOTIFY testStringChanged)
    
    public:
        explicit ComplexData(QObject *parent = 0);
    
        QString testString() const;
        void setTestString(const QString &testString);
    
    signals:
        void testStringChanged(QString);
    
    private:
        QString mTestString;
    };
    

    Implementation of ComplexData.cpp

    ComplexData::ComplexData(QObject *parent) : QObject(parent) {
        mTestString = "This is a test string from CPP!";
    }
    
    QString ComplexData::testString() const {
        return mTestString;
    }
    
    void ComplexData::setTestString(const QString &testString) {
        mTestString = testString;
        emit testStringChanged(testString);
    }
    

    My "SomeData" gets this new "ComplexData" as a member variable:

    class SomeData
    {
    public:
        SomeData(const QString &headerText);
    
        QString headerText() const;
        ComplexData* complexData() const;
    
    private:
        QString mHeaderText;
        ComplexData* mComplexData; //::  Has to be a pointer for some reason
    };
    

    Implementation of SomeData.cpp:

    SomeData::SomeData(const QString &headerText)
        : mHeaderText(headerText)
    {
        mComplexData = new ComplexData(); //:: I just create the object here for Testing could be passed into Constructor though
    }
    
    ComplexData* SomeData::complexData() const {
        return mComplexData;
    }
    

    And finally in my AbstractListModel 'SomeListModel.cpp' data function now looks like this:

    QVariant SomeListModel::data(const QModelIndex &index, int role) const {
        if (index.row() < 0 || index.row() >= mSolutions.count())
            return QVariant();
    
        const SomeData &someData= mData[index.row()];
    
        switch (role) {
        case HeaderText:
            return someData.headerText();
        break;
    
        case ComplexData:
            return QVariant::fromValue( someData.complexData() );
        break;
        }
    
        return QVariant();
    }
    

    With this I can use this in QML like:

    Text {
          text: model.complexData.text
    }
    

    Okay, all fine and dandy. I basically just moved some data to another class. I could've just kept on having this in the SomeData class. What I really need to know is if it is possible to return the whole "ComplexData" as a QML of type "Text", is that a QTextField type in CPP?

    So instead in the data function of SomeListModel I would return say:

    case ComplexData:
            return QVariant::fromValue( someData.complexData() ); //::  This returns a formatted textfield setup from C++ to be used in QML instead.
    break;
    

    Not even sure if that is possible though. I am getting a bit confused here :/


  • Moderators

    @Placeable What do you mean by "formatted textfield setup" ?



  • @p3c0 said in Best Practice to Add Dynamic QML from C++ data:

    @Placeable What do you mean by "formatted textfield setup" ?

    In CPP I want to create a set of Texts to be used in QML - I read data from a JSON file and need to create Texts for QML dynamically. All data is stored on the CPP side so let's say I need to create a view with 3 Texts and another with 0 Texts. That is what I am struggling to do.

    So I am wondering if it is possible to create these Texts in CPP (Whatever their CPP variant might be, I dunno) and then return this to be populated in said QML view by changing the QAbstractListModel class somehow.


  • Moderators

    @Placeable Do you mean something like dynamic object creation in QML ?
    And in your case you want the QML component's code(Text) will come from CPP ?

    http://doc.qt.io/qt-5/qtqml-javascript-dynamicobjectcreation.html#creating-an-object-from-a-string-of-qml



  • @p3c0
    Exactly so, dynamically. Basically I want to setup these Texts on the CPP side (Font size, formatting as such etc) and let QML now: Hey here is a Text for you to use, have fun! Oh by the way here's another Text to use for this ListItem. But that ListItem over there you will get no Text to use at all! Hah!

    So a ListItem could have a variable amount of Texts that I also need to somehow tell my View Delegate.


  • Moderators

    @Placeable I have done something similar my project here. So what that particular code does is it creates a QQuickItem from a base QML template as shown here. Then sets some color and font on it. So you can try to do something similar. But remember QQmlComponent requires QQmlEngine. This is the same with which you must have loaded the QML initially. Also remember that dynamic QQuickItem also requires a visual parent which is set using setParent.



  • @p3c0 This is great, I like how to create QML from CPP. I came up with a different solution though which I think works but not sure if it is "best practice" - I'd like to experiment the way you did it as well.

    Here's how I have done it now to create "dynamic" Texts in QML from CPP data:

    In my QAbstractListModel data function I have this field that returns a QVariant:

    case TextData:
            return QVariant::fromValue( someData.textList() );
    break;
    

    The implementation of that returns a QList of QObject:

    QList<QObject*> SomeData::textList() const {
        return mTextList;
    }
    

    I can populate this QList with my TextData class that derives QObject

    TextData.cpp:

    class TextData : public QObject {
        Q_OBJECT
    
        Q_PROPERTY(QString text READ testString WRITE setTestString NOTIFY testStringChanged)
    
    public:
        explicit TextData(QObject *parent = 0);
    
        QString testString() const;
        void setTestString(const QString &testString);
    
    signals:
        void testStringChanged(QString);
    
    private:
        QString mTestString;
    };
    

    Then in QML I can do something like this now to create QML Texts and populate them from the CPP data:

    Component.onCompleted: {
        var arr = model.textData;
        var component = Qt.createComponent("SomeTextLayout.qml");
        for ( var i = 0; i < arr.length; i++ ) {
           var txtObject = arr[i];
           var txtQml = component.createObject(someParentId);
           txtQml.text = txtObject.text;
           ... //Etc fill in more props from the txtObject
        }
    }
    

    Again how this is performance wise I am not sure I am inclined to do the QML markup on the CPP side as well.
    I'll mark this topic as Solved as I think there is a lot of great input here that would help anyone else in the future.

    Thanks everyone!


  • Moderators

    @Placeable Beware of Component.onCompleted.

    The order of running the onCompleted handlers is undefined.

    Due to this it could be possible that your initialized components may be not be available when required. May be create them when you require them.



  • @p3c0

    I am not sure what this means. From my testing I see no issues. You mean there is a chance onCompleted is finished before the nested component.createObject(..) is created for this.

    How would one solve this preferably?


  • Moderators

    @Placeable

    If you find no issues then continue. Just a warning if you find some odd behavior during your implementaion. In some of my cases I found that onCompleted is not the most reliable place for initialization of components. Sometimes it triggered earlier causing problems to the objects which were intialized in it and which were dependent on others.



  • @p3c0 Alrighty, I'll bare that in mind. Thanks again!


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