Best Practice to Add Dynamic QML from C++ data

[kshegunov]

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.

[VRonin]

@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?

[kshegunov]

@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.

[VRonin]

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

[kshegunov]

@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
            }
        }
    }
}

[VRonin]

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

[kshegunov]

@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

[Placeable]

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 :/

[p3c0]

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

[Placeable]

@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.

[p3c0]

@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

[Placeable]

@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.

[p3c0]

@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.

[Placeable]

@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!

[p3c0]

@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.

[Placeable]

@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?

[p3c0]

@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.

[Placeable]

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

[VRonin]

Since this was something new from me and the issue popped up again after a while
I prepared a small wiki article that summarises the technique