How to create n Objects dynamically to populate StackView

fcarney

@sandro4912 said in How to create n Objects dynamically to populate StackView:

QQmlListProperty

It looks like it is repeating, but not creating modelData. Not sure if QQmlListProperty is 100% supported, but it should be creating an "index" variable you can use.

       Repeater{
            model: randomQuestions
            delegate: QuizPage{
                question: randomQuestions[index]
            }
        }

sandro4912

I tried that but it also insist that index is not existing:

qrc:/qml/quiz/QuizPage.qml:51: TypeError: Cannot read property 'id' of undefined
qrc:/qml/quiz/QuizPage.qml:56: TypeError: Cannot read property 'askedQuestion' of undefined
qrc:/qml/quiz/QuizPage.qml:63: TypeError: Cannot read property 'picture' of undefined
qrc:/qml/quiz/Quiz.qml:0: ReferenceError: index is not defined
qrc:/qml/quiz/QuizPage.qml:51: TypeError: Cannot read property 'id' of undefined
qrc:/qml/quiz/QuizPage.qml:56: TypeError: Cannot read property 'askedQuestion' of undefined
qrc:/qml/quiz/QuizPage.qml:63: TypeError: Cannot read property 'picture' of undefined
qrc:/qml/quiz/Quiz.qml:0: ReferenceError: index is not defined
qrc:/qml/quiz/QuizPage.qml:51: TypeError: Cannot read property 'id' of undefined
qrc:/qml/quiz/QuizPage.qml:56: TypeError: Cannot read property 'askedQuestion' of undefined
qrc:/qml/quiz/QuizPage.qml:63: TypeError: Cannot read property 'picture' of undefined
qrc:/qml/quiz/Quiz.qml:0: ReferenceError: index is not defined
qrc:/qml/quiz/QuizPage.qml:51: TypeError: Cannot read property 'id' of undefined
qrc:/qml/quiz/QuizPage.qml:56: TypeError: Cannot read property 'askedQuestion' of undefined
qrc:/qml/quiz/QuizPage.qml:63: TypeError: Cannot read property 'picture' of undefined
qrc:/qml/quiz/Quiz.qml:0: ReferenceError: index is not defined
qrc:/qml/quiz/QuizPage.qml:51: TypeError: Cannot read property 'id' of undefined
qrc:/qml/quiz/QuizPage.qml:56: TypeError: Cannot read property 'askedQuestion' of undefined
qrc:/qml/quiz/QuizPage.qml:63: TypeError: Cannot read property 'picture' of undefined
qrc:/qml/quiz/Quiz.qml:0: ReferenceError: index is not defined
qrc:/qml/quiz/QuizPage.qml:51: TypeError: Cannot read property 'id' of undefined
qrc:/qml/quiz/QuizPage.qml:56: TypeError: Cannot read property 'askedQuestion' of undefined
qrc:/qml/quiz/QuizPage.qml:63: TypeError: Cannot read property 'picture' of undefined
qrc:/qml/quiz/Quiz.qml:0: ReferenceError: index is not defined
qrc:/qml/quiz/QuizPage.qml:51: TypeError: Cannot read property 'id' of undefined
qrc:/qml/quiz/QuizPage.qml:56: TypeError: Cannot read property 'askedQuestion' of undefined
qrc:/qml/quiz/QuizPage.qml:63: TypeError: Cannot read property 'picture' of undefined
qrc:/qml/quiz/Quiz.qml:0: ReferenceError: index is not defined
qrc:/qml/quiz/QuizPage.qml:51: TypeError: Cannot read property 'id' of undefined
qrc:/qml/quiz/QuizPage.qml:56: TypeError: Cannot read property 'askedQuestion' of undefined
qrc:/qml/quiz/QuizPage.qml:63: TypeError: Cannot read property 'picture' of undefined
qrc:/qml/quiz/Quiz.qml:30: ReferenceError: index is not defined
qrc:/qml/quiz/QuizPage.qml:51: TypeError: Cannot read property 'id' of undefined
qrc:/qml/quiz/QuizPage.qml:56: TypeError: Cannot read property 'askedQuestion' of undefined
qrc:/qml/quiz/QuizPage.qml:63: TypeError: Cannot read property 'picture' of undefined
qrc:/qml/quiz/Quiz.qml:30: ReferenceError: index is not defined
qrc:/qml/quiz/QuizPage.qml:51: TypeError: Cannot read property 'id' of undefined
qrc:/qml/quiz/QuizPage.qml:56: TypeError: Cannot read property 'askedQuestion' of undefined
qrc:/qml/quiz/QuizPage.qml:63: TypeError: Cannot read property 'picture' of undefined
qrc:/qml/quiz/Quiz.qml:30: ReferenceError: index is not defined

Some more background information of random Questions.

A Question is defined like this:

class Question : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int id READ getId CONSTANT)
    Q_PROPERTY(QString askedQuestion READ getAskedQuestion CONSTANT)
    Q_PROPERTY(QString answer1 READ getAnswer1 CONSTANT)
    Q_PROPERTY(QString answer2 READ getAnswer2 CONSTANT)
    Q_PROPERTY(QString answer3 READ getAnswer3 CONSTANT)
    Q_PROPERTY(QString answer4 READ getAnswer4 CONSTANT)
    Q_PROPERTY(int correctAnswer READ getCorrectAnswer CONSTANT)
    Q_PROPERTY(QString picture READ getPicture CONSTANT)
public:
    enum class Correct{
        Answer1 = 1,
        Answer2 = 2,
        Answer3 = 3,
        Answer4 = 4
    };

    Question() = default;

    Question(int id,
             QString askedQuestion,
             QString answer1,
             QString answer2,
             QString answer3,
             QString answer4,
             Correct correctAnswer,
             QString picture = QString{});

    int getId() const;
    QString getAskedQuestion() const;
    QString getAnswer1() const;
    QString getAnswer2() const;
    QString getAnswer3() const;
    QString getAnswer4() const;
    int getCorrectAnswer() const;
    QString getPicture() const;

private:
    int mId;
    QString mAskedQuestion;
    QString mAnswer1;
    QString mAnswer2;
    QString mAnswer3;
    QString mAnswer4;
    Correct mCorrectAnswer;
    QString mPicture;
};

In annother class which has sql connection I read the randomQuestions:

class QuestionSqlTableModel : public QSqlTableModel
{
    Q_OBJECT
    Q_PROPERTY(QQmlListProperty<Question> randomQuestions
               READ getRandomQuestions
               NOTIFY randomQuestionsChanged)
public:
    explicit QuestionSqlTableModel(QObject *parent = nullptr,
                                   const QSqlDatabase &db = QSqlDatabase());

    Q_INVOKABLE QVariant data(const QModelIndex &index, int role) const override;


    Q_INVOKABLE bool addNewEntry(const QString& askedQuestion,
        const QString& answer1,
        const QString& answer2,
        const QString& answer3,
        const QString& answer4,
        int correctAnswer,
        const QString& picturePath);

    Q_INVOKABLE void generateNewRandomQuestions(int count);

    QQmlListProperty <Question> getRandomQuestions();

signals:
    void randomQuestionsChanged();

private:
    void appendRandomQuestion(Question *question);
    int randomQuestionsCount() const;
    Question* randomQuestionAt(int index) const;
    void randomQuestionsClear();

    static void appendRandomQuestion(
            QQmlListProperty<Question>* list, Question *question);
    static int randomQuestionsCount(QQmlListProperty<Question>* list);
    static Question* randomQuestionAt(
            QQmlListProperty<Question>* list, int index);
    static void randomQuestionsClear(QQmlListProperty<Question>* list);

    QVector<Question*> mRandomQuestions;
};

They get registered like this:

    qmlRegisterAnonymousType<Question>("sandro.custom.types",1);

    auto context = engine.rootContext();
    context->setContextProperty("questionSqlTableModel", questionSqlTableModel);

I pass the random questions like this to the Quiz:

        ToolButton {
            text: qsTr("New Quiz")
            icon.name: "address-book-new"
            onClicked: {
                questionSqlTableModel.generateNewRandomQuestions(10)
                loader.setSource("quiz/Quiz.qml",
                                 {"randomQuestions":
                                     questionSqlTableModel.randomQuestions})
            }
        }

What I could already do is show a single page likes this:

import QtQuick 2.15
import QtQuick.Controls 2.15

Item {
    id: quiz

    required property var randomQuestions
    width: parent.width
    height: parent.height

    QuizPage{
        id: quizPage
        anchors.fill: parent
        question: randomQuestions[0]
    }
}

This works without problem so I really wonder why the delegate solution does not work.

For completion here is QuizPage:

import QtQuick 2.15
import QtQuick.Layouts 1.15
import QtQuick.Controls 2.15

Item{
    id: root

    required property var question

    property var __shuffledAnswers
    property bool __correctAnswer: false

    signal answeredCorrectly()
    signal answeredWrong()

    width: parent.width
    height: parent.height
    anchors.fill: parent


    Rectangle {
        id: dialog

        width: parent.width
        height: parent.height

        Component.onCompleted: {
            checkButton.enabled = false;
            nextQuestionButton.enabled = false;

            root.__shuffledAnswers = makeAnswerArray(question.answer1,
                                                question.answer2,
                                                question.answer3,
                                                question.answer4);
            shuffleArray(root.__shuffledAnswers);

            answer1TextField.text = root.__shuffledAnswers[0];
            answer2TextField.text = root.__shuffledAnswers[1];
            answer3TextField.text = root.__shuffledAnswers[2];
            answer4TextField.text = root.__shuffledAnswers[3];
        }

        ButtonGroup {
            id: radioGroup
        }

        ColumnLayout{
            anchors.fill: parent

            RowLayout{
                Text{
                    text: qsTr("Question: " + question.id)
                }
            }
            RowLayout{
                Text{
                    text: qsTr(question.askedQuestion)
                }
            }
            RowLayout{
                Image{
                    id: image

                    source: question.picture.length > 0 ?
                                "data:image/png;base64," + question.picture: ""
                }
            }
            RowLayout{
                RadioButton{
                    id: radioButtonAnswer1
                    ButtonGroup.group: radioGroup
                    onCheckedChanged: {
                        if(checked) {
                            checkButton.enabled = true
                            if(root.question.correctAnswer === 1) {
                                __correctAnswer = true
                            }
                            else {
                                __correctAnswer = false
                            }
                        }
                    }
                }
                AnswerTextField{
                    Layout.fillWidth: true
                    id: answer1TextField
                }
            }
            RowLayout{
                RadioButton{
                    id: radioButtonAnswer2
                    ButtonGroup.group: radioGroup
                    onCheckedChanged: {
                        if(checked) {
                            checkButton.enabled = true
                            if(root.question.correctAnswer === 2) {
                                __correctAnswer = true
                            }
                            else {
                                __correctAnswer = false
                            }
                        }
                    }
                }
                AnswerTextField{
                    Layout.fillWidth: true
                    id: answer2TextField
                }
            }
            RowLayout{
                RadioButton{
                    id: radioButtonAnswer3
                    ButtonGroup.group: radioGroup
                    onCheckedChanged: {
                        if(checked) {
                            checkButton.enabled = true
                            if(root.question.correctAnswer === 3) {
                                __correctAnswer = true
                            }
                            else {
                                __correctAnswer = false
                            }
                        }
                    }
                }
                AnswerTextField{
                    Layout.fillWidth: true
                    id: answer3TextField
                }
            }
            RowLayout{
                RadioButton{
                    id: radioButtonAnswer4
                    ButtonGroup.group: radioGroup
                    onCheckedChanged: {
                        if(checked) {
                            checkButton.enabled = true
                            if(root.question.correctAnswer === 4) {
                                __correctAnswer = true
                            }
                            else {
                                __correctAnswer = false
                            }
                        }
                    }
                }
                AnswerTextField{
                    Layout.fillWidth: true
                    id: answer4TextField
                }
            }
            RowLayout{
                Layout.alignment: Qt.AlignRight
                Button{
                    id: checkButton
                    text: qsTr("Check Answer")
                    onPressed: {
                        markAnswers(root.question.correctAnswer);
                        disableRadioButtons();
                        enabled = false
                        nextQuestionButton.enabled = true
                    }
                }
                Button{
                    id: nextQuestionButton
                    text: qsTr("Next Question")
                    onPressed: {
                        if(root.__correctAnswer) {
                            root.answeredCorrectly()
                        }
                        else {
                            root.answeredWrong()
                        }
                    }
                }
            }
        }
    }

    function makeAnswerArray(answer1, answer2, answer3, answer4)
    {
        return [answer1, answer2, answer3, answer4];
    }

    function shuffleArray(array)
    {
        for (var i = array.length - 1; i > 0; i--) {
            var j = Math.floor(Math.random() * (i + 1));
            var temp = array[i];
            array[i] = array[j];
            array[j] = temp;
        }
    }

    readonly property string correctAnswerColor: "#99FFCC"
    readonly property string wrongAnswerColor: "#FF9999"

    function markAnswers(correctAnswer)
    {
        switch(correctAnswer)
        {
            case 1:
                markAnswersIfAnswer1IsCorrect();
                break;
            case 2:
                markAnswersIfAnswer1IsCorrect();
                break;
            case 3:
                markAnswersIfAnswer1IsCorrect();
                break;
            case 4:
                markAnswersIfAnswer1IsCorrect();
                break;
            default:
                console.assert(false, "Invalid Value for correctAnswer: "
                                + correctAnswer)

        }
    }

    function markAnswersIfAnswer1IsCorrect()
    {
        answer1TextField.backgroundColor = correctAnswerColor
        answer2TextField.backgroundColor = wrongAnswerColor
        answer3TextField.backgroundColor = wrongAnswerColor
        answer4TextField.backgroundColor = wrongAnswerColor
    }

    function markAnswersIfAnswer2IsCorrect()
    {
        answer1TextField.backgroundColor = wrongAnswerColor
        answer2TextField.backgroundColor = correctAnswerColor
        answer3TextField.backgroundColor = wrongAnswerColor
        answer4TextField.backgroundColor = wrongAnswerColor
    }

    function markAnswersIfAnswer3IsCorrect()
    {
        answer1TextField.backgroundColor = wrongAnswerColor
        answer2TextField.backgroundColor = wrongAnswerColor
        answer3TextField.backgroundColor = correctAnswerColor
        answer4TextField.backgroundColor = wrongAnswerColor
    }

    function markAnswersIfAnswer4IsCorrect()
    {
        answer1TextField.backgroundColor = wrongAnswerColor
        answer2TextField.backgroundColor = wrongAnswerColor
        answer3TextField.backgroundColor = wrongAnswerColor
        answer4TextField.backgroundColor = correctAnswerColor
    }

    function disableRadioButtons()
    {
        radioButtonAnswer1.enabled = false
        radioButtonAnswer2.enabled = false
        radioButtonAnswer3.enabled = false
        radioButtonAnswer4.enabled = false
    }
}

sandro4912

I remembered I had this index issue before and asked it on stack overflow:
https://stackoverflow.com/questions/62484078/required-property-not-working-with-repeater

So solution is in QuizPage add:

required property int index

Than index can be accessed. Weired but thats the way.

fcarney

What does your repeater code look like?
It cannot inject a property into an object. It has to be set inside the repeater statement.

sandro4912

Repeater code is like you suggested:

SwipeView{
       id: quizSwipeView
       interactive: false
       anchors.fill: parent

       Repeater{
           model: randomQuestions
           delegate: QuizPage{
               question: randomQuestions[index]
           }
       }
   }

Item{
    id: root

    required property var question
    required property int index
//....
}

If I drop the required in question i can skip defining the index it is very weired behaviour.

See also here: https://stackoverflow.com/questions/62484078/required-property-not-working-with-repeater

fcarney

@sandro4912 said in How to create n Objects dynamically to populate StackView:

Item{
id: root

required property var question
required property int index

//....
}

What is the reason for this property index inside your object? This doesn't make sense if you are using "question" as your property.

fcarney

@sandro4912 said in How to create n Objects dynamically to populate StackView:

Repeater{
model: randomQuestions
delegate: QuizPage{
question: randomQuestions[index]
}
}

What does this do?

Repeater{
           model: randomQuestions
           delegate: Item{
              Component.onCompleted: {
                console.log(index)
              }
           }
       }

This "should" print out the index for each entry in randomQuestions. If it doesn't then you need a data model Repeater can use.

sandro4912

this

        Repeater{
           model: randomQuestions
           delegate: Item{
              Component.onCompleted: {
                console.log(index)
              }
           }
       }

gives:

qml: 0
qml: 1
qml: 2
qml: 3
qml: 4
qml: 5
qml: 6
qml: 7
qml: 8
qml: 9

Even without the added index property.

But if I do:

Repeater{
         model: randomQuestions
         delegate: QuizPage{
             question: randomQuestions[index]
         }
     }

It only works with adding into QuizPage:

required property int index

Otherwise:

qrc:/qml/quiz/QuizPage.qml:52: TypeError: Cannot read property 'id' of undefined
qrc:/qml/quiz/QuizPage.qml:57: TypeError: Cannot read property 'askedQuestion' of undefined
qrc:/qml/quiz/QuizPage.qml:64: TypeError: Cannot read property 'picture' of undefined
qrc:/qml/quiz/Quiz.qml:0: ReferenceError: index is not defined

fcarney

@sandro4912 said in How to create n Objects dynamically to populate StackView:

required property int index

Why are you doing this? This makes zero sense to create a require property when all your info is in your question property.

fcarney

@sandro4912 said in How to create n Objects dynamically to populate StackView:

question.answer1

Also, for every usage of question inside of QuizPage do this:
Change this:
question.answer1
To this:
root.question.answer1

I think you may be having scoping issues with the question property.

sandro4912

@fcarney

check out this:

https://stackoverflow.com/questions/62484078/required-property-not-working-with-repeater

The issue is if you have a required property in an item you put into an repeater it only gets access to the index in the weired way that you add an index as required property. I also don't understand why it is necessary.

You can try it out by copy the project on git hub and remove the line index:
https://github.com/SandroWissmann/Quiz

fcarney

The problem they had is that you have to use the delegate property to get access to the properties the Repeater adds to the delegate.

If the "required" part makes it not work, then get rid of it.

fcarney

@sandro4912 I ran your program. I removed the property index from QuizPage and got rid of "required" from question. It then showed the quiz correctly. When I clicked next question it crashed. Not sure what was wrong there.

sandro4912

Of course I could change

required property var question

to

property var question

but I thought it would be good to make it required to indicate the page only works with a question supplied. To me that all sounds like a weired bug that index is only available when you don't make a property required.

so

required property var index

looks like a workarround.

sandro4912

@fcarney
you are right it does not work anymore. I will check that issue tomorrow and then come back to here.

fcarney

@sandro4912 said in How to create n Objects dynamically to populate StackView:

To me that all sounds like a weired bug that index is only available when you don't make a property required.

That issue on SO was because the OP was not using "delegate" inside of Repeater. The relevance to "required" was due to the repeater not being able to set the property.

GrecKo

@fcarney said in How to create n Objects dynamically to populate StackView:

That issue on SO was because the OP was not using "delegate" inside of Repeater. The relevance to "required" was due to the repeater not being able to set the property.

No, it was not because of not using delegate, as a comment said delegate is a default property so you can omit it.
Repeater { Item {} } is the same as Repeater { delegate : Item {} }.

The problem was indeed because of the required property. Since Qt 5.15, if a delegate has a required property, the view won't assign the delegate context properties based on the roles of the model. It will only assign roles corresponding to the required properties present in the delegate.

https://doc.qt.io/qt-5/qtquick-modelviewsdata-modelview.html#models

To get finer control over which roles are accessible, and to make delegates more self-contained and usable outside of views, required properties can be used. If a delegate contains required properties, the named roles are not provided. Instead, the QML engine will check if the name of a required property matches that of a model role. If so, that property will be bound to the corresponding value from the model.
[...]
Note: model, index, and modelData roles are not accessible if the delegate contains required properties, unless it has also required properties with matching names.

That's why you can't access index or modelData if you don't declare them as required property in QuizPage.

And instead of index, you should use modelData :

delegate: QuizPage{
    required property var modelData
    question: modelData
}

Note that QQmlListProperty is not meant to expose C++ data to QML, it is meant to allow the QML to populate a C++ list from QML.
You should use QList<QObject*>, QVariantList or even better QAbstractListModel if you need a dynamic model. QQmlListProperty does work, but it's not its job.

fcarney

@GrecKo
No wonder I was going crazy trying to understand this problem. Thanks for shedding light on this.

sandro4912

@GrecKo

Thanks for clarify. I was very confused how to expose my randomQuestions correctly.

Now comes the big Question.

Currently it is like this:

All Questions are in a class Derived from QSqlTableModel.

Now for RandomQuestions currently I create with an additional function in that class the QQmlListProperty<Questions>.

Wouldn't it the best to implement QSortFilterProxModel which picks um the required random questions to expose that to QML?