accessing aggregates (QVector of a struct)

kshegunov

You can refer to the object by id. Like this:

function loadRackDataFirst() {
    bottle1.cellText = changeConsumablesViewModel.getBottleListQv()[0].name
}

However you should convert to QString.

fcarney

Repeater {
  model: changeConsumablesViewModel.getBottleListQv() // or use property, a property will have a signal to update if the list changes
  Bottle {
    cellText: modelData.name
  }
}

If you want more interaction then a model might be more appropriate.

modelData Search for modelData on that page to understand where it comes from.

mzimmers

@kshegunov the "name" field isn't accessible as I've currently implemented it. I think I may have over-designed this. I've made several changes since my earlier posts, so let me recap my code:

The struct:

struct BottleData {
  Q_GADGET
public:
  uint32_t m_volume;               // amount in bottle (in uL)
  uint32_t m_amountNeeded;         // amount needed for synth (in uL)
  int m_position;                  // still figuring this one out
  std::string m_name;              // name of the reagent
  ReagentBottleType m_bottleType;  // bottle type.
  Q_PROPERTY(uint32_t volume MEMBER m_volume)
  Q_PROPERTY(uint32_t amountNeeded MEMBER m_amountNeeded)
  Q_PROPERTY(int position MEMBER m_position)
  Q_PROPERTY(std::string name MEMBER m_name)
  Q_PROPERTY(ReagentBottleType bottleType MEMBER m_bottleType)
};
Q_DECLARE_METATYPE(BottleData)

The class:

typedef QVector<BottleData> BottleDataList;

class BottleList : public QObject
{
    Q_OBJECT
private:
    BottleDataList m_bottleList;
public:
    explicit BottleList(QObject *parent = nullptr);
    Q_PROPERTY(QVariantList qvl READ getBottleListQv)
    QVariantList getBottleListQv();
    ...

From reading the docs, I was under the impression that I wouldn't need a getter for the fields in the struct, because I used the MEMBER macro. Did I misinterpret this?

mzimmers

@fcarney that looks really powerful. The complete definition of each model is like this:

        Bottle {
            id: bottle1
            cellX: 25
            cellY: 105
            cellHeight: 75
            cellWidth: 75
            bottleScaleFactor: scaleFactor
            cellText: "W7"
            cellColor: "red"
        }

If I use the repeater to load the names, can I alter the individual bottles afterwards?

Thanks...

PS: I'm aware that there's a lot of ugly hard-coding in here; I was going to address that after I got the connections working.

fcarney

If I use the repeater to load the names, can I alter the individual bottles afterwards?

It only alters the copy given to the Repeater. It has no way to get that data back.

I have not used Q_GADGETs before. What does it print out when you console.log(modelData)? Curious as what QML thinks that objects is.

fcarney

@mzimmers said in accessing aggregates (QVector of a struct):

From reading the docs, I was under the impression that I wouldn't need a getter for the fields in the struct, because I used the MEMBER macro. Did I misinterpret this?

I "think" so. DOH! I meant to agree to the MEMBER macro doing that, not you misinterpreting this.

mzimmers

@fcarney my console.log isn't working for this app, so I can't tell you. I discovered Q_GADGET from some online searching; in simplest terms, it's a lightweight version of Q_OBJECT (no signals/slots).

I'm not at all concerned with updating anything other than my display. But now you have me wondering where I should really define all those values. (I would rather not use JSON, but that's probably the right way to do this.)

fcarney

@mzimmers If you want your display to interact with the data then a full blown QAbstractListModel would be a better fit. Then each piece of the BottleData object could be its own role. With a setData routine you can edit that model.

kshegunov

@mzimmers said in accessing aggregates (QVector of a struct):

Q_DECLARE_METATYPE(BottleData)

is already done by the Q_GADGET so it's superfluous.

Switch

std::string m_name;

to QString.

Register the type with QML (qmlRegisterType) if you intend to create instances of it from there.

From reading the docs, I was under the impression that I wouldn't need a getter for the fields in the struct, because I used the MEMBER macro. Did I misinterpret this?

Nope, this is correct as far as I recall.

If I use the repeater to load the names, can I alter the individual bottles afterwards?

I don't think so, but I'm a noobster with QML. I believe you can imperatively create the items like this (untested):

Component {
        id: component
        Bottle { cellText: "default text" }
        onCompleted: changeConsumablesViewModel.getBottleListQv().forEach(element => function(element)  {
           createObject(parentItemId, { cellText: element.name });
        }, this);
}

or something akin.

mzimmers

OK, so why doesn't this work?

    Rectangle {
        id: rack
        function getBottleName(i) {
            return changeConsumablesViewModel.getBottleListQv()[i].m_name
        }
        Bottle {
            cellText: rack.getBottleName(0)// "W7"
        }

I get this error on the line with the "return" statement:

TypeError: Cannot read property 'm_name' of undefined

kshegunov

@mzimmers said in accessing aggregates (QVector of a struct):

TypeError: Cannot read property 'm_name' of undefined

Your property is called name, also check the elements you get in that array. Side note: you may need to wait for the component to be fully loaded before doing that (but take with a grain of salt).

mzimmers

@kshegunov said in accessing aggregates (QVector of a struct):

@mzimmers said in accessing aggregates (QVector of a struct):

TypeError: Cannot read property 'm_name' of undefined

Your property is called name, also check the elements you get in that array.

Tried with "name" -- same error.

Side note: you may need to wait for the component to be fully loaded before doing that (but take with a grain of salt).

Actually, I think you're on to it here. For some reason, the people who wrote this app (I'm just maintaining it) load all the QML files up-front, rather than as-needed. I think the problem is that this is an empty vector when this function is first called...I need to think of how best to handle that.

kshegunov

@mzimmers said in accessing aggregates (QVector of a struct):

Actually, I think you're on to it here. For some reason, the people who wrote this app (I'm just maintaining it) load all the QML files up-front, rather than as-needed. I think the problem is that this is an empty vector when this function is first called...I need to think of how best to handle that.

Check that through the console. If you want, you can try to wait for the component by adding[1]:

Component.onCompleted: <js code to do w/e>

[1]: https://doc.qt.io/qt-5/qml-qtqml-component.html#completed-signal

fcarney

Okay, I tested it. I cannot get Q_GADGET to work so I used Q_OBJECT:
class in main.cpp:

using ReagentBottleType = int;
class BottleData : public QObject
{
    Q_OBJECT

    Q_PROPERTY(int volume MEMBER m_volume NOTIFY somethingChanged)
    Q_PROPERTY(int amountNeeded MEMBER m_amountNeeded NOTIFY somethingChanged)
    Q_PROPERTY(int position MEMBER m_position NOTIFY somethingChanged)
    Q_PROPERTY(std::string name MEMBER m_name NOTIFY somethingChanged)
    Q_PROPERTY(ReagentBottleType bottleType MEMBER m_bottleType NOTIFY somethingChanged)
public:
    BottleData(QObject* parent=nullptr)
        : QObject(parent)
    {
        connect(this, &BottleData::somethingChanged, [this](){
            qDebug() << "somethingChanged" << m_position << QString::fromStdString(m_name) << m_volume;
        });
    }

    int m_volume=0;               // amount in bottle (in uL)
    int m_amountNeeded=0;         // amount needed for synth (in uL)
    int m_position=0;                  // still figuring this one out
    std::string m_name="";              // name of the reagent
    int m_bottleType=0;  // bottle type.

signals:
    void somethingChanged();
};

setting contextProperty in main.cpp for testing:

TestObj testobj;
auto context = engine.rootContext();
context->setContextProperty("varlisttestobj", &testobj);

QML to exercise the object in C++:

Column {
        anchors.top: listview1.bottom
        Repeater {
            model: varlisttestobj.varList

            Row {
                id: bottledelegate

                width: 50
                spacing: 20

                Component.onCompleted: console.log(modelData, modelData.position, modelData.volume)

                Timer {
                    interval: 1000
                    repeat: true
                    running: true
                    onTriggered: {
                        modelData.volume += 1
                    }
                }

                Text {
                    text: modelData.position
                    height: 20
                }
                Text {
                    text: modelData.volume
                    height: 20
                }
            }
        }
    }

QML did not like uint32_t at all. So you will have to find another type that it likes on that page with compatible QML types I linked earlier.

fcarney

Oops, I forgot test object in main.cpp:

class TestObj : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QVariantList varList READ varList NOTIFY varListChanged)
    //Q_PROPERTY(QObjectList objList READ objList NOTIFY objListChanged)

public:
    TestObj(QObject* parent=nullptr)
        : QObject(parent)
    {
        for(int count=0; count<10; ++count){
            auto bottle = new BottleData();
            bottle->m_volume = count*10;
            bottle->m_position = count;
            m_bottleData.append(bottle);
        }

        emit varListChanged();
    }

    QVariantList varList(){
        QVariantList list;
        for(auto bottleData: qAsConst(m_bottleData)){
            list.append(QVariant::fromValue(bottleData));
        }
        return list;
    }
    /*
    QObjectList objList(){
        QObjectList list;
        for(auto bottleData: qAsConst(m_bottleData)){
            list.append(bottleData);
        }
        return list;
    }
    */


signals:
    void varListChanged();

private:
    QVector<BottleData*> m_bottleData;

};

fcarney

I didn't expect it to trigger the somethingChanged signal in the BottleData object.

fcarney

Trying to assign to std::string didnt work either:

modelData.name = "fred"

error:

Error: Cannot assign QString to an unregistered type

kshegunov

Okay, so here you go:

types.h

#ifndef TYPES_H
#define TYPES_H

#include <QObject>
#include <QVector>
#include <QVariant>
#include <QVariantList>

struct Bottle
{
    Q_GADGET

    Q_PROPERTY(QString name MEMBER m_name)
    Q_PROPERTY(qreal size MEMBER m_size)

public:
    QString m_name;
    qreal m_size;
};

class DataSource : public QObject
{
    Q_OBJECT

public:
    Q_INVOKABLE QVariantList getData();
};


#endif // TYPES_H

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>

#include "types.h"

QVariantList DataSource::getData()
{
    return {
        QVariant::fromValue<Bottle>({ "First bottle", 0.75 }),
        QVariant::fromValue<Bottle>({ "Second bottle", 0.70 })
    };
}

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    engine.addImportPath(QStringLiteral("qrc:/"));
    QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated, &app, [url] (QObject * obj, const QUrl & objUrl) -> void {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);

    DataSource source;
    engine.rootContext()->setContextProperty(QStringLiteral("DataSource"), &source);

    qmlRegisterUncreatableType<Bottle>("Example", 1, 0, "Bottle", "");

    engine.load(url);

    return app.exec();
}

main.qml

import QtQuick 2.15
import QtQuick.Window 2.15
import QtQuick.Controls 2.15

import Example 1.0

Window {
    id: mainWindow
    visible: true
    width: 300
    height: 200
    title: qsTr("Some title")

    Component {
        id: template
        Text {
            text: "defaultText"
        }
    }

    Column {
       Component.onCompleted: DataSource.getData().forEach(function(element) {
           template.createObject(this, { text: element.name });
       }, this)
    }
}

PS. QML is damn annoying ...

mzimmers

@kshegunov said in accessing aggregates (QVector of a struct):

PS. QML is damn annoying ...

Heh...no argument there, but if I'm reading the tea leaves correctly, it's here to stay.

I understand your C++ code, but I'm still trying to figure out what your QML results in. What exactly is this doing?

Component {
        id: template
        Text {
            text: "defaultText"
        }
    }

    Column {
       Component.onCompleted: DataSource.getData().forEach(function(element) {
           template.createObject(this, { text: element.name });
       }, this)
    }

kshegunov

@mzimmers said in accessing aggregates (QVector of a struct):

I understand your C++ code, but I'm still trying to figure out what your QML results in. What exactly is this doing?

Well, I'm leveraging the fact that the QML engine is a glorified object factory. You know the widgets' basics, so with QtQuick it's the same story, more or less. It goes roughly like this:

All visual items are derived from QQuickItem, these include the stuff you see in the QML file definition like Text and Column.
Component does not(!) derive from QQuickItem, it is a QObject (or rather QQmlComponent) that's supposed to create your QQuickItems by calling QQmlComponent::createObject and passing it the correct visual parent and maybe a set of property values.
When you load a QML file, this is what the engine does - it reads and instantiates the component your file represents, hence it creates all the visual and non-visual items described in said file. It parents everything by the way the items are nested (think of it as a big QObject tree). All the visual items are (re)parented to the root QQuickItem so they get painted correctly.
After the object tree's loaded and the objects are instantiated you get the completed signal emitted and propagated through your items (visual and non-visual), so you can do w/e.
In the above code you attach to the completed signal, then you call the getData to retrieve the list of structs, then forEach of the elements of the array you execute the anonymous function. The function is a dummy mostly. It just needs to go get the reference to the template component (which isn't a visual item, as mentioned) and call it's createObject to create the Text item contained. The first argument of the method is the visual parent (which is the Column in this case) and the second parameter is the set of properties to pass along, which is just the text value retrieved from the structure.

PS. This code's tested (unlike most of the snippets I provide) so you can directly plug it in your project/app and play with it as you wish.