Receiving c++ signals with arguments in QML QList<QPair> - qt

Question

I'm trying to figure out how to get QList> from C++ signal in QML, i'm only getting either QVariant(RecordList, ) or QVariant(QList, ). Tried with different supported sequence type and they work perfectly (QList. I'll appreciate if somebody can help me to understand my error. Kind regards.
jsonreaderasync.h
typedef QPair<qreal,qreal>Record;
typedef QList<Record>RecordList;
class JsonReaderAsync : public QObject
{
Q_OBJECT
public:
explicit JsonReaderAsync(QObject *parent = nullptr);
Q_INVOKABLE void start(const QString& fileName);
signals:
void started();
//void finished(QList<qreal> record);
void finished(RecordList record);
//void finished(QList<Record> record);
};
jsonreaderasync.cpp
#include <QDebug>
#include "jsonreaderasync.h"
Q_DECLARE_METATYPE(Record)
Q_DECLARE_METATYPE(RecordList)
//Q_DECLARE_METATYPE(QList<Record>)
JsonReaderAsync::JsonReaderAsync(QObject *parent) : QObject(parent)
{
qRegisterMetaType<Record>("Record");
qRegisterMetaType<RecordList>("RecordList"); // qml prints QVariant(RecordList, )
//qRegisterMetaType<QList<Record>>("QList<Record>"); //qml prints QVariant(QList<Record>, )
}
void JsonReaderAsync::start(const QString &fileName)
{
QList<Record> record;
record.append(qMakePair(1,1));
record.append(qMakePair(1,2));
record.append(qMakePair(1,3));
// QList<qreal> foo;
// foo.append(1);
// foo.append(1);
// foo.append(1);
emit finished(record);
}
main.cpp
#include "jsonreaderasync.h"
typedef QPair<qreal,qreal>Record;
typedef QList<QPair<qreal,qreal>>RecordList;
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QQmlApplicationEngine *engine = new QQmlApplicationEngine;
JsonReaderAsync* dataReaderAsync = new JsonReaderAsync();
engine->rootContext()->setContextProperty("JsonReaderAsync", dataReaderAsync);
engine->load(QUrl(QStringLiteral("main.qml")));
return app.exec();
}
main.qml
import QtQuick 2.12
import QtQuick.Dialogs 1.2
import QtQuick.Layouts 1.13
import QtQuick.Window 2.13
import QtQuick.Controls 2.12
import QtCharts 2.3
Window {
id: appWindow
visible: true
minimumWidth : 400
minimumHeight: 400
Connections
{
target: JsonReaderAsync
onStarted:{
console.log("onStarted")
}
onFinished:{
console.log("onFinished")
console.log(record)
console.log(record[0])
}
}
Button {
width : 40
height: 40
anchors.centerIn: parent
onClicked: {
JsonReaderAsync.start("")
}
}
}

Answer 1

In QML, the meta-type system is the only way that QML engine can access C++ structures from a QML environment.
That is, only Predefined C++ Types and custom objects that have Q_PROPERTY declarations could access from QML environment.
Here's my recommend (simplest) modification:
jsonreaderasync.h
#include <QObject>
#include <QVariant>
class Record : public QPair<qreal, qreal> {
Q_GADGET
Q_PROPERTY(qreal first MEMBER first CONSTANT FINAL)
Q_PROPERTY(qreal second MEMBER second CONSTANT FINAL)
public:
Record() = default;
Record(qreal a, qreal b) : QPair<qreal, qreal>(a, b) {}
};
class JsonReaderAsync : public QObject
{
Q_OBJECT
public:
explicit JsonReaderAsync(QObject *parent = nullptr);
Q_INVOKABLE void start(const QString& fileName);
signals:
void started();
void finished(QVariantList record); // RecordList
};
jsonreaderasync.cpp
#include <QDebug>
#include "jsonreaderasync.h"
JsonReaderAsync::JsonReaderAsync(QObject *parent) : QObject(parent)
{
qRegisterMetaType<Record>("Record");
}
void JsonReaderAsync::start(const QString &fileName)
{
QVariantList record;
record.append(QVariant::fromValue(Record(1,1)));
record.append(QVariant::fromValue(Record(1,2)));
record.append(QVariant::fromValue(Record(1,3)));
emit finished(record);
}
Now you can access records from QML:
onFinished: {
for (var i = 0; i < record.length; ++i)
console.log(record[i].first + "->" + record[i].second);
}
Note that converting between C++ objects and QML objects does incur a bit overhead, if these codes are performance sensitive, please consider using C++ Data Models.

Answer 2

While #GPBeta solution works, I wanted more flexibility for Qml supported pairs. I tried to work with templates, but Q_GADGET doesn't support it. There might be a smart wrapper (Template + QVariant) solution, I guess... Nonetheless, here is my approach to the problem:
class PairQml {
Q_GADGET
Q_PROPERTY(QVariant first MEMBER first CONSTANT FINAL)
Q_PROPERTY(QVariant second MEMBER second CONSTANT FINAL)
public:
PairQml() = default;
PairQml(QVariant f, QVariant s): first(f), second(s) {}
QVariant first;
QVariant second;
};
Don't forget to register: qRegisterMetaType<PairQml>("PairQml");