Read C++ QVector of structs in QML - c++

Question

In my C++ class I have
struct trackPoint {
QString lat;
QString lon;
QString elevation;
};
QVector<trackPoint> trackPoints;
In QML I want to access this as a multi-dimensional array of lon,lat pairs
[[0,1],[1,1],[2,1]]
Is this possible using the Q_Property mechanism? As I am pretty sure that structs cannot be exposed to QML?
I've tied:-
Q_PROPERTY(QVector<trackPoint> trackPoints READ gpx)
With a method:-
QVector<trackPoint> GPXFileIO::gpx() const {
return trackPoints;
}
But this gives me the error:-
QMetaProperty::read: Unable to handle unregistered datatype 'QVector<trackPoint>' for property 'GPXFileIO::trackPoints'

Answer 1

A simple way to expose a struct to QML is using Q_GADGET with Q_PROPERTY so we can get each element of the structure, they will not be part of an array. On the other hand QVector is supporting a number of elements with QString, int, QUrl, etc. but not for new types, in which case QVariantList should be used.
main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include <QVector>
struct TrackPoint {
Q_GADGET
Q_PROPERTY(qreal lat MEMBER lat)
Q_PROPERTY(qreal lon MEMBER lon)
Q_PROPERTY(qreal elevation MEMBER elevation)
public:
qreal lat;
qreal lon;
qreal elevation;
};
class TrackClass: public QObject
{
Q_OBJECT
Q_PROPERTY(QVariantList trackpoints READ gpx)
public:
TrackClass(QObject *parent=nullptr):QObject(parent){
trackPoints << TrackPoint{10, 10, 10} << TrackPoint{11, 11, 11};
}
QVariantList gpx() const{
QVariantList l;
for(const TrackPoint & p: trackPoints){
l << QVariant::fromValue(p);
}
return l;
}
private:
QVector<TrackPoint> trackPoints;
};
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
TrackClass track;
QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty("track", &track);
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
if (engine.rootObjects().isEmpty())
return -1;
return app.exec();
}
#include "main.moc"
main.qml
import QtQuick 2.9
import QtQuick.Window 2.2
Window {
visible: true
width: 640
height: 480
title: qsTr("Hello World")
Component.onCompleted: {
for( var i in track.trackpoints){
var p = track.trackpoints[i];
console.log("lat: ", p.lat, "lon: ", p.lon, "elevation: ", p.elevation)
}
}
}
Output:
qml: lat: 10 lon: 10 elevation: 10
qml: lat: 11 lon: 11 elevation: 11

Answer 2

Without adding the complexity of gadgets, I find pretty straight forward the usage of QVariantList of QVariantMap, as Qvariant.
This is how I do it:
Q_PROPERTY(QVariant trackpoints READ gpx NOTIFY gpxChanged)
QVariant TrackClass::gpx() const
{
QVariantList itemsList;
for(const TrackPoint &p : trackPoints)
{
QVariantMap itemMap;
itemMap.insert("lat", p.lat);
itemMap.insert("lon", p.lon);
itemMap.insert("elevation", p.elevation);
itemsList.append(itemMap);
}
return QVariant::fromValue(itemsList);
}
Then in QML you can use trackpoints as model and access item fields by name.
It is a good practice to also add a NOTIFY signal, to be called when your QVector changes.