Adding/ Removing markers in quickwidget openstreet map - c++

Question

I'm using QQuickwidget in my QT5 gui. I've loaded an openstreet map on it by adding qml files accordingly.
By adding a function in the qml file, I call it through the cpp file by using QMetaObject::invokeMethod. This function adds markers at certain coordinates. This is because I want to control my markers through c++ code itself.
My problem: I am able to load the map and the markers successfully, but I'm not able to remove the markers. My aim is to show the path of a moving object on a map. Therefore, using a timer, I want to update it's position periodically. Every t_samp seconds, I want to remove the marker and add it somewhere else.
the 'item' created in the addMarker code is added to the map by using map.addMapItem(item). By appending map.removeMapItem(item) to the function, the marker disappears. But, the problem is, I can't seem to be able to access 'item' outside the function. Therefore, I cant use map.removeMapItem since I cant input the marker.
I also tried making the function output 'item', so that I can then use it to add/remove the marker. Unfortunately, I dont know the data type of item, and hence cant receive it.
mapview.qml:
import QtQuick 2.12
import QtLocation 5.12
import QtPositioning 5.12
Item {
id: window
Plugin
{
id: mapPlugin
name:"osm"
}
function addMarker(latitude, longitude)
{
var component= Qt.createComponent("qrc:///qml/marker.qml")
var item= component.createObject(window, {coordinate: QtPositioning.coordinate(latitude,longitude)})
map.addMapItem(item)
}
Map
{
id: map
anchors.fill: parent
plugin: mapPlugin
center: QtPositioning.coordinate(15.4561,73.8021);
zoomLevel: 14
}
}
marker.qml:
import QtQuick 2.12
import QtLocation 5.12
import QtPositioning 5.12
MapQuickItem
{
id: marker
anchorPoint.x: marker.width / 4
anchorPoint.y: marker.height
sourceItem: Image
{
id: icon
source: "qrc:///images/mapmark.png"
sourceSize.width: 50
sourceSize.height: 50
}
}
mainwindow.cpp: (only relevant snippet)
QObject* target= qobject_cast<QObject*>(ui->quickWidget->rootObject());
QString functionName= "addMarker";
QMetaObject::invokeMethod(target,functionName.toUtf8().constData(), Qt::AutoConnection, Q_ARG(QVariant, 15.4561), Q_ARG(QVariant,73.8021));

Answer 1

Instead of exporting the QML marker to C++, it is best to export a QObject from C++ to QML, and since you want to handle several markers you must use a model.
Explanation of the approach:
The MVC pattern is Qt's natural way of handling a lot of information, and for this it implements views such as MapItemView, and models that can be created based on QAbstractXXXModel. So only the responsibility is to specialize the classes for the objective, such as implementing the logic of just keeping n elements and if there is a new element, remove the oldest one.
Why is it better to export a QObject to QML? The life cycle of the objects in QML are handled by QML, so in your case you could access the markers at a given time QML could delete it so that the pointer in C ++ would append non-reserved memory. Another advantage is that the Q_PROPERTY are recognized in QML and the data type is known by QML and C ++, unlike if you export a QML object to C ++ since only the properties of QObject or QQuickItem will be used. Also when exported using setContextProperty the QObject is global. The disadvantage is that more code is added. For more details read Interacting with QML from C++.
markermodel.h
#ifndef MARKERMODEL_H
#define MARKERMODEL_H
#include <QAbstractListModel>
#include <QGeoCoordinate>
class MarkerModel : public QAbstractListModel
{
Q_OBJECT
Q_PROPERTY(QGeoCoordinate current READ current NOTIFY currentChanged)
public:
enum MarkerRoles{
PositionRole = Qt::UserRole + 1000,
};
explicit MarkerModel(QObject *parent = nullptr);
void moveMarker(const QGeoCoordinate & coordinate);
int rowCount(const QModelIndex &parent = QModelIndex()) const override;
QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;
QHash<int, QByteArray> roleNames() const override;
int maxMarkers() const;
void setMaxMarkers(int maxMarkers=0);
QGeoCoordinate current() const;
signals:
void currentChanged();
private:
void insert(int row, const QGeoCoordinate & coordinate);
void removeLastMarker();
QList<QGeoCoordinate> m_markers;
QGeoCoordinate m_current;
int m_maxMarkers;
};
#endif // MARKERMODEL_H
markermodel.cpp
#include "markermodel.h"
MarkerModel::MarkerModel(QObject *parent)
: QAbstractListModel(parent),
m_maxMarkers(0)
{
}
void MarkerModel::moveMarker(const QGeoCoordinate &coordinate)
{
QGeoCoordinate last = m_current;
m_current = coordinate;
Q_EMIT currentChanged();
if(!last.isValid())
return;
if(m_maxMarkers == 0){
insert(0, last);
return;
}
if(rowCount() >= m_maxMarkers){
while (rowCount() >= m_maxMarkers)
removeLastMarker();
removeLastMarker();
}
insert(0, last);
}
int MarkerModel::maxMarkers() const
{
return m_maxMarkers;
}
void MarkerModel::setMaxMarkers(int maxMarkers)
{
m_maxMarkers = maxMarkers > 1 ? maxMarkers: 0;
}
QGeoCoordinate MarkerModel::current() const
{
return m_current;
}
int MarkerModel::rowCount(const QModelIndex &parent) const
{
if (parent.isValid())
return 0;
return m_markers.count();
}
QVariant MarkerModel::data(const QModelIndex &index, int role) const
{
if (!index.isValid())
return QVariant();
if(role == PositionRole)
return QVariant::fromValue(m_markers[index.row()]);
return QVariant();
}
QHash<int, QByteArray> MarkerModel::roleNames() const
{
QHash<int, QByteArray> roles;
roles[PositionRole] = "position";
return roles;
}
void MarkerModel::insert(int row, const QGeoCoordinate & coordinate)
{
beginInsertRows(QModelIndex(), row, row);
m_markers.insert(row, coordinate);
endInsertRows();
}
void MarkerModel::removeLastMarker()
{
if(m_markers.isEmpty())
return;
beginRemoveRows(QModelIndex(), rowCount()-1, rowCount()-1);
m_markers.removeLast();
endRemoveRows();
}
mainwindow.h
// ...
MarkerModel marker_model;
// ...
mainwindow.cpp
// ...
ui->quickWidget->rootContext()->setContextProperty("marker_model", &marker_model);
ui->quickWidget->setSource(QUrl("qrc:/mapview.qml"));
// ...
main.qml
// ...
Map{
id: map
anchors.fill: parent
plugin: mapPlugin
center: QtPositioning.coordinate(15.4561,73.8021);
zoomLevel: 14
Marker{
coordinate: marker_model.current
}
MapItemView{
model: marker_model
delegate: Marker{
coordinate: model.position
}
}
}
// ...
The complete example is here