QML QTQuick ChartView pass pointer to C++ - c++

Question

I'm trying to make an Oscilloscope like Qt Quick application, based on the qtcharts-qmloscilloscope-example here
In this example the traces (a QTQuick ChartView) are pre-allocated in the QML and updated via a timer.
I would like to be able to add and remove traces at runtime.
The existing application passes a reference to the underlying data array as QAbstractSeries of QPointF objects. This action is triggered on a Timer, thus:
Timer {
id: refreshTimer
interval: 1 / 1 * 1000 // 1 Hz
running: dataSource.isRunning
repeat: true
onTriggered: {
dataSource.updateTime();
//dataSource.update(chartView.series(0));
//dataSource.update(chartView.series(1));
//dataSource.update(chartView.series(2));
dataSource.update(chartView);
}
}
And the existing update method looks like this:
void DataSource::update(QAbstractSeries * series)
{
...
}
This is OK if you only want a fixed number of traces and they are all updated individually. But I would like to be able to add traces as they are turned on and off.
I've tried to pass the chartView ID to an update(QChartView *) function but this always breaks with a null pointer.
Q_INVOKABLE void DataSource::update(QChartView * view)
{
...
}
I've also tried using window->findChildren at the top level and passing that to the instance of DataSource. That gets a valid pointer but of type QQuickItem. If I cast that to a QChartView I also get a null pointer.
How to I correctly pass a pointer to a QChartView object to C++?

Answer 1

ChartView is not a QChartView but a QQuickItem so casting does not work.
So you can not access the methods directly but use the QMetaObject as shown below:
helper.h
#ifndef HELPER_H
#define HELPER_H
#include <QObject>
class QQuickItem;
class Helper : public QObject
{
Q_OBJECT
public:
using QObject::QObject;
Q_INVOKABLE void createSerie(QQuickItem *chartview);
Q_INVOKABLE void removeAllSeries(QQuickItem *chartview);
};
#endif // HELPER_H
helper.cpp
#include "helper.h"
#include <QAbstractAxis>
#include <QAbstractSeries>
#include <QLineSeries>
#include <QMetaObject>
#include <QQuickItem>
#include <random>
#include <cstring>
QT_CHARTS_USE_NAMESPACE
void Helper::createSerie(QQuickItem *chartview){
if(!chartview)
return;
const QMetaObject *mo = chartview->metaObject();
if(std::strcmp(mo->className(), "QtCharts::DeclarativeChart") != 0)
return;
int ix = mo->indexOfEnumerator("SeriesType");
QMetaEnum me = mo->enumerator(ix);
int type = me.keyToValue("SeriesTypeLine");
QAbstractAxis *axis_x = nullptr;
QMetaObject::invokeMethod(chartview, "axisX", Qt::DirectConnection,
Q_RETURN_ARG(QAbstractAxis *, axis_x));
QAbstractAxis *axis_y = nullptr;
QMetaObject::invokeMethod(chartview, "axisY", Qt::DirectConnection,
Q_RETURN_ARG(QAbstractAxis *, axis_y));
QAbstractSeries *serie = nullptr;
QMetaObject::invokeMethod(chartview, "createSeries", Qt::DirectConnection,
Q_RETURN_ARG(QAbstractSeries *, serie),
Q_ARG(int, type),
Q_ARG(QString, "serie from c++"),
Q_ARG(QAbstractAxis *, axis_x),
Q_ARG(QAbstractAxis *, axis_y));
if(QLineSeries *line_serie = qobject_cast<QLineSeries *>(serie)){
static std::default_random_engine e;
static std::uniform_real_distribution<> dis(0, 3);
for(int i=0; i < 14; i++){
line_serie->append(i, dis(e));
}
}
}
void Helper::removeAllSeries(QQuickItem *chartview){
if(!chartview)
return;
const QMetaObject *mo = chartview->metaObject();
if(std::strcmp(mo->className(), "QtCharts::DeclarativeChart") != 0)
return;
QMetaObject::invokeMethod(chartview, "removeAllSeries", Qt::DirectConnection);
}
main.qml
import QtQuick 2.14
import QtQuick.Window 2.14
import QtQuick.Layouts 1.14
import QtQuick.Controls 2.4
import QtCharts 2.14
Window {
visible: true
width: 640
height: 480
title: qsTr("Hello World")
ColumnLayout{
anchors.fill: parent
RowLayout{
Button{
text: "Create serie"
Layout.fillWidth: true
onClicked: helper.createSerie(chartview)
}
Button{
text: "Clear series"
Layout.fillWidth: true
onClicked: helper.removeAllSeries(chartview);
}
}
ChartView {
id: chartview
title: "Line"
antialiasing: true
Layout.fillWidth: true
Layout.fillHeight: true
LineSeries {
name: "LineSeries"
XYPoint { x: 0; y: 0 }
XYPoint { x: 3; y: 2.1 }
XYPoint { x: 8; y: 3.3 }
XYPoint { x: 10; y: 2.1 }
XYPoint { x: 11; y: 4.9 }
XYPoint { x: 12; y: 3.0 }
XYPoint { x: 13; y: 3.3 }
}
axes: [
ValueAxis{
id: xAxis
min: 1.0
max: 15.0
},
ValueAxis{
id: yAxis
min: 0.0
max: 5.0
}
]
}
}
}
In the following link is the complete code.

Answer 2

Better variant exists without using QMetaObject::invokeMethod(): it is possible to use QChart directly. Just need to extend following solution "Push QML ChartView updates from c++" a bit:
*.h
public:
Q_INVOKABLE void setSeries(QAbstractSeries *series);
[...]
private:
QXYSeries *mSeries;
QChart *mChart;
[...]
*.cpp
void DataSource::setSeries(QAbstractSeries *series)
{
if (series) {
mSeries = static_cast<QXYSeries *>(series);
mChart = mSeries->chart();
}
}