displaying a C++ QChart in a QML ChartView delegate

devDawg

Hello all,

I am looking to visually display several real-time QCharts that are updated with simulation data. I should have no problem with this portion; what I am confused about is the C++/QML interface.

Disclaimer: I am using Qt 5.9.3 for a specific device.

I have read through this LineChart example: https://doc.qt.io/qt-5.11/qtcharts-linechart-example.html

And also the ChartView documentation: https://doc.qt.io/qt-5.11/qml-qtcharts-chartview.html

My current approach is to use the following class as a context property:

#include <QtCharts>
#include <QLineSeries>
#include <QProcess>
#include <QQmlComponent>
#include <QList>
#include <QStringList>
#include <QString>

/*This class will be responsible for one specific module. Depending on the page, this model will contain a select number of params.
 * For example, on the summary page, we only want a brief overview of Priority 1 measurements, so the data models will be less compact.
 * Whereas on the following pages that detail full modules, the models will be larger.
 */


class DataModel: public QObject
{

    friend class QAbstractListModel;

    Q_OBJECT


public:
    explicit DataModel(QObject * parent = nullptr);
    explicit DataModel(const QString file); //this is our main constructor here.
    explicit DataModel(DataModel&&); //move constructor
    explicit DataModel(const DataModel&); //copy constructor

    DataModel& operator=(const DataModel&);
    DataModel& operator=(DataModel&&);

    Q_INVOKABLE QString getTextBodyAt(int i);
    Q_INVOKABLE DataModule *getModuleAt(int i);
    Q_INVOKABLE int getModulesLength() { return modules.length(); }
    Q_INVOKABLE int getGraphsLength() { return graphs.length(); }
    Q_INVOKABLE DataPiece *getDataAt(int index) const;
    Q_INVOKABLE QLineSeries* getSeriesAt(int index) const {return series.at(index);}
    Q_INVOKABLE int getDataCount();

    int timeToSeconds(QTime* time) const;
    void addModule(DataModule* mod_);
    void addDataGraph(DataPiece* piece);
    void replaceDataGraph(DataPiece* piece, int index); //replaces the graph at the given index with a new graph for the supplied DataPiece.
    void removeDataGraphAt(int i); //removes the graph at the given index.
    const QVector<DataModule*> getModel() const;  
    const QVector<DataPiece*> getGraphs() {return graphs;}
    const QVector<QLineSeries*> getSeries() {return series;}
    const QVector<QTime*> getTimes() {return timers;}

    ~DataModel();
signals:
    void pieceChanged(DataPiece*); //this signal might not be used anywhere.
    void pieceAdded(DataPiece*);
    void pieceReplaced(DataPiece*);

public slots:
    void onClose();
    void on_pieceAdded(DataPiece *piece);
    void on_pieceReplaced(DataPiece *piece);

private:
    QVector<DataModule*> modules; //separates Modules into Rectangles. Spacing modifiable.
    QVector<DataPiece*> graphs; //for every DataPiece, we will also have a QLineSeries and a QTimer.
    QVector<QLineSeries*> series;
    QVector<QTime*> timers;


};

Allowing me to import QLineSeries into my ListView of ChartViews here:

//this will display our charts.
                    ListView {
                        objectName: "chartListView"
                        id: chartListView
                        x: 0
                        y: 130
                        width: 350
                        height: 350

                        anchors.top: controlRect.bottom
                        orientation: Qt.Vertical

                        /*
                            Procedure:
                            -load a DataPiece in to GraphModel. (name, units, value, maximum, minimum)
                            -Set up ChartView & LineSeries according to that DataPiece.
                         */

                        model: dataModel.getGraphsLength()

                        delegate: ChartView {

                            objectName: "chartView"
                            width: 350
                            height: 350
                            title: graphModel.getDataAt(index).getModuleName() + " " + graphModel.getDataAt(index).getName() + " vs. time"
                            property LineSeries imported_data
                            //data:

                            //need to import our data from dataModel. This is on the to-do list.
                        }
                    }

-Is this a good approach? I am unsure if, in my context property class, if I should have a QVector of QCharts, or of QLineSeries.

-In addition, I noticed in QML that ChartView has a property called "data", to which the ChartView documentation makes zero mention. Is this something I could use to import data to my delegate? If so, what type does "data" want? (LineSeries?)

-Given that my axes will remain static for a given ChartView, I am thinking that these should be set up in my QML delegate. Is my thinking correct on this?

Note: I should add that it is intended for the user to be able to add or remove graphs from the ListView of ChartViews.

I apologize for the load of questions, I am just sort of in the dark as to how I should do this the proper way. The Qt documentation is lacking on this topic.

Thanks in advance.

devDawg

Still actively working to make this thing work. I would appreciate any comments on what I am trying to do, and if there is an easier or better way to do it.

Thanks

devDawg

Okay, so I have completely set up my data + signal handlers for new data in C++.

My current problem is providing the correct format to the delegate of my ListView that will hold my dynamic set of ChartViews.

Here is the .qml code:

ListView {
                        objectName: "chartListView"
                        id: chartListView
                        x: 0
                        y: 130
                        width: 350
                        height: 350

                        anchors.top: controlRect.bottom
                        orientation: Qt.Vertical

                        /*
                            Procedure:
                            -load a DataPiece in to GraphModel. (name, units, value, maximum, minimum)
                            -Set up ChartView & LineSeries according to that DataPiece.
                         */

                        model: dataModel.getGraphsLength()
                        delegate: Component {
                                ChartView { data: dataModel.getChartAt(index) }

                        } //data handling is done in C++.

                    }

dataModel is a C++ class that I have registered as a context property.
The error message I am getting from QML is : Error: Unknown method return type: QChart*. It points to where I am trying to set my ListView delegate.

datamodel.h:

class DataModel: public QObject
{

    friend class QAbstractListModel;

    Q_OBJECT


public:
    explicit DataModel(QObject * parent = nullptr);
    explicit DataModel(const QString file); //this is our main constructor here.
    explicit DataModel(DataModel&&); //move constructor
    explicit DataModel(const DataModel&); //copy constructor

    DataModel& operator=(const DataModel&);
    DataModel& operator=(DataModel&&);

    Q_INVOKABLE QString getTextBodyAt(int i);
    Q_INVOKABLE DataModule *getModuleAt(int i);
    Q_INVOKABLE int getModulesLength() { return modules.length(); }
    Q_INVOKABLE int getGraphsLength() { return graphs.length(); }
    Q_INVOKABLE DataPiece *getDataAt(int index) const;
    Q_INVOKABLE QChart* getChartAt(int index) const {return charts.at(index)->chart();}
    Q_INVOKABLE int getDataCount();

    //depending on the needed accuracy, we will have to look at which data type is needed. Probably won't end up being an int in all likelihood.
    Q_INVOKABLE int getCurrentTimeAt(int index);

    long timeToSeconds(QTime* time) const;
    void addModule(DataModule* mod_);
    void addDataGraph(DataPiece* piece);
    void replaceDataGraph(DataPiece* piece, int index); //replaces the graph at the given index with a new graph for the supplied DataPiece.
    void removeDataGraphAt(int i); //removes the graph at the given index.
    const QVector<DataModule*> getModel() const;  
    const QVector<DataPiece*> getGraphs() {return graphs;}
    const QVector<QTime*> getTimes() {return timers;}

    ~DataModel();
signals:
    void pieceChanged(DataPiece*); //this signal might not be used anywhere.
    void pieceAdded(DataPiece*);
    void pieceReplaced(DataPiece*);

public slots:
    void onClose();
    void on_pieceAdded(DataPiece *piece);
    void on_pieceReplaced(DataPiece *piece);
    void on_processError(QProcess::ProcessError error);
    void on_newValue(QString val);

private:
    QVector<DataModule*> modules; //separates Modules into Rectangles. Spacing modifiable.
    QVector<DataPiece*> graphs; //for every DataPiece, we will also have a QLineSeries and a QTimer.
    QVector<QChartView*> charts;
    QVector<QTime*> timers;


};

So, should I be setting the delegate to be of type ChartView or Component? If I were to set it as a Component, how would I go about assigning its visual content to be my C++ ChartView at the index?

SGaist

Hi,

First thing: QObject based class are not copiable.

QChart is a widget, so if you want to use it from QtQuick, you should take a look at KDAB's Declarative Widgets module

devDawg

@SGaist Sounds good. After wrestling with the QtCharts API since early this morning, I am kicking it to the curb and going with QCustomPlot. Seems to be a much cleaner, more efficient, and easier to use. I can't imagine I will have the same problems with QCustomPlot as I was with QtCharts.

Nonetheless, I appreciate the response!

SGaist

AFAIK, lots of people are satisfied with QCustomPlot.

Depending on what you want/need, Qwt might also be of interest.