Simple Qt question(s) (QObject parenting model, QLinearGradient, plotArea( ) )

[Dean95]

Hello, I'm beginner at Qt and currently learning from Mastering Qt5 book and I got stuck. There is a piece of code and I don't understand some things.

#include "MemoryWidget.h"
#include "SysInfo.h"        // Getting system informations
#include <QtCharts/QAreaSeries>
#include <QPen>
#include <QLinearGradient>

using namespace QtCharts;

const int CHART_X_RANGE_COUNT = 50;
const int CHART_X_RANGE_MAX = CHART_X_RANGE_COUNT - 1;
const int COLOR_DARK_BLUE = 0x209fdf;
const int COLOR_LIGHT_BLUE = 0xbfdfef;
const int PEN_WIDTH = 3;

MemoryWidget::MemoryWidget(QWidget *parent) :
    SysInfoWidget(parent),
    mSeries(new QLineSeries(this)),
    mPointPositionX(0)
{
    QPen pen(COLOR_DARK_BLUE);
    pen.setWidth(PEN_WIDTH);

    QLinearGradient gradient(QPointF(0, 0), QPointF(0, 1));
    gradient.setColorAt(1.0, COLOR_DARK_BLUE);
    gradient.setColorAt(0.0, COLOR_LIGHT_BLUE);
    gradient.setCoordinateMode(QGradient::ObjectBoundingMode);

    QAreaSeries *areaSeries = new QAreaSeries(mSeries);
    areaSeries->setPen(pen);
    areaSeries->setBrush(gradient);

    QChart *chart = chartView().chart();
    chart->addSeries(areaSeries);
    chart->setTitle("Memory used");
    chart->createDefaultAxes();
    chart->axisX()->setRange(0, CHART_X_RANGE_MAX);
    chart->axisX()->setVisible(false);
    chart->axisY()->setRange(0, 100);
}

void MemoryWidget::updateSeries()
{
    double memoryUsed = SysInfo::instance().memoryUsed();
    mSeries->append(mPointPositionX++, memoryUsed);
    if (mSeries->count() > CHART_X_RANGE_COUNT)
    {
        QChart *chart = chartView().chart();
        chart->scroll(chart->plotArea().width() / CHART_X_RANGE_MAX, 0);
        mSeries->remove(0);
    }
}

I don't understand the whole QLinearGradient piece of code.
What are the QPointF values in

QLinearGradient gradient(QPointF(0, 0), QPointF(0, 1));

and in the following code, who will be the parent:

QAreaSeries *areaSeries = new QAreaSeries(mSeries);

And finally, in MemoryWidget::updateSeries() method, why is

chart->scroll(chart->plotArea().width() / CHART_X_RANGE_MAX, 0);

the solution if i want scroll the x axis by one point.
Anyway, the book says:
"The syntax chart->scroll(chart->plotArea().width() / CHART_X_RANGE_MAX, 0) will then scroll to the latest point on the X axis and nothing on Y. The chart->scroll(dx, dy) expects coordinates expressed in our series coordinates. That is the reason why we have to retrieve the char->plotArea() divided by CHART_X_RANGE_MAX , the X axis unit."

Thank you, and sorry for my bad english

[jsulm]

@Dean95 said in Simple Qt question(s) (QObject parenting model, QLinearGradient, plotArea( ) ):

QLinearGradient gradient(QPointF(0, 0), QPointF(0, 1));

Take a look at documentation: http://doc.qt.io/qt-5/qlineargradient.html#QLinearGradient-1 and http://doc.qt.io/qt-5/qlineargradient.html

QAreaSeries *areaSeries = new QAreaSeries(mSeries);

mSeries will be the parent as it is passed as parent.

[Dean95]

@jsulm Thanks, but i still do not understand the following piece of code

chart->scroll(chart->plotArea().width() / CHART_X_RANGE_MAX, 0);

[mrjj]

@Dean95

• chart->scroll(chart->plotArea().width() / CHART_X_RANGE_MAX, 0);

Which part of it
It calls
void QChart::scroll(qreal dx, qreal dy)
that makes it scroll

so it calculates the scroll amount

say plotArea().width() is 1000
it will scroll 1000/49
( from
const int CHART_X_RANGE_COUNT = 50;
const int CHART_X_RANGE_MAX = CHART_X_RANGE_COUNT - 1; )