Question

I am trying to implement a widget in Qt that has 2 child widgets of its own: one is a render area where I draw some points and connect lines between them and the other one is a ListBox where I would like to insert the list of all the points I drew with their coordinates from the render area. The 2 widgets where added with Qt Designer. Here is my code until now:

renderarea.h:

class RenderArea : public QWidget
{
    Q_OBJECT

public:
    RenderArea(QWidget *parent = 0);
    QPoint point;
    QList<QPoint> list;

protected:
    void mousePressEvent(QMouseEvent *);
    void paintEvent(QPaintEvent *event);
    void updateList(QPoint p);
};

renderarea.cpp:

RenderArea::RenderArea(QWidget *parent)
    : QWidget(parent)
{
    setBackgroundRole(QPalette::Base);
    setAutoFillBackground(true);
}

void RenderArea::mousePressEvent(QMouseEvent *e)
{
    point = e->pos();
    updateList(point);
    this->update();
}

void RenderArea::updateList(QPoint p)
{
    list.append(p);
}
void RenderArea::paintEvent(QPaintEvent * /* event */)
{
    QPainter painter(this);
    painter.setPen(QPen(Qt::black,2));
    for (int i = 0; i < list.size(); ++i)
        painter.drawPoint(list[i]);
    if (list.size()>1)
        for(int j = 0; j < list.size()-1; ++j)
                painter.drawLine(list[j], list[j+1]);
}

paintwidget.h:

class PaintWidget : public QWidget
{
    Q_OBJECT

public:
    explicit PaintWidget(QWidget *parent = 0);
    ~PaintWidget();

private:
    Ui::PaintWidget *ui;
};

paintwidget.cpp:

PaintWidget::PaintWidget(QWidget *parent) :
        QWidget(parent),
        ui(new Ui::PaintWidget)
{
    ui->setupUi(this);
}

PaintWidget::~PaintWidget()
{
    delete ui;
}

My question is how to transmit from the render area widget to my ListBox that I drew another point and it should be displayed along with its coordinates in the list?

Answer 1

OrcunC gave you a good advice.
If your are new to signal/slots implementation here some hints you can start from.

renderarea.h

signal:
    void pointAdded(QPoint*);

renderarea.cpp

void RenderArea::updateList(QPoint p)
{
    list.append(p);
    emit pointAdded(&list.back());
}

listbox.h

public slots:
    void onPointAdded(QPoint*);

listbox.cpp

void ListBox::onPointAdded(QPoint* point)
{
    //lets assume your ListBox is a QListWidget
    addItem( QString::number(point->x()) + "," + QString::number(point->y()))
}

somewhere instance of ListBox and RenderArea are accessible

QObject::connect( renderArea, SIGNAL(pointAdded(QPoint*),
                  listBox,    SLOT(onPointAdded(QPoint*)));

NOTE: nameing is very important for readability and maintenance the void RenderArea::updateList(QPoint p) in this case it’s more void RenderArea::addPoint( const QPoint& p) (also notice the const reference telling the compiler that we are not changing p event if we have it’s reference)