CODEWITHSUNDEEP
c++qtqt4
user10904879
user10904879

Reputation: 1

Unable to call PaintEvent in Qt (C++)

I want to make a Qt application draw an Ellipse (a circle). I’m using a two file setup - main.cpp and ui_mainwindow.h. I have defined a QGraphicsView and a QPainter on the Ui MainWindow class:

class Ui MainWindow {
public:
    ...
    QGraphicsView *radarGraphicsView;
    QPainter *radarPainter;
    ...

I have as well initialized the QGraphicsView:

void setupUi(QMainWindow *MainWindow) {
    ...
    radarGraphicsView = new QGraphicsView(centralWidget);
    ...

I have defined a paintEvent in the same class as well:

void paintEvent(QPaintEvent *event) {
    radarPainter = new QPainter(radarGraphicsView);
    radarPainter->setPen(Qt::green);
    radarPainter->drawEllipse(10, 10, 10, 10); }

Anyways, in result I cannot see any ellipse being drawn in the QGraphicsView, and I assume I’m supposed to call the paintEvent, but I’ve found no such example in a setup like mine (Qmake). How am I supposed to call the paintEvent for the Ellipse to be drawn?

Upvotes: 0

Views: 2452

Answers (1)

arsdever
arsdever

Reputation: 1262

void paintEvent(QPaintEvent*) override; is a virtual member function of QWidget class and it's used to draw on widgets derived from QWidget. In case you want to draw an ellipse into QWidget you have to override that like following.

class MyClass : public DerivedFromQWidget
{
  ...

  protected: // or any other
    void paintEvent(QPaintEvent* event){
      QPainter painter(this);
      painter.setPen(Qt::black);
      painter.setBrush(Qt::red);
      painter.drawEllipse(ellipse);

      ...

      DerivedFromQWidget::paintEvent(event);
    }

  ...

};

The paintEvent function will be called continuous while it's implementer is shown.

I believe the following class can help you to solve the problem

// ellipse_drawer.h

class EllipseDrawer : public QWidget
{
public:
  EllipseDrawer(QWidget* parent = nullptr);

protected:
  void paintEvent(QPaintEvent*);
};

// ellipse_drawer.cpp

EllipseDrawer::EllipseDrawer(QWidget* parent)
  : QWidget(parent)
{
  setFixedSize(200, 200);
}

void EllipseDrawer::paintEvent(QPaintEvent*)
{
  QPainter painter(this);
  painter.setPen(Qt::black);
  painter.setBrush(Qt::red);
  painter.drawEllipse(rect());
}

// my_main_window.h

class MyMainWindow : QMainWindow
{
  MyMainWindow(QWidget* parent = nullptr);
};

// my_main_window.cpp

MyMainWindow::MyMainWindow(QWidgat* parent)
  : QMainWindow(parent)
{
  setCentralWidget(new EllipseDrawer());
}

// in main.cpp

int main(int argc, char** argv)
{
  QApplication app(argc, argv);
  MyMainWindow widget;
  widget.show();
  return app.exec();
}

Upvotes: 2

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