Qt call to update(QRegion&) not triggering call to paintEvent(..)

Question

I was playing around with some code from the Qt tutorial and I am stuck because one of my QWidget inherited classes calls update(QRegion&) but it does not trigger a call to paintEvent(..) - essentially I am trying to do a simple refactoring of the tutorial source code to add multiple shots using a CannonShot class in the 12th chapter of the Qt tutorial (see the link: http://doc.trolltech.com/4.3/tutorial-t12.html)

The CannonField update() still works correctly and causes its paintEvent re-implementation to be called. Its only CannonShot's update() that does not call its own paintEvent. (I've confirmed that it does get before and after the update call). Here are some snippets:

class CannonShot : public QWidget
{
   Q_OBJECT
public:
   CannonShot(QWidget *parent = 0);
   int angle() const { return shotAngle; }
   int force() const { return shotForce; }
   void shoot();
private slots:
   void moveShot();
   void plotShot();

signals:
   void hit();
   void missed();
   void angleChanged(int);
   void forceChanged(int);
...
...
protected:
   paintEvent(QPaintEvent*);

private:
   int shotTimerCount;
   QTimer* autoShotTimer;
   float shotAngle;
   float shotForce;
};

Here's a snippet of the constructor of CannonShot and where its update() is called:

CannonShot::CannonShot(QWidget *parent): QWidget(parent)
{
   shotTimerCount = 0;
   autoShotTimer = new QTimer(this);
   connect(autoShotTimer, SIGNAL(timeout()), this, SLOT(moveShot()));
}

void CannonShot::shoot()
{
   if (autoShotTimer->isActive())
   return;
   shotTimerCount = 0;
   shotAngle = CannonField::angle();
   shotForce = CannonField::force();
   autoShotTimer->start(5);
}

void CannonShot::moveShot()
{
   QRegion region = shotRect();
   ++shotTimerCount;

   QRect shotR = shotRect();
   if (shotR.x() > width() || shotR.y() > height()) {
      autoShotTimer->stop();
      emit missed();
   }
   else {
      region = region.unite(shotR);
   }
   update(region); //-Checked that code reaches before and after this call!
}

void CannonShot::paintEvent(QPaintEvent*)
{
   //-Never gets here!!
   QPainter painter(this);
   QColor color;
   //-Do whatever
}

Here is also a brief snippet of CannonField that contains the shots and calls shoot() to fire them.

class CannonField : public QWidget
{
   Q_OBJECT

public:
   CannonField(QWidget *parent = 0);
   ~CannonField();
   static int angle() { return currentAngle; }
   static int force() { return currentForce; }

public slots:
   void setAngle(int angle);
   void setForce(int force);
   void shoot();

signals:
   void hit();
   void missed();
...
...

protected:
   void paintEvent(QPaintEvent *event);

public:
   static int currentAngle;
   static int currentForce;

private:
   QVector<CannonShot*> shots;
};

And here is a snippet of CannonField's source where the shots are fired.

void CannonField::shoot()
{
   //-Added this argument since I though after searching that CannonShot 
   // needs to have a QWidget with some dimensions..
   shots.push_back(new CannonShot(this));
   (shots.back())->shoot();
}

Finally the driver looks as:

class MyWidget : public QWidget {
public:
   MyWidget(QWidget *parent = 0);
};

MyWidget::MyWidget(QWidget *parent): QWidget(parent)
{
   QPushButton *quit = new QPushButton(tr("&Quit"));
   quit->setFont(QFont("Times", 18, QFont::Bold));
   connect(quit, SIGNAL(clicked()), qApp, SLOT(quit()));
   LCDRange *angle = new LCDRange(tr("&ANGLE"));
   angle->setRange(0, 90);
   LCDRange *force = new LCDRange(tr("&FORCE"));
   force->setRange(10, 50);
   CannonField *cannonField = new CannonField(this);
   connect(angle, SIGNAL(valueChanged(int)),
          cannonField, SLOT(setAngle(int)));
   connect(cannonField, SIGNAL(angleChanged(int)),
          angle, SLOT(setValue(int)));
   connect(force, SIGNAL(valueChanged(int)),
          cannonField, SLOT(setForce(int)));
   connect(cannonField, SIGNAL(forceChanged(int)),
          force, SLOT(setValue(int)));
   QPushButton *shoot = new QPushButton(tr("&Shoot"));
   shoot->setFont(QFont("Times", 18, QFont::Bold));
   connect(shoot, SIGNAL(clicked()), cannonField, SLOT(shoot()));      
   ...
   ...
   //-Other layout stuff
}

int main(int argc, char *argv[])
{
   QApplication app(argc, argv);
   MyWidget widget;
   widget.setMinimumSize(300, 300);
   widget.setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Fixed);
   widget.setGeometry(100, 100, 500, 355);
   widget.show();
   return app.exec();
}

Barbaris Barbaris · Accepted Answer · 2011-04-18T19:20:52

If QRegion::isEmpty() is true, it won't call QWidget::paintEvent(). I suspect it may be happening in your case.

Qt call to update(QRegion&) not triggering call to paintEvent(..)

3 Answers