Newbie-Question: update does not trigger paintEvent

[janitor]

Hello there,

I'm new to Qt and just working my way through the tutorials and docs. But now I am stuck on a seemingly simple problem. I hope that you can help me out.

So I was trying to hack one of Qt's tutorials a bit: http://doc.trolltech.com/4.3/tutorial-t10.html
I created a custom widget like this

#include "forceindicator.h"
 
CannonField::CannonField(QWidget *parent)
    : QWidget(parent)
{
/* stuff */
 
    indicator = new ForceIndicator;
    overlayLayout = new QGridLayout(this);
    overlayLayout->addWidget(indicator,0,1,Qt::AlignRight | Qt::AlignTop);
    this->setLayout(overlayLayout);
/* more stuff */
}

#include "forceindicator.h"

CannonField::CannonField(QWidget *parent)
    : QWidget(parent)
{
/* stuff */

    indicator = new ForceIndicator;
    overlayLayout = new QGridLayout(this);
    overlayLayout->addWidget(indicator,0,1,Qt::AlignRight | Qt::AlignTop);
    this->setLayout(overlayLayout);
/* more stuff */
}

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Now this child widget "indicator" has a slot "setForce" which is triggered by some events. Within this slot (see below) update() is called.
I was thinking that this in turn would trigger a paint event and call my paintEvent(QPaintEvent *) method. However this is not working. The slot is called alright and everything before and after my update() calls is executed (checked with some couts), but the paintEvent (again couts) is not called.

I tried repaint instead - same result. Triggering the paintEvent explicitely does call my custom function.

So, what stupid mistake am I missing out on?

Thanks for your help!

Here is the header of my custom widget

#ifndef FORCEINDICATOR_H
#define FORCEINDICATOR_H
 
#include <QWidget>
 
class ForceIndicator : public QWidget
{
    Q_OBJECT
 
public:
    ForceIndicator(QWidget *parent = 0);
 
    int force() const {return currentForce;}
 
public slots:
    void setForce(int force);
 
signals:
    void forceChanged(int newForce);
 
protected:
    void paintEvent(QPaintEvent *event);
 
private:
    int currentForce;
};
 
#endif

#ifndef FORCEINDICATOR_H
#define FORCEINDICATOR_H

#include <QWidget>

class ForceIndicator : public QWidget
{
    Q_OBJECT

public:
    ForceIndicator(QWidget *parent = 0);

    int force() const {return currentForce;}

public slots:
    void setForce(int force);

signals:
    void forceChanged(int newForce);

protected:
    void paintEvent(QPaintEvent *event);

private:
    int currentForce;
};

#endif

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And here are parts of the body
#

include <QPainter>
#include <iostream>
#include "forceindicator.h"
 
void ForceIndicator::setForce(int force)
{
  if (currentForce == force)
    return;  
  std::cout << "bla" << std::endl;
  currentForce = force;
 
  update();
  std::cout << "blabla" << std::endl;
  //emit forceChanged(currentForce);
}
 
 
void ForceIndicator::paintEvent(QPaintEvent* /*event*/)
{
  std::cout << "blub" << std::endl;
  QPainter painter(this);
 
/* stuff */
 
}

include <QPainter>
#include <iostream>
#include "forceindicator.h"

void ForceIndicator::setForce(int force)
{
  if (currentForce == force)
    return;  
  std::cout << "bla" << std::endl;
  currentForce = force;
  
  update();
  std::cout << "blabla" << std::endl;
  //emit forceChanged(currentForce);
}


void ForceIndicator::paintEvent(QPaintEvent* /*event*/)
{
  std::cout << "blub" << std::endl;
  QPainter painter(this);
  
/* stuff */

}

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[nish]

as said in tutorial... update() will not call the paintEvent() everytime.
but repaint() should call it.

i have not used cout<< in any of my qt programs. try this...

#include<QDebug>
#include<QTime>
 
void ForceIndicator::paintEvent(QPaintEvent* /*event*/)
{
  //std::cout << "blub" << std::endl;
  qWarning()<<"Current Time ="<<QTime::currentTime();
 
  QPainter painter(this);
 
  //paint the current time somewhere it is visible in your widget.
  painter.drawText(50,50,QTime::currentTime().toString());
 
/* stuff */
 
}

#include<QDebug>
#include<QTime>

void ForceIndicator::paintEvent(QPaintEvent* /*event*/)
{
  //std::cout << "blub" << std::endl;
  qWarning()<<"Current Time ="<<QTime::currentTime();
  
  QPainter painter(this);
  
  //paint the current time somewhere it is visible in your widget.
  painter.drawText(50,50,QTime::currentTime().toString());
  
/* stuff */
 
}

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[janitor]

as said in tutorial... update() will not call the paintEvent() everytime.
but repaint() should call it.

Well, as I wrote - I tried using repaint() and it doesn't change anything.
Also using qWarning() instead of cout doesn't help, the paintEvent simply isn't called.

Could it be something in the way I create the widget? It's a child of this CannonField widget (see original post)

indicator = new ForceIndicator(this);

indicator = new ForceIndicator(this);

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and then added to a layout (see below). I also omitted passing this as a parent, but that doesn't affect the behavior.
The constructor right now by the way is just:

ForceIndicator::ForceIndicator(QWidget* parent)
    : QWidget(parent)
{
}

ForceIndicator::ForceIndicator(QWidget* parent)
    : QWidget(parent)
{
}

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Thanks for your help!

[wysota]

Could you prepare a minimal compilable example reproducing the problem?

[janitor]

Thanks a lot for your replies!

While boiling it down to a minimal example I actually found my mistake. I never assigned my custom child widget a geometry or a minimum size or the like. So apparently it just had size zero and that's why the paintEvent was never even called.

[gab74]

Post deleted wrong section...sorry