QPixmap on QLabel doesn't show correctly - c++

Question

I am trying to draw some shape with QPainter class and save it to disk. As far as I know the easiest way is to use QPainter to draw into a QPixmap, visualize in the pixmap though a QLabel, and use QPixmap::save.
But when I run this test I see only a little black box inside the QWidget.
MyWidget::MyWidget()
{
std::cout << "MyWidget > ." << std::endl;
l = new QLabel();
l->setParent(this);
pixmap = new QPixmap(460, 480);
painter = new QPainter(pixmap);
}
MyWidget::~MyWidget()
{
delete pixmap;
delete painter;
}
void MyWidget::paintEvent(QPaintEvent *event)
{
std::cout << "dudee" << std::endl;
painter->begin(pixmap);
painter->drawLine(1,1,100,100);
QPen myPen(Qt::black, 2, Qt::SolidLine);
painter->setPen(myPen);
painter->drawLine(100,100,100,1);
painter->setRenderHint(QPainter::Antialiasing, true);
painter->setPen(QPen(Qt::black, 3, Qt::DashDotLine, Qt::RoundCap));
painter->setBrush(QBrush(Qt::green, Qt::SolidPattern));
painter->drawEllipse(200, 80, 400, 240);
painter->end();
l->setPixmap(*pixmap);
}
I have tried to add some l->update() but it doesn't change anything..
EDIT:
It should be an animation. I get the animation work through a QTimer that call every n msec the function for draw (not the paintEvent as the answer suggest)

Answer 1

You need instance of QPainter only during painting something. You don't need to keep it as class member.
Pixmap may be declared as class member, not as pointer.
Paint should be done once. It is bad idea to draw you external pixmap inside paintEvent, because you don't know, when exactly paintEvent will be called (and how much times).
You must not set pixmap for a label inside paint event, because call of l->setPixmap forces your widget to update => you will get infinite loop of draw->set->update->draw...
Solution:
Create somewhere a pixmap and paint on it necessary content.
Set content to a label, when it necessary (for example, after drawing).
Do not call update() - it will be called automatically, when you will set pixmap to label.
EDITED code:
Simple class for edited question:
AnimationSample.h
#ifndef ANIMATIONSAMPLE_H
#define ANIMATIONSAMPLE_H
#include <QWidget>
#include <QPixmap>
#include <QLabel>
#include <QPointer>
#include <QTimer>
class AnimationSample
: public QWidget
{
Q_OBJECT
public:
AnimationSample( QWidget *parent = NULL );
~AnimationSample();
private slots:
void onTick();
private:
QPointer< QLabel > m_label;
QPointer< QTimer > m_timer;
int m_salt;
};
#endif // ANIMATIONSAMPLE_H
AnimationSample.cpp
#include "AnimationSample.h"
#include <QPixmap>
#include <QPainter>
AnimationSample::AnimationSample( QWidget *parent )
: QWidget( parent )
, m_salt( 1 )
{
m_label = new QLabel( this );
m_label->setFixedSize( 100, 100 );
m_timer = new QTimer( this );
connect( m_timer, SIGNAL( timeout() ), SLOT( onTick() ) );
m_timer->start( 250 );
}
AnimationSample::~AnimationSample()
{
}
void AnimationSample::onTick()
{
QPixmap pic( 100, 100 );
QPainter p( &pic );
QPen myPen( Qt::black, 2, Qt::SolidLine );
p.setPen( myPen );
p.drawLine( 0, 0, m_salt, m_salt );
m_salt = (m_salt + 1) % 100;
m_label->setPixmap( pic );
}