Qt Beginner QPainter and QRect

Question 1

The paint event is the method that is called by the paint system when a widget needs to be redrawn. That is why simply naming your own method does not work. It is never called by the paint system.

You really should be using the QPaintEvent. It gives you the rect that needs to be drawn. This rect will be based upon the size of the widget, so instead of using an explicit rect in your paint event, set your widget to the right size. A paint event will be generated should your widget ever move, resize, etc.

void MyWidget::paintEvent(QPaintEvent *event)
{
    QRect rect = event->rect();
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);
    painter.setPen(Qt::black);
    painter.drawText(rect, Qt::AlignCenter,
                      "Data");
    painter.drawRect(rect);
}

Now if you want to separate your paint logic into another method, that is fine. But you would need to have it called from the paint event:

void MyWidget::paintEvent(QPaintEvent *event)
{
    QRect rect = event->rect();
    draw(rect);
}

void MyWidget::draw(QRect &rect)
{
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);
    painter.setPen(Qt::black);
    painter.drawText(rect, Qt::AlignCenter,
                      "Data");
    painter.drawRect(rect);
}

If you want to completely bypass the paint events as you said, and only want to create a static rectangle to display, one way is to just draw it once to a pixmap and display it in a QLabel:

QPixMap pix(200,100);
QPainter painter(&pix);
// do paint operations
painter.end()
someLabel.setPixmap(pix)

Question 2

Any data that your paintEvent() needs should be accessible as fields of the containing class, in your case, private fields of MyWidget. These private fields can be exposed to clients of MyWidget via "setters" which would set the data values before calling update() on MyWidget which will trigger a call to paintEvent().

Question 3

This playlist contains the best Qt tutorials , starting tutorial 74 would be useful for you (Qpainter and QPen), tutorial 75 is how to draw rectangles using QRect.

Question 4

As well @Mat told you: the "event" is the correct way to launch a painter.
QPainter can only be evoked after a QPaintEvent event, which carries the safe region where the object may be drawn.

So you must find another strategy to transport your data, to help I will propose a method simple, which can be adjusted to many cases.

widget.cpp

#include <QtGui>
#include "widget.h"

#define MIN_DCX    (0.1)
#define MAX_DCX    (5.0)

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{    
    dcx=MIN_DCX;
    setFixedSize(170, 100);
}

void Widget::paintEvent(QPaintEvent *event)
{
    Q_UNUSED(event); 
    QPainter painter;
    painter.begin(this);
    painter.setRenderHint(QPainter::Antialiasing);
    painter.setPen(Qt::black);
    pcx=dcx*2;
    QRect rect = QRect(50-dcx,25-dcx,60+pcx,40+pcx);
    painter.drawText(rect, Qt::AlignCenter,printData);
    painter.drawRect(rect);
    painter.end();

}

void Widget::setPrintData(QString value){
   printData = value;
   dcx=(dcx>MAX_DCX)?MIN_DCX:dcx+MIN_DCX;
}

widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent);
    void setPrintData(QString value);

protected:
    void paintEvent(QPaintEvent *event);

private:
    QString printData;
    float dcx;
    float pcx;
};


#endif

window.cpp

#include <QtGui>
#include "widget.h"
#include "window.h"

#define MAX_SDCX  20

Window::Window()
    : QWidget()
{
    gobject = new Widget(this);

    textMode=1;
    rectMode=1;
    gobject->setPrintData(msgs[textMode]);

    QGridLayout *layout = new QGridLayout;
    layout->addWidget(gobject, 0, 0);
    setLayout(layout);

    QTimer *timer = new QTimer(this);
    connect(timer, SIGNAL(timeout()), this, SLOT(dataOnAir()));
    timer->start(10);

    setWindowTitle(tr("Rect Shaking"));
}



void Window::dataOnAir(){
    if((++rectMode)>MAX_SDCX){
        rectMode=0;
        textMode^=1;
    }
    gobject->setPrintData(msgs[textMode]);
    gobject->repaint();
}

window.h

#ifndef WINDOW_H
#define WINDOW_H

#include <QWidget>
#include "widget.h"

class Window : public QWidget
{
    Q_OBJECT

public:
    Window();

private slots:
    void dataOnAir();

private:
    Widget *gobject;
    const QString msgs[2] = {"Hello","World"};
    int textMode;
    int rectMode;
};

#endif

main.cpp

#include <QApplication>
#include "window.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    Window window;
    window.show();
    return app.exec();
}

As you can see in the code is executed a timer, outside the object "widget"

every 10ms sends a repaint the widget to redraw a "rect" with a different size and every 20 cycles (200ms) changes the text "hello" for "world"

In this example you can see that in any way need overwrite the QPainterDevice architecture.

You may also notice that the "event" within the "paintEvent" is silenced and not used directly, but it is essential to execute a sequence QPainter.

Question 5

Overriding the paintEvent() function of a widget enables you to customize the widget and this function is called periodically to redraw the widget. Therefore any drawing should be made in this function. However overriding paintEvent() may cause some performance issues. I would prefer using a QGraphicsScene and QGraphicsView then I would add a rectangle to the scene which is the common way of doing this kind of drawing stuff. Please check the GraphicsView Framework

http://qt-project.org/doc/qt-4.8/graphicsview.html