Qt | About the repaint event handler paintEvent()

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Foreword:

The paintEvent() function provided by QWidget is a pure virtual function, which must be reimplemented when subclasses inheriting from it want to repaint.

Situations where a repaint event is raised:

1. When the window control is displayed for the first time, the system automatically generates a drawing event.
2. When the repaint() and update() functions are called.
3. When the window control is obscured by other widgets and then displayed again, a repaint event is generated for the hidden area.
4. When resizing the window.

The paintEvent() function is a highly optimized function that has been automatically turned on and implemented a double buffering mechanism, so repainting in Qt will not cause any flickering on the screen.

repaint() function:

• repaint() is the fastest to cause a repaint operation. In an emergency , you can call repaint() when you need to repaint immediately .
• But repaint() cannot be called in the paintEvent() function, or cause an infinite loop.

update() function:

• After the update() is called , it will not be redrawn immediately , but the redraw event will be put into the main loop and dispatched uniformly by the Event Loop of the main() main function.
• update() is optimized before calling paintEvent(), if update() is called many times, finally these update() will be merged into a big repaint event and added to the message queue, and finally only this big update () is executed once .

Compared with update(), repaint() is generally enough to call update(). When update() cannot meet the requirements, try to use repaint().

Implement drawing operations in paintEvent():

Commonly used tools for drawing include the brush class QPen, the brush class QBrush, and the font class QFont, etc., which all inherit from the QPainter class.

• QPainter can draw various basic graphics.
• The QPen class is used to draw the edges of geometric figures, consisting of parameters such as color, width, and line style.
• The QBrush class is a palette for filling geometry, consisting of a color and a fill style.
• The QFont class is used for text drawing and consists of font properties.

example:

void Widget::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);
    painter.drawLine(10, 100, 30, 300);//画线
    painter.setPen(Qt::red);
    painter.drawRect(10, 10, 100, 100);//红色矩形框
    painter.setPen(QPen(Qt::green, 5));
    painter.setBrush(Qt::blue);
    painter.drawEllipse(100, 10, 200, 200);//绿边蓝色填充椭圆
}
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For other specific usage, please refer to the help manual.