分层思想的进一步深入：绘制复杂的图形，其实只需要基本元素的上一层接口，就能再复杂也可以绘制

// .h 文件
#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

QT_BEGIN_NAMESPACE
namespace Ui { class Widget; }
QT_END_NAMESPACE

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();

    void initPaint(QPainter *p);

    void Draw_Element(QPainter *p, QPoint point, float size);
    void Draw_Four_Rect_Element(QPainter *p, QPoint point, float size);
    void Draw_Factory(QPainter *p, QPoint point, float size);
    void Draw(QPainter *p, QPoint point, float size);
private:
    Ui::Widget *ui;

    // QPaintDevice interface
protected:
    virtual void initPainter(QPainter *painter) const;
    void paintEvent(QPaintEvent*);

};
#endif // WIDGET_H

// .cpp 文件
#include "widget.h"
#include "ui_widget.h"

#include <QPainter>
#include <QPoint>
#include <QRect>


Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    this->resize(1000,600);
}

Widget::~Widget()
{
    delete ui;
}

void Widget::initPaint(QPainter *p)
{
    p->begin(this);
    p->setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform); //抗锯齿和使用平滑转换算法
}

void Widget::initPainter(QPainter *painter) const
{
    painter->setPen(Qt::black);
}

void Widget::Draw_Element(QPainter* p,QPoint point,float size)
{
    p->drawRect(QRect(point,QPoint(point.x() + size,point.y() + size)));
}

void Widget::Draw_Four_Rect_Element(QPainter* p,QPoint point,float size)
{
    // 左上角
    Draw_Element(p,point,size);

    // 右上角
    point.setX(point.x() + size * 2);
    Draw_Element(p,point,size);

    // 中间
    point.setX(point.x() - size);
    point.setY(point.y() + size);
    Draw_Element(p,point,size);

    // 左下角
    point.setX(point.x() - size);
    point.setY(point.y() + size);
    Draw_Element(p,point,size);

    // 右下角
    point.setX(point.x() + size * 2);
    Draw_Element(p,point,size);
}


void Widget::Draw_Factory(QPainter* p,QPoint point,float size)
{
    int origin_size = size;
    size = size / 3;
    // 左上角
    Draw_Four_Rect_Element(p,point,size);

    // 右上角
    point.setX(point.x() + origin_size * 2);
    Draw_Four_Rect_Element(p,point,size);

    // 中间
    point.setX(point.x() - origin_size);
    point.setY(point.y() + origin_size);
    Draw_Four_Rect_Element(p,point,size);

    // 左下角
    point.setX(point.x() - origin_size);
    point.setY(point.y() + origin_size);
    Draw_Four_Rect_Element(p,point,size);

    // 右下角
    point.setX(point.x() + origin_size * 2);
    Draw_Four_Rect_Element(p,point,size);
}

void Widget::Draw(QPainter* p,QPoint point,float size)
{
    int origin_size = size;
    size = size / 3;
    // 左上角
    Draw_Factory(p,point,size);

    // 右上角
    point.setX(point.x() + origin_size * 2);
    Draw_Factory(p,point,size);

    // 中间
    point.setX(point.x() - origin_size);
    point.setY(point.y() + origin_size);
    Draw_Factory(p,point,size);

    // 左下角
    point.setX(point.x() - origin_size);
    point.setY(point.y() + origin_size);
    Draw_Factory(p,point,size);

    // 右下角
    point.setX(point.x() + origin_size * 2);
    Draw_Factory(p,point,size);
}

void Widget::paintEvent(QPaintEvent *)
{
    /*
     * 10px rect 10px rect 10px rect 10px rect 10px
     * */

    QPainter p;
    int point_x = 0;
    int point_y = this->height() / 4;

    initPaint(&p);

    // 第一个基础图形
    point_x += 10;
    QPoint point(point_x,point_y);
    Draw_Element(&p,point,200);

    // 第二个图形
    point.setX(point.x() + 200 + 10);
    Draw_Four_Rect_Element(&p,point,200 / 3);

    point.setX(point.x() + 200 + 10);
    Draw_Factory(&p,point,200 / 3);

    point.setX(point.x() + 200 + 10);
    Draw(&p,point,200 / 3);


    p.end();
}