画图出现问题如下：

QWidget::paintEngine: Should no longer be called
QPainter::begin: Paint device returned engine == 0, type: 1

QPainter::setPen: Painter not active
QPainter::setPen: Painter not active
QPainter::end: Painter not active, aborted

QWidget::paintEngine: Should no longer be called
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::setPen: Painter not active
QPainter::setPen: Painter not active
QPainter::end: Painter not active, aborted
0
QPainter::setFont: Painter not active
QPainter::setPen: Painter not active
QPainter::setPen: Painter not active
QPainter::end: Painter not active, aborted
0

解决方法：在Mainwindow的类中重载paintEvent()函数，在函数小括号后添加override，之后即便报错，也能够画出我们要求的图像，不仅可以，也可以更新图像。

源代码mainwindow.h如下：

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

#include<QPainter>

#include<QWidget>

#include<QFont>

#include<QPen>

#include<QGraphicsView>

#include<QPaintEvent>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

    //void paintEvent(QPaintEvent *event);

    QPaintEvent *e;//如果去掉星号会变成error

    QPainter painter2;

private:
    Ui::MainWindow *ui;

protected:

    void paintEvent(QPaintEvent *event)override;

};
#endif // MAINWINDOW_H

mainwidow.cpp

#include "mainwindow.h"
#include "./ui_mainwindow.h"

#include<iostream>

using namespace std;

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)

{
    //QPainter p(this); //QWidget::paintEngine: Should no longer be called


    ui->setupUi(this);

    paintEvent(this->e);


}

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

//void MainWindow::painter(QPainter *painter1){
void MainWindow::paintEvent(QPaintEvent *event){

     QPainter painter2(this);

     cout<<painter2.isActive()<<endl;


    int BASESIZE = 8;
    int SIZE = 4;
    int gate = 1;
    int enemyNum = 23;

    //painter2.begin(ui->centralwidget); //QWidget::paintEngine: Should no longer be called


    //painter2.beginNativePainting();

    painter2.begin(this);

    QPoint x(560,55);


    painter2.setCompositionMode(QPainter::CompositionMode_SourceOver);
    painter2.fillRect(100 - 20, 100 - 20,
                                  80, 80, QColor(255,0,0,255));

    //设置字体属性
       QFont font;//字体属性类对象，可以设置字体的属性
       font.setPixelSize(18);//设置字体大小
       font.setUnderline(true);//设置下划线
       font.setOverline(true);//设置上划线
       font.setItalic(true);//设置斜体
       font.setBold(true);//设置加粗
       font.setCapitalization(QFont::SmallCaps);//设置字母大小写
       font.setLetterSpacing(QFont::AbsoluteSpacing,10);//设置字符间距
       painter2.setFont(font);//使用字体属性
       painter2.setPen(Qt::red);//设置画笔颜色
       painter2.rotate(10);
       painter2.drawText(6*SIZE/4,BASESIZE,"第"+QString::number(gate)+"关,现存敌人"+QString::number(enemyNum));


       QColor color(0,0,0,255);
       QPen pen;
       pen.setWidth(4);
       pen.setColor(color);
       painter2.setPen(pen);
       painter2.drawLine(QPoint(560-400,55),QPoint(620-400,55));
       painter2.drawLine(QPoint(590-400,45),QPoint(620-400,55));
       painter2.drawLine(QPoint(590-400,65),QPoint(620-400,55));


      painter2.end();


      // this->update(); //此处没有意义

}

在主程序中调用画图函数，main.cpp不做改动：

#include "mainwindow.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

效果图如下，里面有三个图像：

如有问题，请您留言。