问不擦除组件的Qt绘图

EN

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    Ui::MainWindow *ui;

protected:
    void paintEvent(QPaintEvent *e);
    void setBackGroundColorToBlack();
};

#endif // MAINWINDOW_H

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

#include <QtGui>
    MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    setBackGroundColorToBlack();
}

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


void MainWindow::paintEvent(QPaintEvent *event) {
    setUpdatesEnabled(false);
    QPainter painterObj;
    painterObj.begin(this);
    painterObj.setPen(QPen(Qt::green, 2, Qt::SolidLine, Qt::RoundCap));
    painterObj.drawEllipse(0, 0, 318, 390);//456
    painterObj.drawEllipse(53, 65, 212, 260);//304
    painterObj.drawEllipse(106, 130, 106, 130);//152

    painterObj.end();
}



void MainWindow::setBackGroundColorToBlack() {
    QPalette pal = palette();

    // set black background
    pal.setColor(QPalette::Background, Qt::black);
    this->setAutoFillBackground(true); // This enables the qt to fill the         background before the paint event.
    this->setPalette(pal);
    //update();
}