Scene is not centered after changing size of rect

 

//mainwindow.h

#include <QMainWindow>
#include <QGraphicsScene>

namespace Ui {
    class MainWindow;
}

class MainWindow : public QMainWindow {
    Q_OBJECT
public:
    MainWindow(QWidget *parent = 0);
    ~MainWindow();

protected:
    void changeEvent(QEvent *e);
		QGraphicsScene *scene;

private:
    Ui::MainWindow *ui;

	private slots:
		void click123();
};


//mainwindow.cpp

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

		scene = new QGraphicsScene();
		scene->addRect(0, 0, 400, 50);

		ui->graphicsView->setScene(scene);

		connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(click123()));

}

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

void MainWindow::changeEvent(QEvent *e)
{
    QMainWindow::changeEvent(e);
    switch (e->type()) {
    case QEvent::LanguageChange:
        ui->retranslateUi(this);
        break;
    default:
        break;
    }
}

void MainWindow::click123() {

	scene->clear();

	scene->addRect(0, 0, 100, 50);
}

 

void MainWindow::click123() {

	scene->clear();

	scene->addRect(0, 0, 500, 50);
}