setMask do not affects on DeclarativeView based window

#include <QtGui/QApplication>
#include "mainwindow.h"

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

	return a.exec();
}

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QDeclarativeView>

class MainWindow : public QDeclarativeView
{
public:
	explicit MainWindow(QWidget *parent = 0);
};

#endif // MAINWINDOW_H

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

MainWindow::MainWindow(QWidget *parent) :
	QDeclarativeView(parent)
{
	this->setMask(QRegion(0,0, this->size().width(), this->size().height(), QRegion::Ellipse ));
}