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Source code

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

this->myView->setFixedSize(400, 300); 

this->setCentralWidget(this->myView); 

this->myScene = new QGraphicsScene; 

this->myScene->setSceneRect(this->myView->rect()); 

this->myView->setScene(this->myScene); 
//why I have to call these function to diable scrollbar,otherwise they will show no matter the view and scene size 
this->myView->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff); 
this->myView->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff); 

} 

MainWindow::~MainWindow() 
{ 
delete ui; 
} 
void MainWindow::on_actionAdd_triggered() 
{ 
QGraphicsTextItem *pitem = new QGraphicsTextItem("new Text"); 

pitem->setFlags(QGraphicsItem::ItemIsMovable|QGraphicsItem::ItemIsSelectable); 

this->myScene->addItem(pitem); 
} 

void MainWindow::on_actionClear_triggered() 
{ 
this->myScene->clear(); 
} 

void MainWindow::on_actionZoomin_triggered() 
{ 
this->myView->adjustSize(); 
} 

void MainWindow::resizeEvent(QResizeEvent *event) 
{ 
//if I call this fitInView, there will be a margin in view. why 
this->myView->fitInView(0,0,400,300,Qt::KeepAspectRatio); 

}