Drawing a rectangle (or anything) on QGraphicsView - qt

Question

I'm using QtCreator to build an interface application.
I'm just getting used to Qt and toying around trying to draw stuff on a QtGraphicsView.
Since I created my interface with the editor, I am retrieving my objects in the code like so (please tell me if this is wrong).
this->m_graphView = this->findChild<QGraphicsView *>("graphicsView");
this->m_graphScene = this->m_graphView->scene();
I have buttons on the interface and already created slots to react to the clicked event.
I'm just trying to draw something (anything) on the graphics view that is on my MainWindow (geometry : [(10,10), 320x240]).
I've been reading examples online and I can't make anything work.
My current code is as follows :
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDebug>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
this->m_graphView = this->findChild<QGraphicsView *>("graphicsView");
this->m_graphScene = this->m_graphView->scene();
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::on_btnDown_clicked()
{
qDebug() << "STUB : button DOWN";
m_graphScene->addLine(0, 0, 42, 42, QPen(QBrush(Qt::black),1));
m_graphView->show();
}
void MainWindow::on_btnLeft_clicked()
{
qDebug() << "STUB : button LEFT";
}
void MainWindow::on_btnUp_clicked()
{
qDebug() << "STUB : button UP";
}
void MainWindow::on_btnRight_clicked()
{
qDebug() << "STUB : button RIGHT";
}
void MainWindow::on_btnShoot_clicked()
{
qDebug() << "STUB : button SHOOT";
}
But annoyingly, it doesn't draw anything and I even get this error when the addLine method is called
QGraphicsScene::addItem: item has already been added to this scene
What's wrong in my code and/or my ways of doing things?
I just want to draw something but can't make it, thank you.

Answer 1

retrieving widget in form
you can get graphicsView pointer (and its scene) more easier.
"ui" member has the pointer to widgets arranged in .form file.
(If you please, see "ui_mainwindow.h" file)
// assign to pointer
QGraphicsView *view = ui->graphicsView;
view->...
// or directly (I like this)
ui->graphicsView->...
so, Mainwindow class don't need "m_graphicsView" member.
graphics view
QGraphicsView need to set scene.(It has no scene at first)
We have to create QGraphicsScene ourselves.
So Mainwindow class need "m_graphicsScene" member.
m_graphicsScene = new QGraphicsScene(this);
ui->graphicsView->setScene(m_graphicsScene);
drawing more easier
If you just want to draw something, you can override "paintEvent" method.
PaintEvent is QWidget's virtual method.
in .h file:
protected:
void paintEvent(QPaintEvent *event);
in .cpp file:
void MainWindow::paintEvent(QPaintEvent *event)
{
// unuse
Q_UNUSED(event);
// pass "this" pointer to painter
QPainter painter(this);
// setPen
// QPen can take "Qt::black" directly as first arg (without QBrush() constructor)
painter.setPen(QPen(Qt::black), 1);
// draw line
painter.drawLine(0, 0, 42, 42);
}
please enjoy Qt!