CODEWITHSUNDEEP
qtdrawingqt-creator
dominicbri7
dominicbri7

Reputation: 2599

Drawing a rectangle (or anything) on QGraphicsView

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.

Upvotes: 2

Views: 15776

Answers (1)

Taurano
Taurano

Reputation: 41

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!

Upvotes: 4

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