Displaying a text in a QGraphicsScene

Sayanel

This is a newbie I am trying to create a scene with Qt to display what I want (lets say for now just a text), but with the code describing the content being in another class.

I have a mainwindows, created with QtCreator gui, which contains the widget "graphicView"
In my mainwindows.cpp, I can do :

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QtWidgets>
#include <QGraphicsScene>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QGraphicsScene *scene = new QGraphicsScene();
    scene->addText("Hi");
    ui->graphicsView->setScene(scene);
}

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

And it will display the text ('Hi!'). Now what I want to have is a class MyClass in charge of what is inside the widget graphicsView, and do in mainwindows.cpp:

MyClass foo;
ui->graphicsView->setScene(foo.scene);

with MyClass.h having

QGraphicsScene *scene;

and MyClass.cpp, in the constructor of the class :

scene = new QGraphicsScene(this);
scene->addtext("Hi again!");

But when running it it doesn't work, it doesn't display anything.
If I add this at the end of the mainwindows.cpp

std::cout << foo.scene->items().size() << std::endl;

I can see that my scene do contains one element, but for some reason it isn't displayed. Any hint why ?
Thanks

P.S.: Where shall I post newbie question ?

SGaist

Hi and welcome to devnet,
@Sayanel said in Displaying a text in a QGraphicsScene:

MyClass foo;
ui->graphicsView->setScene(foo.scene);

What it is the lifetime of foo ?

If it's declared in the same method where you make the ui call, then it will be destroyed before anything else happens.

Sayanel

If think that is is, foo is deleted at the end of the mainwindow constructor. Thank you sir ! I just have to do in the mainwindow.cpp:

    MyClass *foo = new MyClass;
    ui->graphicsView->setScene(foo->scene);

SGaist

Except that now you are leaking memory as this object pointer gets lost.

You should make it a member of your MainWindow class and handle it's destruction properly. Or use e.g QScopedPointer to store it.

Sayanel

What if I just declare foo as a child of mainwindow

MyClass *foo = new MyClass(this);

it will be destructed when I close my mainwindow ?

SGaist

If MyClass is a QObject based class. But if you need that kind of stuff for a pretty simple task the way you want to do it. you might want to reconsider its design.

Sayanel

Well MyClass inherits from QWidget
The idea I had was to have MyClass handle everything inside my 'graphicsView' -in order to not have it pollute the main windows-. This way I can interact with the scene using MyClass method and I can have it have slots.
The idea is to crate a zone (foo.scene, displayed on the widget graphicsView) where I will be able to put all the object I want at the position I want.

I found this example (https://doc.qt.io/qt-5/qtwidgets-graphicsview-chip-example.html) which I think does what I want (via its class 'View')