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')