showFullScreen

Docs for showFullScreen state:
"Shows the widget in full-screen mode.
Calling this function only affects windows."

Check whether your QGraphicsView' isWindow() method returns true. If not, I believe you need to specify the flag Qt::Window in the constructor.
Or you make a QDialog or QMainWindow (which are a window by default), and insert the QGraphicsView as a full-size child window.

The QGraphicsView never maximizes.

fitInView(QApplication::desktop()->availableGeometry(-1), Qt::IgnoreAspectRatio);

because your image should already be fully zoomed in from the constructor.

Second, try to change the line in showEvent to the following

fitInView(myGraphicsPixmapItem, Qt::IgnoreAspectRatio);

Finally, what do you mean by "it doesn't work": Is the whole window not maximized? Or is the image not large enough within the window?

fitInView(this->scene->sceneRect(), Qt::IgnoreAspectRatio);

The 'this' pointer is the QGraphicsView. I have tryed this lines, but it doesn't work:

void imageView::mouseDoubleClickEvent(QMouseEvent *event) {
    this->setWindowState(Qt::WindowFullScreen);
    this->showFullScreen();
    fitInView(QApplication::desktop()->availableGeometry(-1), Qt::IgnoreAspectRatio);
}

imageView is the QGraphicsView.

Do you know what could be the problem?? Thank you again!!

The image is a QGraphicPixmapItem in a QGraphicsScene in a QGraphicsView.

I tryed this line in the mouseDoubleClickEvent():

void imageView::mouseDoubleClickEvent(QMouseEvent *event) {
    fitInView(QApplication::desktop()->availableGeometry(-1), Qt::IgnoreAspectRatio);
}

but it doesn't works.

Thank you very much

If you show the image within a QGraphicsItem (e.g. a QGraphicsPixmap item), you can use QGraphicsView::fitInView to show this image in full view. Before that, you can remember the position and zoom level by saving QGraphicsView::transform().
If you

This is that I want:

When I doubleclick the image, I want to show it fullscreen and, doubleclicking again, I want to restore the default interface. Do you understand me? I know my english is not very good.