Question

In Qt Creator, I have a couple of widgets declared like so:

Header File:

    class MapViewer : public QGraphicsView
{
    Q_OBJECT

public:
    explicit MapViewer(QGraphicsScene *scene, QWidget *parent = 0);
    ~MapViewer();

public slots:
    void mousePressEvent(QMouseEvent *event);

};

// Declaration for the map editor window.
class MapEditor : public QMainWindow
{
    Q_OBJECT

public:
    explicit MapEditor(QWidget *parent = 0);
    ~MapEditor();

public:
    QLayout *editorLayout;
    QPushButton *btn;
    QGraphicsScene *mapScene;
    MapViewer *mapView;

private:
    Ui::MapEditor *ui;
};

CPP File:

  MapEditor::MapEditor(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MapEditor)
{

    ui->setupUi(this);
    this->setWindowTitle("2DXY :: Map Editor");
    this->setGeometry(10,10,1170,750);
    editorLayout = new QVBoxLayout; // Create a new layout
    this->setLayout(editorLayout); // Set the widget's layout to our newly created layout.

    mapScene = new QGraphicsScene(); // Create a new graphics scene to draw upon.

    mapView = new MapViewer(mapScene,this); // Create a new graphics view to display our scene - set its parent to 'this' so that it doesn't open in a new window.
    mapView->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    mapView->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    mapView->setGeometry(20,20,1178,546); // Width first, then height.

AND:

void MapViewer::mousePressEvent(QMouseEvent *event)
{
    // Show an empty message box, just to check that the event handler works!
    QMessageBox *notification = new QMessageBox();
    notification->show();
    notification->exec();
    // Some how access the same QGraphicsScene and View (mapScene, mapView) as above, so
    // I can update their contents on the open form / window.

}

And as you can see, I wish to access the Graphics Scene again to update it, then redraw it (or whatever). But I’m not able to access the graphics scene at all, despite a few hours of trial and error with declaring widgets in different ways.

I know that the listener itself works, because if it’s set to open a new message box, or output to the debug window, then it works, it’s just that I can’t access the widgets I’ve already defined.

I feel that there is a (relatively) easy solution to this problem, and that I’m just missing something obvious.

Answer 1

void MapViewer::mousePressEvent(QMouseEvent *event)
{
    // Show an empty message box, just to check that the event handler works!
    QMessageBox *notification = new QMessageBox();
    notification->show();
    notification->exec();

    //  To add something whenever the user clicks, you don't need the view,
    //  just the scene.
    scene()->addItem( new MyItem() );
}

Remember MapViewer derives from QGraphicsView and the view must know about the scene it belongs to – so it has a scene() method to return it, which you inherited.