Prevent QPixmap from being moved out of QGraphicsView

[Macok]

I'd like to write very simple game.
Here's a part of code:

QGraphicsScene *scene=new QGraphicsScene;
QGraphicsPixmapItem *pixmap=scene->addPixmap(QPixmap("img.png"));
ui->graphicsView->setScene(scene);

QGraphicsScene *scene=new QGraphicsScene;
QGraphicsPixmapItem *pixmap=scene->addPixmap(QPixmap("img.png"));
ui->graphicsView->setScene(scene);

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pixmap is a creature that can be moved by player using WSAD keys.
I did it by this way:

void MainWindow::keyPressEvent(QKeyEvent *event){
    switch(event->key()){
        case Qt::Key_D:
            pixmap->setPos(pixmap->pos().x()+1, pixmap->pos().y());
        break;
        case Qt::Key_A:
            pixmap->setPos(pixmap->pos().x()-1, pixmap->pos().y());
        break;
        (...)
    }
}

void MainWindow::keyPressEvent(QKeyEvent *event){
    switch(event->key()){
        case Qt::Key_D:
            pixmap->setPos(pixmap->pos().x()+1, pixmap->pos().y());
        break;
        case Qt::Key_A:
            pixmap->setPos(pixmap->pos().x()-1, pixmap->pos().y());
        break;
        (...)
    }
}

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Now i want to prevent pixmap from being moved out of QGraphicsView.
How can I do it?
Thanks in advance.

[radek.z]

try to use

bool QGraphicsItem::collidesWithItem ( const QGraphicsItem * other, Qt::ItemSelectionMode mode = Qt::IntersectsItemShape ) const [virtual]

[Macok]

Thanks, but what should I pass as arguments of this function?

[koral]

Hello, here are some thoughts:
if you're making a sprite-based game, you should either:
- have a QGraphicsScene that has the size of the game and use QGraphicsView scrollbars/scrolling to pan/zoom the scene
- use a fixed Screen-size QGraphicsScene and do do your own code to 'scroll' the items (or the 'insert at right edga, scroll all items left, and remove at left edge type of thing)

I assume you're using 1, so the QGraphicsScene has the size of the game area and your 'player' sprite is in there.
In this case:

A. when moving the player, just don't move it out of the scene
QRect sceneRect = scene()->sceneRect().toRect();
player->setPos(qBound(sceneRect.left(), player->pos(), sceneRect.right() - player_width));
this keeps the X coordinate between scene rect boundaries (assuming the item has its origin in the top-left corner)

B. the QGraphicsView can scroll to see the player
just call player->ensureVisible(); and the view will scroll

Good luck ;-)

[jpn]

Take a look at QGraphicsItem::itemChange(). Notice the example.

[Macok]

Thanks for the replies.

@jpn
Based on that example I wrote something like this:

void MainWindow::keyPressEvent(QKeyEvent *event){
    switch(event->key()){
        case Qt::Key_D:
            pixmap->setPos(pixmap->pos().x()+5, pixmap->pos().y());
        break;
        case Qt::Key_A:
            pixmap->setPos(pixmap->pos().x()-5, pixmap->pos().y());
        break;
        case Qt::Key_W:
            pixmap->setPos(pixmap->pos().x(), pixmap->pos().y()-5);
        break;
        case Qt::Key_S:
            pixmap->setPos(pixmap->pos().x(), pixmap->pos().y()+5);
        break;
    }
 
    if(!scene->sceneRect().contains(pixmap->pos())){
        QMessageBox box;
        box.setText("Player is out of the screen!");
        box.exec();
    }
}

void MainWindow::keyPressEvent(QKeyEvent *event){
    switch(event->key()){
        case Qt::Key_D:
            pixmap->setPos(pixmap->pos().x()+5, pixmap->pos().y());
        break;
        case Qt::Key_A:
            pixmap->setPos(pixmap->pos().x()-5, pixmap->pos().y());
        break;
        case Qt::Key_W:
            pixmap->setPos(pixmap->pos().x(), pixmap->pos().y()-5);
        break;
        case Qt::Key_S:
            pixmap->setPos(pixmap->pos().x(), pixmap->pos().y()+5);
        break;
    }

    if(!scene->sceneRect().contains(pixmap->pos())){
        QMessageBox box;
        box.setText("Player is out of the screen!");
        box.exec();
    }
}

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But it doesn't work.
I think the problem is that everytime I move pixmap, QGraphicsScene expands, so scene->sceneRect().contains(pixmap->pos()) will always return true.

[radek.z]

bool QGraphicsItem::collidesWithItem ( const QGraphicsItem * other, Qt::ItemSelectionMode mode = Qt::IntersectsItemShape ) const [virtual]

as a QGraphicsItem you can pass QGraphicsRectItem(scene->sceneRect())
It means that you create graphics item with sboundary same as of graphicsscene has and then you check if your pixmap icollides with this RectItem

[jpn]

Yes, the scene will expand automatically by default, but shouldn't you define the game board area if that's the desired behavior? Something like:

void MyGraphicsView::resizeEvent(QResizeEvent* event)
{
    QGraphicsview::resizeEvent(event);
    if (scene())
        scene()->setSceneRect(QRectF(QPointF(0,0), event->size()));
}

void MyGraphicsView::resizeEvent(QResizeEvent* event)
{
    QGraphicsview::resizeEvent(event);
    if (scene())
        scene()->setSceneRect(QRectF(QPointF(0,0), event->size()));
}

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[Macok]

When I reimplemented QGraphicsView::resizeEvent program crashes imidiately after being started.
In QtCreator's console I have:

The program has unexpectedly finished.

[jpn]

See the backtrace from the debugger.