Draw an element in a static location relative to QGraphicsView

Question

I would like to draw a notification that always appears in the upper right corner of my QGraphicsView . However, the positions of QGraphicsItems indicated in the coordinates of the scene, so if the user draws / zooms in to view another part of the scene, this notification will go off the screen.

I realized that I could simulate this behavior by moving and scaling the notification at any time when the current view changes. But it seems terribly inefficient and not at all elegant.

It seems that QGraphicsView should support this behavior. The docs mention the ItemIsPanel flag, which sounds encouraging, but doesn't mention anything about the static layout in the view. ItemIgnoresTransformations will also help with scaling / scaling, but not panning.

Is there a built-in Qt function that supports this behavior?

Answer 1

The naive decision that the notification is part of the original scene is bad - it violates the separation of the model’s presentation. You can have several views, all of which are displayed on one scene, but usually only on one of them can appear a notification at will.

Another easy way to do this is to overlay a QWidget notification on top of your view. The problem is that on some architectures, overlaying regular QWidgets on top of accelerated QGLWidgets will make the former disappear. Please note that the QGraphicsView widget may be a QGLWidget!

So the only portable solution is to explicitly paint on top of everything else in the QGraphicsSceneView viewport() .

Below is a complete example.

 // main.cpp #include <QApplication> #include <QGraphicsScene> #include <QGraphicsView> #include <QGraphicsItem> qreal rnd() { return qrand() / (float)RAND_MAX; } class OverlaidGraphicsView : public QGraphicsView { Q_OBJECT QGraphicsScene * m_overlayScene; public: explicit OverlaidGraphicsView(QWidget* parent = 0) : QGraphicsView(parent), m_overlayScene(NULL) {} explicit OverlaidGraphicsView(QGraphicsScene * scene = 0, QWidget * parent = 0) : QGraphicsView(scene, parent), m_overlayScene(NULL) {} void setOverlayScene(QGraphicsScene * scene) { if (scene == m_overlayScene) return; m_overlayScene = scene; connect(scene, SIGNAL(changed(QList<QRectF>)), SLOT(overlayChanged())); update(); } QGraphicsScene * overlayScene() const { return m_overlayScene; } void paintEvent(QPaintEvent *ev) { QGraphicsView::paintEvent(ev); if (m_overlayScene) paintOverlay(); } virtual void paintOverlay() { QPainter p(viewport()); p.setRenderHints(renderHints()); m_overlayScene->render(&p, viewport()->rect()); } Q_SLOT void overlayChanged() { update(); } }; class Window : public QWidget { QGraphicsScene scene, notification; OverlaidGraphicsView * view; QGraphicsSimpleTextItem * item; int timerId; int time; public: Window() : view(new OverlaidGraphicsView(&scene, this)), timerId(-1), time(0) { for (int i = 0; i < 20; ++ i) { qreal w = rnd()*0.3, h = rnd()*0.3; scene.addEllipse(rnd()*(1-w), rnd()*(1-h), w, h, QPen(Qt::red), QBrush(Qt::lightGray)); } view->fitInView(0, 0, 1, 1); view->setResizeAnchor(QGraphicsView::AnchorViewCenter); view->setRenderHint(QPainter::Antialiasing); view->setOverlayScene(&notification); item = new QGraphicsSimpleTextItem(); item->setPen(QPen(Qt::blue)); item->setBrush(Qt::NoBrush); item->setPos(95, 0); notification.addItem(item); notification.addRect(0, 0, 100, 0, Qt::NoPen, Qt::NoBrush); // strut timerId = startTimer(1000); QTimerEvent ev(timerId); timerEvent(&ev); } void resizeEvent(QResizeEvent * ev) { view->resize(size()); view->fitInView(0, 0, 1, 1, Qt::KeepAspectRatio); QWidget::resizeEvent(ev); } void timerEvent(QTimerEvent * ev) { if (ev->timerId() != timerId) return; item->setText(QString::number(time++)); } }; int main(int argc, char ** argv) { QApplication a(argc, argv); Window window; window.show(); a.exec(); } #include "main.moc"