CODEWITHSUNDEEP
qtqt4qgraphicsview
speedwell
speedwell

Reputation: 655

QGraphicsScene::changed() always returns a single rect sized to the app window

In a Qt 4.7.1 Windows app, a slot that's connected to QGraphicsScene::changed() is fired as expected but the dirty region count is always 1 and the rect size I get is always the same as my app window. I tried calling QGraphicsView::setViewportUpdateMode(QGraphicsView::MinimalViewportUpdate); but that didn't help.

Is there a way to tell Qt to only give me the area(s) of the page that changed?

Upvotes: 0

Views: 2026

Answers (1)

Stephen Chu
Stephen Chu

Reputation: 12832

An update in a QGRaphicsView is different from the one in a QGraphicsScene. Update in the view is caused by the need to repaint the view. With or without changing the scene. This typical is from window (resize) and view changes (scroll). An change in the scene will also trigger an update to the view.

A change in a scene is the change of the content of the scene. Like adding or removing a item, scaling or translating of the transformation. This will emit the changed() signal. All views displaying that scene will also update themselves for the display.

For example. Scrolling a view around will not generate any scene update since nothing in the scene changed. The paint() function of items in the scene will be called to repaint. But no changed() signal will be emitted from the scene.

If you changed the scale of the scene for instance, the whole scene changed. In addition to the whole repaint, the scene will emit changed() signal and indicates the whole scene changed. But if you add a new item to the scene, changed() should indicate only the rect of the new item.

If you want to know what part of the scene need to be repainted, in addition to calling QGraphicsView::setViewportUpdateMode(), you need to install a event filter to the view and check for QEvent::Paint. Note that the region and rect in QPaintEvent is in local coordinate of the view, which can be different from the scene. But QGraphicsView has many mapping functions to do the conversion.

Upvotes: 2

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