QGraphicsScene::clear() and destructor performance

Question

I am using a QGraphicsScene to display a very large number of items, often more than 100,000. When I call delete the scene, it takes an unusually long time to complete, 10 seconds or more.

A bit of experimenting revealed that the QGraphicsScene::clear function is the slow part. If I clear first and then delete, the clear is slow but the delete is fast.

Why is the QGraphicsScene destructor/clear function so slow, and is there any way I can speed it up?

score 5 · Answer 1 · answered May 13 '15 at 14:20

I discovered that the problem relates to the fact that my QGraphicsScene was being displayed in a QGraphicsView. If I first called view->setScene(0) to remove my scene from the view, then the scene would delete much more quickly.

I figured it out by using callgrind to see which functions were being called during QGraphicsScene::clear, and I saw that QGraphicsView functions were taking up the time. I'm not sure, but I suspect QGraphicsView is updating something about itself for each deleted object.

Most likely your scene uses the spatial index, and the index is being continuously updated. You could also temporarily disable the index with the same effect, I think. — Kuba hasn't forgotten Monica, May 14 '15 at 15:55

QGraphicsScene::clear() and destructor performance

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