void QGraphicsScene::changed ( const QList<QRectF> & region ) [signal]
This signal is emitted by QGraphicsScene when control reaches the event loop, if the scene content changes. The region parameter contains a list of scene rectangles that indicate the area that has been changed.
если я правильно понял, то можно так во view:
connect(_scene, SIGNAL(changed(QList<QRectF>)),this, SLOT(sceneChanged(QList<QRectF>)));
а в слоте сделать save метод в источник