Item selection

The scene supports the ability of selecting items, similar to how you select files in a file manager. To be selectable, an item must have the QGraphicsItem::ItemIsSelectable flag turned on. Try to add parent->setFlag(QGraphicsItem::ItemIsSelectable, true) to the createComplexItem() function we created earlier. Now, if you run the application and click on a rectangle, it is selected, which is indicated by dashed lines:

You can use the Ctrl button to select multiple items at once. Alternatively, you can call view.setDragMode(QGraphicsView::RubberBandDrag) to activate the rubber band selection for the view.

Another useful drag mode of the Graphics View is ScrollHandDrag. It allows you to scroll the view by dragging the scene with the left mouse button, without the need to use scrollbars.

Besides that, there are different ways to select items programmatically. There is the item's QGraphicsItem::setSelected() function, which takes a bool value to toggle the selection state on or off, or you can call QGraphicsScene::setSelectionArea() on the scene, which takes a QPainterPath parameter as an argument, in which case all items within the area are selected.

With the scene's QGraphicsScene::selectedItems() function, you can query the actual selected items. The function returns a QList holding QGraphicsItem pointers to the selected items. For example, calling QList::count() on that list will give you the number of selected items. To clear the selection, call QGraphicsScene::clearSelection(). To query the selection state of an item, use QGraphicsItem::isSelected(), which returns true if the item is selected and false otherwise.

Another interesting flag of GraphicsItem is ItemIsMovable. It enables you to drag the item within the scene by holding it with the left mouse button, effectively changing the pos() of the item. Try to add parent->setFlag(QGraphicsItem::ItemIsMovable, true) to our createComplexItem function and drag around the rectangles.