QGraphicsItemGroup Class Reference
[QtGui module]

Detailed Description

The QGraphicsItemGroup class provides treating a group of items as one.

A QGraphicsItemGroup is a special type of compound item that treats itself and all its children as one item (i.e., all events and geometries for all children are merged together). It's common to use item groups in presentation tools, when the user wants to group several smaller items into one big item in order to simplify moving and copying of items.

If all you want is to store items inside other items, you can use any QGraphicsItem directly by passing a suitable parent to setParentItem().

The boundingRect() function of QGraphicsItemGroup returns the bounding rectangle of all items in the item group. In addition, item groups have handlesChildEvents() enabled by default, so all events sent to a member of the group go to the item group (i.e., selecting one item in a group will select them all). QGraphicsItemGroup ignores the ItemIgnoresTransformations flag on its children (i.e., with respect to the geometry of the group item, the children are treated as if they were transformable).

There are two ways to construct an item group. The easiest and most common approach is to pass a list of items (e.g., all selected items) to QGraphicsScene::createItemGroup(), which returns a new QGraphicsItemGroup item. The other approach is to manually construct a QGraphicsItemGroup item, add it to the scene calling QGraphicsScene::addItem(), and then add items to the group manually, one at a time by calling addToGroup(). To dismantle ("ungroup") an item group, you can either call QGraphicsScene::destroyItemGroup(), or you can manually remove all items from the group by calling removeFromGroup().

 // Group all selected items together
 QGraphicsItemGroup *group = scene->createItemGroup(scene->selecteditems());

 // Destroy the group, and delete the group item
 scene->destroyItemGroup(group);

The operation of adding and removing items preserves the items' scene-relative position and transformation, as opposed to calling setParentItem(), where only the child item's parent-relative position and transformation are kept.

The addtoGroup() function reparents the target item to this item group, keeping the item's position and transformation intact relative to the scene. Visually, this means that items added via addToGroup() will remain completely unchanged as a result of this operation, regardless of the item or the group's current position or transformation; although the item's position and matrix are likely to change.

The removeFromGroup() function has similar semantics to setParentItem(); it reparents the item to the parent item of the item group. As with addToGroup(), the item's scene-relative position and transformation remain intact.

See also QGraphicsItem and The Graphics View Framework.

Member Function Documentation

QGraphicsItemGroup::QGraphicsItemGroup ( QGraphicsItem * parent = 0 )

Constructs a QGraphicsItemGroup. parent is passed to QGraphicsItem's constructor.

See also QGraphicsScene::addItem().

QGraphicsItemGroup::~QGraphicsItemGroup ()

Destroys the QGraphicsItemGroup.

void QGraphicsItemGroup::addToGroup ( QGraphicsItem * item )

Adds the given item to this item group. The item will be reparented to this group, but its position and transformation relative to the scene will stay intact.

See also removeFromGroup() and QGraphicsScene::createItemGroup().

void QGraphicsItemGroup::removeFromGroup ( QGraphicsItem * item )

Removes the specified item from this group. The item will be reparented to this group's parent item, or to 0 if this group has no parent. Its position and transformation relative to the scene will stay intact.

See also addToGroup() and QGraphicsScene::destroyItemGroup().