Adding and Removing of Items

  Each item must be added to a group or layer with the method "add-inter" (see 9.1.3.12) of the group or layer. The item can also be a whole group as it was described in the last subsubsection. The group will try to add the item with the size, which can be set using the method "set-pref-size" (see 9.1.3.15), and the position, which can be set using the method "set-pref-pos" (see 9.1.3.15). (see also next subsection.) The position arguments must be given relatively to the group, because you don't know which position the group will have. If an item doesn't fit into the group at the preferred position, the group will try to add it at another position. If this fails either, the group will try to find a place for the item in one of the group's replacement groups, which can be defined using the methods "add-replacement" (see 9.1.3.12) and "remove-replacement" (see 9.1.3.12). You must remove the item always from the origin group, also if it was replaced, using the method "remove-inter" (see 9.1.3.12). The replacement groups are totally transparent from the sight of the programmer.

Each item, which is added to a group, which is added to a group, ..., which is added to a layer, which is in the list of the manager, is called visible. An item is called visible even if it is totally obscured by a layer. The visibility state of an item can be determined using the method "is-visible". Each item has got two methods, which are called if the item is added to the manager or if it is removed from the manager. These methods can be changed using "set-add-action". These methods are also called if the group or a superior group of the item is removed from/added to the manager.