Adapt an object to different client’s needs through transparently attached role objects, each one representing a role the object has to play in that client’s context. The object manages its role set dynamically. By representing roles as individual objects, different contexts are kept separate and system configuration is simplified.

See 
*** http://st-www.cs.uiuc.edu/users/hanmer/PLoP-97/Proceedings/riehle.pdf
*** http://www.inf.fu-berlin.de/lehre/WS01/SWT/Rollenmuster.pdf (german)

See RoleObject

CategoryPattern CategoryStructuralPatterns