In UML models, signals are model elements that are independent of the classifiers that handle them. Signals specify one-way, asynchronous communications between active objects.
Signals are often used in event-driven systems and distributed computing environments. For example, a communications system might contain a Pager class, whose objects wait for, and respond to, Page signals. Signals differ from other message types in that when an object receives a signal, the object does not need to return anything, but reacts to the receipt of a signal according to the behavior specified by its receptions.
All signals are assumed to have a send( ) operation. A signal’s attributes represent the data it carries in its send operation.
You can add signals to the class diagrams in your model to represent the following functions:
A signal has a name describing its purpose in the system. The UML notation for a signal is a rectangle with two compartments.