Signals

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. Signals can have no other operations.

You can add signals to the class diagrams in your model to represent the following functions:

Trigger events in objects in models that represent event-driven systems
Exceptions thrown by an operation when something unexpected occurs in a software system

A signal has a name describing its purpose in the system. As the following figure illustrates, the UML notation for a signal is a rectangle with two compartments.

A signal named Signal1 with an attribute named attribute1.

The upper compartment contains the «signal» keyword and the signal’s name.
The lower compartment contains the signal’s attributes.

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