Events and Signals

Introduction to Events and Signals in UML

Introduction to the Events and Signals

The different things that happen are called events
Many events occur at the same time and at unexpected times
Events represent important occurrences that have a time (the time when the event happened) and space (where the event happened)
Events are used to model stimulus that can make a state machine to change its state
Events can be of two types -synchronous or asynchronous
In a synchronous event the sender waits for an action from the receiver on the event. For example, pressing a button on the mouse, a phone call, etc.
There is no waiting in an asynchronous event. For example, sending an e-mail, letter, delivery of newspaper, etc.

Events

The kinds of events are

Signals

Signals refer to the objects that are dispatched asynchronously by one object and then received by another
Exceptions and e-mails are an example of internal signals
They may have instances, generalization relationships, attributes and operations
Attributes of a signal serve as its parameters
It may be sent as the action of a state transition in a state machine or the sending of a message in an interaction
Signals are modeled as stereotyped classes and the relationship between an operation and the events by using a dependency relationship, stereotyped as send

Call Events

A call is a synchronous event representing the dispatch of an operation Example: Method calls in programming
It may trigger a state transition in a state machine or it may invoke a method on the target object

Time and Change Events

Time Event represents the passage of time
Modeled by using the keyword 'after' followed by some expression that evaluates to a period of time which can be simple or complex
Change event represents a change in state or the satisfaction of some condition
Modeled by using the keyword 'when' followed by some boolean expression

Sending and Receiving Events

For Synchronous Events:

Sending or receiving is similar to call event
The sender and the receiver are in a meeting (the sender sends the signal and wait for a response from the receiver) for the duration of the operation
When an object calls an operation, the sender sends the operation and then waits for the receiver

For Asynchronous Events:

Sending or receiving is similar to signal event
The sender and receiver do not meet, i.e., the sender sends the signal but does not wait for a response from the receiver
When an object sends a signal, the sender sends the signal and then continues along its flow of control, without waiting for any return from the receiver

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