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.
The kinds of events are
- Signals
- Calls
- The passing of time
- A change in state
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
