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Events (Triggers)

What is an Event?

• Events are incidents or a stimulus by which a state machine can be triggered. Incidents are abstracted as events.

• In state machine events cause transitions (external or internal)

• An event usually has 2 components

  • Signal

    • e.g., TIME_CHANGE

      This event signifies by its signal attribute that the user has pressed a button that changes time.

  • One or more associated values parameters (optional)

    • e.g., dir Direction_t

      ​x
      1
      enum
      2
      {
      3
          UP,
      4
          DOWN
      5
      } Direction_t
      

      This signal has an associated parameter that encodes whether the user has pressed a button that increases time (UP) or decreases time (DOWN).

Example

• Incidents in the operation of a microwave oven:

  1. Opening door (event) $\to$$\to$ Heater off, lights on

  2. Closing door (event) $\to$$\to$ lights off

  3. Set timer (event) $\to$$\to$ manage time

  4. Start (event) $\to$$\to$ heater ON