2. CS 325: Software Engineering February 26, 2015 Object-State Modeling for Event-Driven Systems Event-Driven Systems UML State Machine Diagrams State Pattern

3. CS 325 February 26, 2015 Page 127 Event-Driven Systems Some software systems are dependent on particular events occurring in order to set up the necessary conditions for specific processing to progress. Consider a digital watch system… The watch has four modes: display, set-alarm, stopwatch, and set-time-and-date. The watch has three buttons: mode (for going to the four modes), start-stop (for advancing values in the different modes), and light (for selection, lighting, and stopping the buzzer). Essentially, the entire system can be viewed as a system of events that allow the system to transition between various states. Starting in display mode, pressing the mode button repeatedly takes the watch to set-alarm mode, to stopwatch mode, to set- time-and-date mode, and back to display mode, in that order. For each mode, the light button is used to step through the values that can be set (e.g., in set-time-and-date mode, set the hour, then the minute, then the second, then the month, then the day, and finally the day of the week).

4. CS 325 February 26, 2015 Page 128 UML State Machine Diagrams State Machine Diagrams show the states of an object and the transitions that can cause a change in state. This example models the login process for an on- line banking system.

5. CS 325 February 26, 2015 Page 129 UML State Machine Diagrams A state machine diagram for the digital watch:

6. CS 325 February 26, 2015 Page 130 State Pattern Conventional means of implementing state machines have serious drawbacks… Switch (mode) { Case Display: Case Display: Switch (button): Switch (button): { Case Mode: Case Mode: Case StartStop: Case StartStop: Case Light: Case Light: } Case SetAlarm: Case SetAlarm: Switch (button): Switch (button): { Case Mode: Case Mode: Case StartStop: Case StartStop: Case Light: Case Light: } Case Stopwatch: Case Stopwatch: Switch (button): Switch (button): { Case Mode: Case Mode: Case StartStop: Case StartStop: Case Light: Case Light: } Case SetTimeAndDate: Case SetTimeAndDate: Switch (button): Switch (button): { Case Mode: Case Mode: Case StartStop: Case StartStop: Case Light: Case Light: }} For example, the nested-switch approach is difficult to comprehend, to test, to maintain, and especially to modify.

7. CS 325 February 26, 2015 Page 131 State Pattern The State Pattern is a design pattern to address the implementation of a state machine.

8. CS 325 February 26, 2015 Page 132 State Pattern The State Pattern localizes state-specific behavior in an individual class for each state, and puts all of the behavior for that state in a single object, eliminating the necessity for a set of long, look-alike conditional statements scattered through the program’s code. By aggregating state-specific behavior into state classes with intention-revealing names, it’s easier to find that specific logic, and it’s all in one place, improving cohesion. Extending the behavior is just a matter of defining additional state classes and modifying the linkages between the states.