3-1 State Design Pattern C Sc 335 Rick Mercer

State Design Pattern State is one of the Behavioral patterns It is similar to Strategy Allows an object to alter its behavior when its internal state changes The object will appear to change its class

General Form from Wikipedia, copied from Gof4 “Context” class: Represents the interface to the outside world “State” abstract class: Base class which defines the different states of the “state machine” “Derived” classes from the State class: Defines the implementation of different states

Non Computer Example MyMoodMoodState doSomething() madangryhappy doSomething() state variable Client doSomething()

Another Example from Steve Metsker's Design Patterns Java Workbook Consider the state of a carousel door in a factory Large smart rack that accepts material through a doorway and stores material according to a bar code

Behavior of this manufacturing device There is a single button to operate this door if closed, door begins opening if opening, another click begins closing once open, 2 seconds later (timeout), the door begins closing can be prevented by clicking after open state and before timeout begins Consider two ways to do this If statements State OO design pattern

Transition from one state to another This can get messy. Doing animations? read public void click() { if (state == DoorStateEnum.CLOSED) { setState(DoorStateEnum.OPENING); } else if (state == DoorStateEnum.OPENING || state == DoorStateEnum.STAYOPEN) { setState(DoorStateEnum.CLOSING); } else if (state == DoorStateEnum.OPEN) { setState(DoorStateEnum.STAYOPEN); } else if (state == DoorStateEnum.CLOSING) { setState(DoorStateEnum.OPENING); }

Alternative: The State Pattern Draw a State Diagram first A box representing one of the 5 states CLOSED OPEN… When the carousel door can respond to a button click, or an observable’s update message, draw an arrow to the next state and label the transition with click Or use the transitions complete or timeout

A UML State Diagram also a CSc 473 topic: finite state machine

State Machine characteristics Fixed set of states Closing, opening, closed, open Can only be in one state at a time Can’t be opening and closing at the same time Events are sent to the machine Button clicks, observer notification Each state has a transition Machine changes to the state that transition points

Things to do implement state Define a “context” class to present a single interface Door Define a state abstract base class DoorState Represent different “states” by extending DoorState with DoorOpen, DoorClosed, DoorClosing, … Define state-specific behavior in the appropriate subclasses public class DoorOpening extends DoorState { public void click() { door.setState(door.CLOSING); }

Things to do implement state Maintain current “state” in the “context” class private DoorState state = CLOSED; Change the state of the state machine in context and notify protected void setState(DoorState state) { this.state = state; setChanged(); notifyObservers(); }

Code reverse engineered (demo)

Some of you might use State Complex if statements determine what to do if (thisSprite == running) doRunAnimation(); else if (thisSpite == shooting) doShootingAnimination(); else if (thisSpite == noLongerAlive) doRollOverAnimation();... Or An object can be in one of several states, with different behavior in each state state.doAnimation()