ActionScript 3.0 Design Patterns

364 | Chapter 10: State Pattern

Initially, in lines 6-8, the script instantiates threeStateobjects—one of each of the

two we designed (PlayStateandStopState), and one (state) that acts as a variable to

hold the current state. Because the state machine begins in the Stop state, the state

variable is assigned the Stop state. (This works just like the light switch before you

change it from theoffstate to theon state.)

Next, the two behaviors from the State interface are specified in terms of the current

state’s context (lines 16-23). We’re going to have to add some code to the two state

classes for it to work with the context class, but for now, think of what will happen

in the two different states when those behaviors are executed. For example, in the

Play state, thestartPlay( )method doesn’t do anything, but in the Stop state, it

switches to the Play state.

Finally, add the getter and setter methods (lines 24-40). We need a total of si xmeth-

ods—a set and get function for each of the three state instances. The setters return

nothing and the getters return a State object.

Completing and testing the abstract state machine

To get everything working, we need to revise the state classes to include the refer-

ence to the context—VideoWorks. Save Example 10-3 asStopState.as.

18 state.startPlay( );

19 }

20 public function stopPlay( ):void

21 {

22 state.stopPlay( );

23 }

24 public function setState(state:State):void

25 {

26 trace("A new state is set");

27 this.state=state;

28 }

29 public function getState( ):State

30 {

31 return state;

32 }

33 public function getPlayState( ):State

34 {

35 return this.playState;

36 }

37 public function getStopState( ):State

38 {

39 return this.stopState;

40 }

41 }

42 }

43

Example 10-2. VideoWorks.as (continued)