ActionScript 3.0 Design Patterns

Minimalist Abstract State Pattern | 363

As you can see, the behaviors (methods) have different implementations in the differ-

ent states. When you add more states, all you need to do is add their transitional

behaviors to the interface and create a new concrete state (class) that implements the

behaviors. Each new behavior needs to be added to the existing state classes.

Managing All Those States: Hardworking Context Class

To manage the states and their transitions, you need some kind of management

object—the state engine for your state machine. In Figure 10-3 the bo xlabeled

“Context” is the abstraction of the state engine. The context manages the different

states that make up the state machine and contain the different states. Figure 10-4

shows a more concrete representation of what needs to be transformed:

Creating a context class

Looking at our example of creating a simple video player, we need a context that will

serve to get and set the different states. So, the next phase will be to look at a class

(object) that does just that. This context class should be saved asVideoWorks.as.

First, take a look at the class in Example 10-2, and then we’ll see what’s going on:

Figure 10-4. State Design Pattern applied to video

Example 10-2. VideoWorks.as

1 package

2 {

3 //Context class

4 class VideoWorks

5 {

6 var playState:State;

7 var stopState:State;

8 var state:State;

9 public function VideoWorks( )

10 {

11 trace("Video Player is On");

12 playState = new PlayState(this);

13 stopState = new StopState(this);

14 state=stopState;

15 }

16 public function startPlay( ):void

17 {

VideoWorks

Request

State

startPlay()

stopPlay()

PlayState

startPlay()

stopPlay()

StopState

startPlay()

stopPlay()

state->Handle()