ActionScript 3.0 Design Patterns.pdf - VideoTutorials-bg.com

context. The subsequent chapters use these terms a good deal, and a misunderstanding
at the outset can lead to confusion now and later.
Implementation
For the most part, the term implementation is a noun referring to the details—the
actual code—in a program. So when referring to an implementation, the reference is
to the actual program as it is coded. Implementation is the internal details of a program,
but is often used to refer to a method or class itself.
You may run into some confusion when the keyword implements refers to contracting
with an interface structure. For example, the line:
class MyClass implements IMyInterface
has the effect of passing the interface of IMyInterface to MyClass. To say that MyClass
implements IMyInterface really means that MyClass promises to use all the signatures
of IMyInterface and add the coding details to them. So the phrase MyClass implemented
IMyInterface means that MyClass took all the methods in IMyInterface and
added the code necessary to make them do something while preserving their
signatures.
One of the two main principles of design pattern programming is program to an
interface, not an implementation. However, you must remember that implementation
is employed in different contexts, and its specific meaning depends on these contexts.
Throughout this book, you will see references to implementation used again
and again with different design patterns, and you need to consider the other elements
being discussed where the term is used.
State
The term state is not part of the ActionScript 3.0 lexicon (like implements is), but the
term is used in discussing design patterns. Essentially, state is used to refer to an
object’s current condition. For the most part you will see state used to convey the
value of a key variable. Imagine a class with a single method with a Boolean value set
in the method. The Boolean can either be true or false. So its state is either true or
false—which could represent on/off, allow/disallow, or any number of binary conditions.
The exact meaning of true or false depends on the position of the class in
the rest of the design pattern. A more specific example would be an object that plays
and stops an MP3 file. If the MP3 is playing, it’s in a play state, while if it’s not playing,
it’s in a stop state.
One use of state is in the context of a state machine and the State design pattern you
will be seeing in Chapter 10. A state engine is a data structure made up of a state network
where changes in state affect the entire application. The State design pattern is
centered on an object’s behavior changing when its internal state changes. The term
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