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Figure 9-2. Template Method AbstractClass TemplateMethod() PrimitiveOperationA() PrimitiveOperationB() ConcreteClass PrimitiveOperationA() PrimitiveOperationB() The Template Method design pattern is simple to understand and apply. However, as you’ll see, it has a number of features that make it unique. The real trick to using the Template Method is knowing how to differentiate between the abstract functions that must be overridden, concrete functions used consistently throughout the algorithms that aren’t overridden, and those operations that act as hooks and are optionally overridden. Throughout the examples, you’ll see more precisely how to do this. Key OOP Concepts Used with the Template Method As one of only two design patterns that use inheritance instead of composition in the Behavioral category of design patterns, the Template Method would seem to be a hard sell when it comes to proposing the design pattern as a paragon of good OOP. After all, the second principle put forth by GoF is to favor composition over inheritance. Why even bother with a design pattern that seems to favor inheritance over composition? Further, is the Hollywood Principle unique to the design patterns using inheritance, or is it a general principle usable in both design patterns using composition as well? Why Inheritance and Not Composition? PrimitiveOperationA() PrimitiveOperationB() Whenever a design pattern uses inheritance as a key element instead of composition, you need to consider the reason. To understand the reason, you need to fully understand the principle of favoring composition over inheritance as a general principle in good OOP. The principle’s established because of certain advantages of composition over inheritance, especially the composition advantage of not breaking encapsulation. However, Gamma, Helm, Johnson and Vlissides (GoF) note that inheritance also has certain advantages. One such advantage is that when using subclasses where some but not all operations are overridden, modifying the reused implementation is easier. Because the Template Method design pattern does exactly that—uses some but not all operations that can be overridden to achieve flexibility—it incorporates a key advantage of inheritance over composition. Key OOP Concepts Used with the Template Method | 335
Further, discussing favoring composition over inheritance, GoF note that reuse by inheritance eases the process of making new components that can be composed with old ones. Hence the dictum that composition should be favored over inheritance needs to be tempered by the knowledge that inheritance and composition actually work together. This is not to say that the principle of favoring composition over inheritance is invalid, but rather you need to understand the principle in the context of its development. In the process of learning design patterns and the programming principles surrounding them, do not learn the principles as mantras. Chanting the principles over and over again may help you remember them, but not necessarily understand them. All the principles exist in some context, so be certain to understand the principles’ context to use them effectively. Abstract Functions and Override Flexibility In previous chapters we’ve groused about the fact that ActionScript 3.0 doesn’t have abstract classes. Well, ActionScript 3.0 doesn’t have abstract functions either. Rather than rant about that fact, a more important issue is their use. An abstract function is a way of reducing a function to an idea or concept without content. In actual usage, all this involves is naming the function and placing it in the order you want for your main algorithm in the template method, not unlike the functions in an interface. In languages like Java, you can write: abstract myAbstractFunction( ); and that’s it. In ActionScript 3.0, you can accomplish the same thing using: function myAbstractFunction( ) {} So while you don’t have a function that can be designated abstract, you can effectively create all the abstract functions you want, just as you can develop abstract classes, with ActionScript 3.0. Aside from the fact that ActionScript 3.0 can create functions that act like abstract functions, the more important principle is to understand their use in the context of the Template Method design pattern. Key to all design patterns is the idea of reusability and flexibility. Abstract functions must be overridden, and therein is their flexibility. Whenever a function is overridden, it’s changed. By setting up abstract functions in the parent class, the subclasses can use them as needed to fill out the particular implementation of the template method algorithm. Reuse is fostered by the fact that as long as the algorithm template is applicable, the developer has flexibility because the abstract functions that make up the template method can be modified to suit the application. 336 | Chapter 9: Template Method Pattern
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ActionScript 3.0 Design Patterns
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ActionScript 3.0 Design Patterns Wi
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Bill would like to dedicate this to
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Example: Sprite Factory 84 Example:
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Video Player Concrete State Applica
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Who This Book Is For We wanted to d
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What You Need to Use This Book You
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and OOP. (Working out how to conver
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Safari® Enabled When you see a Saf
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Editors Our development editor, Rob
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Design patterns provide object-orie
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that program. Like pulling a string
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Built-in State Machines: while most
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Figure 1-2. Rocket drawing Center p
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Example 1-1. TestRocket.as package
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skills that would provide the concr
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Example 1-3. PlayVideoAbstract.as (
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Hiding Your Data from Bad Consequen
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Example 1-7. Encap.as (continued) }
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Wait! There’s more! Later in this
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As you can see in Example 1-9, the
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Using Encapsulation and Design Patt
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Example 1-13. QuadPets.as (continue
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Using Interfaces and Abstract Class
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interface. You’ll need an MP3 fil
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Example 1-22. Subclass.as package {
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Example 1-25. ImplementSubChange.as
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Example 1-29. Country.as (continued
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Example 1-32. IBiz.as package { pub
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Example 1-35 has further difference
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In addition to messages in the Outp
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state here is a bit more contextual
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your client dependency leads to une
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Example 1-40. VidPlayer.as (continu
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Doing Composition To understand com
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The clearest way we’ve seen compo
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Example 1-47. Mp3.as (continued) }
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Example 1-54. TestMedia.as (continu
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and ActionScript 3.0. However, the
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Figure 1-10 shows some possible fut
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introduction to OOP, you will find
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Creational Patterns Conceal creatio
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or dependency between the Client an
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The primary responsibility of the f
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The example package contains the Pr
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Example 2-5. CreatorA.as (continued
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instantiate concrete products var p
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Example 2-8. IPrintjob.as package p
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Clients Note that the client doesn
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Extended Example: Color Printing Th
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Example 2-19. NewLowVolPrintCenter.
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Key OOP Concepts Used in the Factor
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to implement default behavior commo
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Example 2-26. ShapeCreator.as packa
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draw filled shapes filledShapeCreat
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checking if the projectile has gone
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Example 2-32. AlienMine.as (continu
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Example 2-34. AlienShip.as } } } }
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Example 2-36. Weapon.as (continued)
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Concrete Creator Classes Until we e
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in teamwork, the Singleton can be u
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To create a class, we need a public
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Example 3-4. Singleton.as package {
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Figure 3-3. Compiler not finding cl
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However, does that mean that only a
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Figure 3-5. Two instance labels and
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Example 3-10. Alert.as } } { Alert.
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However, had we done that, we would
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for playing the selected tune. If i
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MP3 file in the same folder, and re
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shipping costs. It constitutes a go
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open the Convert to Symbol dialog b
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PART III III.Structural Patterns Tr
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Chapter 4 CHAPTER 4 Decorator Patte
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like abstract classes, simply by no
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simple inheritance. Depending on ho
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Minimalist Abstract Decorator To ge
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eral months. Using a Decorator desi
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Example 4-5. DecConB.as package { /
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Applying a Simple Decorator Pattern
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If you want to create more concrete
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Example 4-15. Muff.as package { pub
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Figure 4-6. Paper doll with two dif
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Adding Properties and Methods Up to
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Decorating with Multiple Properties
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} override public function evil( ):
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Example 4-25. Compassion.as package
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Deadly sins Example 4-29. Rage.as p
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Example 4-33. Dogmatisms.as package
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class window in the Dual.fla Proper
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Example 4-37. Soul.as (continued) }
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Imagine that you are responsible fo
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Example 4-41. Accord.as (continued)
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Example 4-45. GPS.as package { publ
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Example 4-48. Deal.as 16 internal v
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Example 4-48. Deal.as 120 { 121 tra
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Figure 4-8. Decorator user interfac
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Chapter 5 CHAPTER 5 Adapter Pattern
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and their functionality to figure o
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Example 5-4. Main.as (continued) }
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Example 5-5. Adaptee.as package { p
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methods and properties to accomplis
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Example 5-9. LegacyCar.as 47 } 48 4
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ignition key of the car (assume tha
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Example 5-11. CarAdapter.as (contin
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Note that the new interface turns t
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var listField = new TextField( ); l
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Example 5-14. ListDisplayField.as 3
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Example 5-15. Main.as (document cla
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Example 5-17. Main.as (document cla
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Summary The adapter pattern allows
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objects and composite objects the s
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the components. We can think of tou
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method by tracing sName. When opera
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oot composite 1 leaf 1 leaf 2 compo
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Executing the following statements
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Example 6-6. The safeRemove( ) meth
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leaf 2 Composite 1 leaf 2 leaf 3 Di
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Example 6-9. Playlist.as (continued
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Example 6-10. Song.as 58 soundChann
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Using Inverse Kinematics Inverse ki
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Example 6-12. Composite.as package
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Example 6-14. Main.as 23 addChild(s
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The following steps relate to Figur
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Example 6-16. Tail.as package { imp
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Note how the snake is constructed u
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Example 6-18 shows the IPlane inter
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Unlike the Shape class, Sprite does
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Figure 6-9. Hierarchical tree struc
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Modifying Components to Reflect Dam
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private function showDamage(evt:Eve
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Behavioral Patterns Class Communica
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Notice the tight coupling between t
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Each envelope encapsulates a partic
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instance is passed to the construct
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Setting a Trigger to Invoke the Com
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Example 7-7. IMacroCommand.as packa
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Example 7-12. ConcreteMacroCommand.
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Example 7-14. TextButton.as package
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Example 7-15. InvokerPanel.as (cont
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Extended Example: Sharing Command O
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changes are seamlessly spread throu
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Example 7-21. IncrementCommandWithU
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Example 7-24. Client code for undoa
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Example 7-26. TextDisplayField.as (
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Example 7-28. Radio.as 44 var url =
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Example 7-30. Client code for the p
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Example 7-31. DynamicControlButtons
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adio, very much like someone progra
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Chapter CHAPTER 8 8 Observer Patter
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Page 313 and 314: Example 8-7. ConcreteSubject.as (co
Page 315 and 316: Example 8-9. TestSub.as (continued)
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Page 323 and 324: Whoaa!, the stock market is at 2987
Page 325 and 326: Example 8-17. ConcreteSubject.as pa
Page 327 and 328: Launching the Space Battle In Examp
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Page 333 and 334: Example 8-19. ChangeHandler.as (con
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Page 341 and 342: Example 8-24. ConcreteObserver.as p
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Page 345 and 346: Example 8-26. QuarterBar.as (contin
Page 347 and 348: Example 8-27. QuarterGraph.as (cont
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Page 351 and 352: Summary The real test for a design
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Page 359 and 360: Figure 9-3. Hollywood principle Min
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Page 367 and 368: Not-So-Concrete Concrete Classes Th
Page 369 and 370: Example 9-11. PlayVideo.as (continu
Page 371 and 372: Example 9-13. PlayMedia .as (contin
Page 373 and 374: When to Hook? The hook’s useful w
Page 375 and 376: Example 9-15. FlightPlan.as (contin
Page 377 and 378: Open a new Flash document, and type
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Page 385 and 386: Example 10-2. VideoWorks.as (contin
Page 387 and 388: To complete the state machine, we n
Page 389 and 390: Example 10-7. State.as package { //
Page 391 and 392: Example 10-10. VideoWorks.as (conti
Page 393 and 394: Example 10-13. TestVid.as package {
Page 395 and 396: Example 10-13. TestVid.as (continue
Page 397 and 398: Example 10-15. StopState.as (contin
Page 399 and 400: Example 10-17. PauseState.as 22 pub
Page 401 and 402: Example 10-19. TestPause.as (contin
Page 403 and 404: Figure 10-6 shows what your video p
Page 405 and 406: Figure 10-7. Statechart with five s
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Example 10-22. PlayState.as 1 packa
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Example 10-24. RecordState.as 1 pac
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Example 10-26. VideoWorks.as (conti
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Example 10-27. TestFMS.as (continue
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Chapter CHAPTER 11 11 Strategy Patt
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say that the Context class (aggrega
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etween the context classes and the
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Example 11-3. ConcreteStrategy.as p
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Adding More Concrete Strategies and
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Example 11-9. Tricks.as (continued)
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Even though we’ve discussed some
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Example 11-18. Bubbles.as package {
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Simply by adding setters to the con
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Contexts for String Strategies You
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Example 11-24. StringWork.as packag
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Example 11-29. SortAll.as package {
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Example 11-33. TestStringStrategy.a
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Using More Than One Pattern PART V
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Chapter 12 CHAPTER 12 Model-View-Co
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triggered by a user interacting wit
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grouped together in a tabbed panel
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Example 12-2. Model.as (continued)
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Example 12-5. View.as (continued) 1
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implementations in the MVC. Our vie
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Example 12-8. CompositeView.as (con
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Example 12-10. CharCodeLeafView.as
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Key OOP Concepts in the MVC Pattern
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Example 12-12. IModel.as (Model int
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Example 12-15. Controller.as (Contr
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Example 12-16. CBView.as 24 update(
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Note that only the combo boxview th
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Example 12-18. INewModel.as (contin
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Example 12-21. RBView.as package {
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add map type select radio button gr
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Example 12-23. CarModel.as (continu
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Example 12-25. RHController.as (con
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Example 12-28. Main.as (document cl
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Example 12-30. GPSView.as package {
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Example 12-32. ChaseController.as (
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Chapter 13 CHAPTER 13 Symmetric Pro
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• Indicate that a remote player h
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Even though both of the solutions a
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Referee 1 Player1 Proxy2 Internet S
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With a single template method, and
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developed with an eye to the order
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Boolean variables to false. (The fa
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Example 13-3. RPS.as (continued) }
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Example 13-4. proxygame.asc (contin
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Example 13-5. SymPlayer1.as package
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Example 13-5. SymPlayer1.as (contin
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Example 13-5. SymPlayer1.as (contin
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In addition to using the Referee to
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Example 13-6. DynamicText.as (conti
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3. Click the Text layer to select i
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A abstract classes, 28, 31, 68, 130
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increment and decrement commands, 2
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E ECOOP, 470 EmailCheck.as, 418 enc
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MapTypeController.as, 454 MapView.a
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Q QuadPets.as, 25 R Radio.as, 273 R
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T Tail.as, 231 Target.as, 180 Templ
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