Template Method Structure. Template Method Example. Template Method Example. Template Method Example. Benefits of Template Method

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1 Design Patterns in Smalltalk CSC591O April 9, 1997 Raleigh, NC Bobby Woolf Senior Member of Technical Staff Knowledge Systems Corp Weston Parkway Cary, North Carolina Copyright (C) 1997, Kyle Brown, Bobby Woolf, and Knowledge Systems Corp. All rights reserved. 1 2 Why Design Patterns and Smalltalk? Design Patterns for Review O Design Patterns help explain Smalltalk s extensive class libraries» Lots of examples in vendor-provided code O Smalltalk raises programmers above the bits & bytes level»they can think about design rather than implementation O Template Method (325) O Composite (163) O Decorator (175) O Chain of Responsibility (223) O Adapter (139) O Observer (293) 3 4 Designing for Extension Template Method (325) O Designing classes that are meant to be subclassed is HARD» Especially when they are concrete classes themselves O Often you want to let a superclass control the way its subclasses will operate» The Hollywood principle 5 O Define the outline of an algorithm in an abstract superclass O Defer some steps to subclasses O Subclasses specialize the algorithm by overridding abstract methods O Template Method uses three types of methods» Concrete operations» Abstract (primitive) operations»hooks 6

2 Template Method Structure Template Method Example AbstractClass templatemethod primitivemethod2 ConcreteClassA primitivemethod2 ConcreteClassB primitivemethod2... self.... self primitivemethod2.... Object>>printString "Answer a String that is an ASCII representation of the receiver." astream astring astring := String new: 20. self printon: (astream := WriteStream on: astring). ^astream contents ConcreteClassC printstring is a Template Method printon: is a hook method 7 8 Template Method Example Template Method Example Object>>printOn: astream "Append the ASCII representation of the receiver to astream. This is the default implementation which prints 'a' ('an') followed by the receiver class name." String>>printOn: astream "Append the receiver as a quoted string to astream doubling all internal single quote characters." astring astring := self class name. (astring at: 1) isvowel iftrue: [astream nextputall: 'an '] iffalse: [astream nextputall: 'a ']. astream nextputall: astring This is the default printon: implementation 9 astream nextput: $'. self do: [ :character astream nextput: character. character = $' iftrue: [astream nextput: character]]. astream nextput: $' so 'abc' printstring yields 'abc', not astring 10 Benefits of Template Method Object-Oriented Trees O Makes classes easier to subclass and specialize» Simpler, smaller methods O Often arises when refactoring common behavior from subclasses» Refactoring to Generalize O The next three patterns are commonly used together in OO programs in tree structures O The structure of the algorithms involved is different from in procedural programming 11 12

3 OO Trees (Top-Down) OO Trees (Bottom-up) OO Trees (doubly-linked) Whole-part structures O Example: Financial portfolio composed of individual stocks and bonds O You would like to have both the portfolio and the individual stocks and bonds act alike in some ways Composite (163) Composite Structure O Compose objects into tree structures to represent part-whole hierarchies O Lets clients treat individual objects and compositions of objects uniformly O Abstract class represents both primitives and their containers AbstractComponent N Composite Leaf 17 18

4 Typical Composite Composite in VisualWorks acomposite VisualComponent VisualPart acomposite DependentPart Wrapper CompositePart View TranslatingWrapper DependentComposite BoundedWrapper CompositeView BorderedWrapper Adding new behavior Decorator (175) O Sometimes you need to customize the behavior of an object at runtime» Subclassing not an option O Example: adding borders or scroll bars to a View O Also known as: Wrapper O Attach additional responsibilities to an object dynamically O A flexible alternative to subclassing for extending functionality O Forwards requests to the component, may perform additional actions O May be nested recursively Decorator Structure Decorator in VisualWorks 1 AbstractComponent VisualComponent VisualPart DependentPart Wrapper CompositePart Decorator Leaf View TranslatingWrapper DependentComposite BoundedWrapper CompositeView BorderedWrapper 23 24

5 Using Tree Structures Chain of Responsibility (223) O How do you handle requests made to an OO tree?» Who is responsible for doing the work?» A tree is NOT an object -- it s a collection of objects O Example: Context sensitive help system O Avoid coupling the sender of a request to its receiver O Request passes through multiple handlers until one handles it O Handler should be ascertained automatically» Who handles request is not determined until request is processed O Set of handlers is dynamic Chain of Responsibility Structure Chain of Responsibility (object) client Client Handler successor handler Handler handler Handler handler ConcreteHandler1 ConcreteHandler2 nil Chain of Responsibility (message) Chain of Responsibility (example) client Handler another ConcreteHandler acontroller view avisualpart container awrapper container ascheduledwindow 29 30

6 Chain of Responsibility (example) Incompatible Interfaces acontroller avisualpart awrapper ascheduled Window bounds compositionboundsfor: compositionboundsfor: bounds O Often you have two objects with incompatible interfaces that need to work together O Example: Electrical Voltage Adaptor Adapter (139) Adapter (Structure) O Also known as: Wrapper O Converts the interface of a class into another interface clients expect O Lets classes with incompatible interfaces work together O Adapter can add functionality the Adaptee doesn t provide O Enables use of several existing subclasses Client OtherTargetImplementor request TargetInterface request (interface) Adapter request adaptee adaptee specificrequest Adaptee specificrequest Maintaining Consistent State Observer (293) O Often you have several objects that need to be kept consistent with the state of another object O Example: multiple windows showing different views of the same object O Also known as: Dependents, Publish- Subscribe O Define a one-to-many dependency» When one object changes state, all dependents are notified automatically» Dependents can update automatically O Decouples Subject from Observer» Hides Observers from Subject, each other 35 36

7 Observer Structure Observer (object) Subject adddependent: removedependent: changed: dependents Observer update: ConcreteSubject : subject ConcreteObserver observerstate update: self observerstate: self subject Subject dependents Observer1 subject Observer2 subject ^ dependents do:[obs obs update: asymbol] Observer (interaction) Summary Subject : anobject Observer1 Observer2 changed: asymbol update: asymbol update: asymbol O Template Method (325) helps you define superclasses for extension O Composite (163) helps you build whole-part structures O Decorator (175) shows how to extend the behavior of runtime objects Summary (2) O Chain of Responsibility (223) handles requests in OO trees O Adapter (139) resolves incompatible interfaces O Observer (293) provides efficient 1-N notification 41