Design Patterns in SmalltalkWe talked about ...IteratorComparing two collectionsSlide 5ProxySlide 7Implementing Proxy with doesNotUnderstand:BridgeFlyweightFlyweights for TextSlide 12Flyweight for CADSlide 14Flyweight as a compound patternChain of ResponsibilitySlide 17Slide 18MementoSlide 20Slide 21Slide 22Patterns Protect from ChangePatterns lead to standard interfacesSmalltalk features impact patternsPatterns and refactoringCommandRefactoring to CommandCompositeRefactoring to a CompositeFaçadeDesign patterns and refactoringDesign Pattern summaryDesign Patterns in SmalltalkInfluenced by programming languageLarge-scale refactoringsWe talked about ...Template MethodState, Strategy, Observer, Mediator, FacadeCommand, Interpreter, VisitorAdapter, Composite, DecoratorFactory Method, Abstract Factory, Builder, Prototype, SingletonIteratorInternal iterator vs external iteratorInternal iterators depend on blocksInternal iterators simplerExternal iterators more powerfulComparing two collections= anObject| first second |self size = anObject size ifFalse: [^false].first := ReadStream on: self.second := ReadStream on: anObject.[first atEnd | second atEnd] whileFalse:[first next = second next ifFalse: [^false]].^first atEnd & second atEndIteratorProvide a way to access the elements of an aggregate object sequentially without exposing its underlying implementation.ConcreteIteratorIteratorFirst()Next()IsDone()CurrentItem()ConcreteAggregateCreateIterator()AggregateCreateIterator()ProxyProvide a surrogate or placeholder for another object to control access to itrepresent an object in a remote address spacecreate expensive objects on demandcheck access rights Proxy has same interface as “real subject”, and forwards operations to itProxySubjectRequestClientProxyRequestrealSubjectRealSubjectRequestImplementing Proxy with doesNotUnderstand:When an object does not understand a message, the message gets converted into doesNotUnderstand: message with the original message as an argumentDefault version of doesNotUnderstand: pops up a debuggerProxy can override doesNotUnderstand:BridgeDecouple an abstraction from its implementation so that the two can vary independently.ImplementorConcreteImp2AbstractionConcreteImp1FlyweightUse sharing to support large numbers of objects efficiently.Separate intrinsic state (state stored in flyweight) from extrinsic state (state passed in as part of context). Minimize extrinsic state. Share flyweights that have the same intrinsic state.Usually requires a factory that detects whether a flyweight exists with a particular intrinsic state and returns it.Flyweights for TextcolumnrowrowrowT h i s i s a s e n t e n c eT h i s a s e n t cFlyweight poolClient of flyweight,which holds context.Context is location.FlyweightFlyweightFactoryGetFlyWeight(key)Flyweight intrinsicOperation(extrinsic)flyweightpoolFlyweight class is usually abstract, with concrete subclasses that define the intrinsic state.Flyweight for CADA VLSI design system must model millions of transistors.This is only possible by sharing structure. Most transistors are part of larger structures (registers, NAND gates, RAM) that designers prefer to think about. Each kind of structure is called a cell.Each cell is interconnected with other cells.Context is location and interconnections.Flyweight for CADTransistorsizeDisplay(loc,connects)CellDisplay(loc,connects)flyweightpoolCompositeCellports, locationsDisplay(loc,connects)CellFactoryGetCell(key)Register(key,Cell)Flyweight as a compound pattern1) Recycling Factory (generalization of Singleton)2) Value Object - immutable objects3) Normalized Object - Separating intrinsic from extrinsicChain of ResponsibilityAvoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.Examples:“inheriting” color from carevent handlers in GUIChain of ResponsibilityHandlerHandleRequest()ConcreteHandler2HandleRequest()ConcreteHandler1HandleRequest()ClientChain of ResponsibilityUsually mixed with other patterns• Composite often has Chain of Responsibility up the tree.• Sometimes request is encoded as a Command• Sometimes request sent to StrategyMementoWithout violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later.MementoGetStateSetStatestateOriginatorSet(Memento)CreateMementostateOriginatorreturn new Memento(state)state = m->GetState()MementoUndo is not enough in the presence of a constraint system.Must go back to same state, not just reverse operation.MementoA C A B C C B A A CX Z X Y Y X X X Z Translator isa state machineinput documentChange part of inputand recompute output.output documentA BProblem: Large structured documents are stored in one form and translated to another form to be printed. How can a small change in the input be converted into a change in the output without retranslating the entire input?MementoFor each input symbol, keep a memento of the translator and the output for that input symbol. Recompute the output until the translator is back in the same state.I S T O P Q R S S S TX Z X Y Y X X X Z next stateA C A B C C B A A CA Boutputinitial stateinputPatterns Protect from ChangeRule: if something is going to change, make it an object.Strategy: make algorithm an object so it can changeState: make state-dependent behavior an object so it can changeIterator: make the way you iterate over an aggregate an object so it can changeFacade: make a subsystem an object so it can changeMediator: make the way objects interact an object so it can changeFactory: make the classes of your products an object so it can changePatterns lead to standard interfacesDecorator, Composite, ProxyIntroduce new objects with same interfaces as old but that add a feature / represent a group / represent a remote objectAdapterFacadeSmalltalk features impact patternsIterator – blocksProxy – doesNotUnderstand:Command – blocks, perform:Prototype – classes as objectsBridge – dynamic typing?Patterns and refactoringCan think of any design pattern as a “big refactoring”Introduce design patterns when you know you need them, not when you think you might need themRefactoring to Patterns by Joshua