Laboratorio di Sistemi Software Design Patterns 2

Transcription

1 TITLE Laboratorio di Sistemi Software Design Patterns 2 Luca Padovani (A-L) Riccardo Solmi (M-Z) 1 Indice degli argomenti Tipi di Design Patterns Creazionali, strutturali, comportamentali Design Patterns Factory Method, Strategy, State, Decorator Composite, Iterator, Visitor Template Method, Abstract Factory, Builder, Singleton Proxy, Adapter, Bridge, Facade Mediator, Observer, Chain of Responsibility Command, Prototype 2

2 Bibliografia UML Linguaggio per modellare un Sistema Software. Design Patterns Elements of Reusable Object-Oriented Software Catalogo di patterns di progettazione Refactoring Improving the Design of Existing Code Catalogo di operazioni di Refactoring Eclipse IDE + UML Plugin Ambiente di programmazione che supporta refactoring e UML. Trovate tutto nelle pagine web relative a questo corso: Ricevimento Metodo 1: il newsgroup Avete a disposizione un newsgroup: unibo.cs.informatica.paradigmiprogrammazione Utilizzatelo il più possibile; se nessuno risponde in due-tre giorni, risponderà uno dei docenti Valuteremo anche la partecipazione al newsgroup Metodo 2: subito dopo le lezioni siamo a vostra disposizione per chiarimenti in aula e alla lavagna, in modo da rispondere a tutti Metodo 3: ricevimento su appuntamento Scrivete a lpadovan@cs.unibo.it (A-L) o a solmi@cs.unibo.it (M-Z) 4

3 Composite Intent Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly Applicability you want to represent part-whole hierarchies of objects. you want clients to be able to ignore the difference between compositions of objects and individual objects. Clients will treat all objects in the composite structure uniformly 5 Composite /2 Structure Participants Component declares the interface for objects in the composition. implements default behavior for the interface common to all classes. declares an interface for accessing and managing its child components. (optional) defines an interface for accessing a component's parent in the recursive structure, and implements it if that's appropriate. Leaf represents leaf objects in the composition. defines behavior for primitive objects Composite Client defines behavior for components having children. stores child components. Implements child-related operations in the Component interface. manipulates objects in the composition through the Component interface. 6

4 Composite /3 Collaborations Clients use the Component class interface to interact with objects in the composite structure. If the recipient is a Leaf, then the request is handled directly. If the recipient is a Composite, then it usually forwa rds requests to its child components, possibly performing additional operations before and/or after forwarding. Consequences defines class hierarchies consisting of primitive objects and composite objects makes the client simple makes it easier to add new kinds of components can make your design overly general 7 Composite example 1 8

5 Composite example 2 Graphics applications like drawing editors and schematic capture systems let users build complex diagrams out of simple components 9 Composite Questions Part 1: How does the Composite pattern help to consolidate system-wide conditional logic? Part 2: Would you use the composite pattern if you did not have a part-whole hierarchy? In other words, if only a few objects have childrenand almost everything else in your collection is a leaf (a leaf can have no children), would you still use the composite pattern to model these objects? 10

6 Iterator Intent Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation Applicability to access an aggregate object's contents without exposing its internal representation. to support multiple traversals of aggregate objects. to provide a uniform interface for traversing different aggregate structures (that is, to support polymorphic iteration). 11 Iterator /2 Structure Participants Iterator defines an interface for accessing and traversing elements. ConcreteIterator implements the Iterator interface. keeps track of the current position in the traversal of the aggregate. Aggregate defines an interface for creating an Iterator object. ConcreteAggregate implements the Iterator creation interface to return an instance of the proper ConcreteIterator. 12

7 Iterator /3 Collaborations A ConcreteIterator keeps track of the current object in the aggregate and can compute the succeeding object in the traversal Consequences It supports variations in the traversal of an aggregate Iterators simplify the Aggregate interface More than one traversal can be pending on an aggregate 13 Iterator example 1 14

8 Iterator example 2 An aggregate object such as a list should give you a way to access its elements without exposing its internal structure 15 Iterator questions Considera composite that contains loan objects. The loan object interface contains a method called "AmountOfLoan()", which returns the current market value of a loan. Given a requirement to extract all loans above, below or in betweena certain amount, would you write or use an Iterator to do this? 16

9 Visitor Intent Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates Applicability an object structure contains many classes of objects with differing interfaces, and you want to perform operations on these objects that depend on their concrete classes many distinct and unrelated operations need to be performed on objects in an object structure, and you want to avoid "polluting" their classes with these operations the classes defining the object structure rarely change, but you often want to define new operations over the structure 17 Visitor /2 Structure 18

10 Visitor /3 Collaborations A client that uses the Visitor pattern must create a ConcreteVisitor object and then traverse the object structure, visiting each element with the visitor When an element is visited, it calls the Visitor operation that corresponds to its class. The element supplies itself as an argument to this operation to let the visitor access its state, if necessary 19 Visitor /4 The following interaction diagram illustrates the collaborations between an object structure, a visitor, and two elements 20

11 Visitor /4 Consequences Visitor makes adding new operations easy A visitor gathers related operations and separates unrelated ones Adding new ConcreteElement classes is hard Visiting across class hierarchies Accumulating state Breaking encapsulation 21 Visitor example 1 22

12 Visitor example 2 Compiler that represents programs as abstract syntax trees 23 Visitor Questions One issue with the Visitor pattern involces cyclicality. When you add a new Visitor, you must make changes to existing code. How would you work around this possible problem? 24