Design Patterns Illustrated

Design Patterns illustrated Herman Peeren May 31st 2010 (DP-illustrations: Nelleke Verhoeff)

Design Patterns ●● recipes against common (OO-) programming problems ●● code reuse: no need to reinvent the wheel ●● common language ●● GOF: 23 “classical” patterns classic, The Book very nice!

The one constant in software development:

The one constant in software development: CHANGE!

The one constant in software development: CHANGE! I knew it ...

Ideal: code as modular black boxes

Wish list and OOP-principles ●● loose coupling: 1 change = ceteris paribus ●● code reuse (is not the same as copy/paste) ●● open for extension, closed for modification ●● encapsulate what varies ●● single responsibility principle ●● program against an interface not against an implementation. Dependency injection ●● prefer composition over inheritance (in fact this is all the same with different words)

Avoid: tight coupling!

It might get you into trouble...

Beware of: Code smells!

some code smells: 1.● duplicate code: DRY 2.● long method, huge class: SRP 3.● combinatorial explosion (similar classes) 4.● conditional complexity, switch statements 5.● “exhibitionistic” classes

Classic pattern categories creational, structural and behavioral patterns: ●● creational: object instantiation ●● structural: larger structures of classes or objects ●● behavioral: interaction and distribution of responsibility

Creational design patterns ●● Factory Method: Allow subclasses to “decide” which class to instantiate. ●● Abstract Factory: Encapsulate a set of analo- gous factories that produce families of objects. ●● Builder: Encapsulate the construction of complex objects from their representation; so, the same building process can create various representations by specifying only type and content. ●● Singleton: Ensure that only a single instance of a class exists and provide a single method for gaining access to it. ●● Prototype: Create an initialized instance for cloning or copying.

Factory Method Provide an interface for the creation of objects. Allow subclasses to “decide” which class to instantiate. c

Abstract Factory Povide an interface for creating families of related or dependent objects. A factory for factories. c

Builder Seperate the construction process (how) of a complex object from the concrete representations (what). c

Singleton Ensure a class only has one instance, and provide a global point of access to it. Oh, I’m so loooooooonly c

Joomla! ●● JFactory: a class with static methods to instantiate objects like JDatabase, JUser, JDocument, JTemplate, etc. ●● most of those methods are singletons Nooku ●● KFactory: any class can be instantiated ●● get() = singleton, tmp() = any instantiation

“Every advantage has its disadvantages” (free to Johan Cruyff, Dutch Football Pattern Designer and Ajax-fan...)

Prototype Make variations on copies of a basic-object. COPY-SERVICE c

Structural design patterns ●● Adapter: Adapt an interface to an expected interface. ●● Bridge: Decouple an interface from its implementation. ●● Composite: Create a tree structure for part-whole hierarchies. ●● Decorator: Extend functionality dynamically. ●● Facade: Simplify usage by defining a high-level interface. ●● Flyweight: Support fine-grained objects efficiently by sharing. ●● Proxy: Represent an object with another object for access control.

Adapter (= Wrapper) c Adapt an interface to an expected interface.

Joomla! ●● new in 1.6: JAdapter and JAdapterInstance ●● JUpdateAdapter extends JAdapterInstance JUpdaterExtension & JUpdaterExtension extend JUpdateAdapter wanted: documentation and examples! what does it adapt?

Bridge Decouple an abstraction from its implementation. COLA 1 LITER 1 LITER COLA 1 LITER 1 LITER COLA MILK COLA MILK COLA MILK c

Composite Create a tree structure for part-whole hierarchies. A node is also a (part of a) tree. Recursive: c

Decorator Add extra functionallity (at runtime), while keeping the interface the same. Matroushka’s... c

Decorator Nooku ●● KPatternDecorator: a general decorator ●● e.g. extended by KDecoratorJoomlaApplication

Facade Provide a general (simpler) interface for a set of interfaces. looks simple c

Flyweight Use one instance of a class to provide many “virtual” instances. c

Proxy Provide a surrogate or placeholder for another object to control access to it. c

Behavioral design patterns ●● Chain of Responsibility: Define a method of passing a request among a chain of objects. ●● Command: Encapsulate a command request in an object. ●● Interpreter: Allow inclusion of language elements in an appli- cation. ●● Iterator: Enable sequential access to collection elements. ●● Mediator: Define simplified communication between classes. ●● Memento: Save and restore the internal state of an object. ●● Observer: Define a scheme for notifying objects of changes to another object. ●● State: Alter the behavior of an object when its state changes. ●● Strategy: Encapsulate an algorithm inside a class. ●● Template Method: Allow subclasses to redefine the steps of an algorithm. ●● Visitor: Define a new operation on a class without changing it.

Command Encapsulate a command request in an object. YOU,DO YOUR TASK! TASK TASK LIGHT LIGHT ON OFF c

Chain of Responsibility c Define a method of passing a request among a chain of objects.

Nooku ●● KCommandChain + KCommandContext, KCommandEvent, KCommandHandler and KCommandInterface ●● N.B.: executes a series of commands instead of passing a command to a series of handlers (like in Tapestry e.g.) ...correct me if I’m wrong....

Interpreter Domain -> (little) language -> grammar -> objects (DSL) HÉ! he means: do this, do that, and after finishing it, go there! c

Iterator Enable sequential access to collection elements, without showing the underlying data-structures (array, list, records, etc) next next c

Mediator c Layer in between: communication via one object.

Memento Save and restore the internal state of an object. ME c

Observer Notify “subscribers” of changes. ME NO ME ME WHO? c

Joomla! ●● JObserver and JObservable ●● JObserver extended by JEvent and that by JPlugin Nooku ●● KPatternObserver and KPatternObservable

State Let an object show other methods after a change of internal state (as if it changes it’s class). in a.....hick......different state, ....hick....I behave differently....hick..... c

Strategy When something can be done in several ways, make those ways interchangeable. POSSI- BILITIES c

Template Method The skeleton of an algorithm is fixed, but parts can be filled in differently. c

Visitor Make a kind of plugin-possibility for methods: in that way methods can be added in runtime. printservice! c

extra: Mixin ●● Used in Nooku (and a.o. in Ruby and Python) ●● a kind of abstract class that is mixed into another object ●● all methods of the mixin are now part of the object ●● handle with care: beware of tight coupling....

Golden OO-principles ●● encapsulate what varies, OCP ●● 1class, 1 responsibility, SRP ●● program against an interface, not against an implementation, DIP ●● prefer composition over inheritance ●● loose coupling, modular black boxes

Some books GOF: 23 “classical” patterns: very nice! classic, The Book handy examples

good start Fowler: extended e.g. with patterns for web Fowler: also known from refactoring combi: refactoring & patterns

Resign Patterns: Ailments of Unsuitable Project-Disoriented Software

Joomla! Could be improved by studying design patterns and applying them. Loose coupling...

Questions? Contact info: Herman Peeren herman@yepr.nl © Yepr The artwork in this presentation is provided under the Creative Commons Public License for noncommercial use. http://creativecommons.org/licenses/by-nc/3.0/legalcode

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Design Patterns Illustrated

by Herman Peeren on Jun 03, 2010

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