MoLabsMedia / design-patterns

design patterns for various languages

Design Patterns

• Design patterns are defined abstractions of relationships, processes, workflows etc.
• Data ( properties etc. ) are static information to be stored ( created ), got, changed or removed.
• Actions ( methods etc. ) are processes of data manipulations in certain context with specific parameters.
• class is a means of data and action informations encapsulation, providing certain name spaces or contexts in execution.

Creational patterns

abstract factory

Creates an instance of several families of classes

• Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
• A hierarchy that encapsulates: many possible "platforms", and the construction of a suite of "products".
• The new operator considered harmful.

builder

Separates object construction from its representation

• Separate the construction of a complex object from its representation so that the same construction process can create different representations.
• Parse a complex representation, create one of several targets.

factory

Creates an instance of several derived classes

• Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
• Defining a "virtual" constructor.
• The new operator considered harmful.

prototype

A fully initialized instance to be copied or cloned

• Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
• Co-opt one instance of a class for use as a breeder of all future instances.
• The new operator considered harmful.

singleton

A class of which only a single instance can exist

• Ensure a class has only one instance, and provide a global point of access to it.
• Encapsulated "just-in-time initialization" or "initialization on first use".

Structural patterns

adapter

Match interfaces of different classes

• Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.
• Wrap an existing class with a new interface.
• Impedance match an old component to a new system

bridge

Separates an object's interface from its implementation

• Decouple an abstraction from its implementation so that the two can vary independently.
• Publish interface in an inheritance hierarchy, and bury implementation in its own inheritance hierarchy.
• Beyond encapsulation, to insulation

composite

A tree structure of simple and composite objects

• Compose objects into tree structures to represent whole-part hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
• Recursive composition
• "Directories contain entries, each of which could be a directory."
• 1-to-many "has a" up the "is a" hierarchy

decorator

Add responsibilities to objects dynamically

• Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
• Client-specified embellishment of a core object by recursively wrapping it.
• Wrapping a gift, putting it in a box, and wrapping the box.

facade

A single class that represents an entire subsystem

• Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.
• Wrap a complicated subsystem with a simpler interface.

flyweight

A fine-grained instance used for efficient sharing

• Use sharing to support large numbers of fine-grained objects efficiently.
• The Motif GUI strategy of replacing heavy-weight widgets with light-weight gadgets.

proxy

An object representing another object

• Provide a surrogate or placeholder for another object to control access to it.
• Use an extra level of indirection to support distributed, controlled, or intelligent access.
• Add a wrapper and delegation to protect the real component from undue complexity.

Behavioral patterns

chain of responsibility

A way of passing a request between a chain of objects

• 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.
• Launch-and-leave requests with a single processing pipeline that contains many possible handlers.
• An object-oriented linked list with recursive traversal.

command

Encapsulate a command request as an object

• Encapsulate a request as an object, thereby letting you parametrize clients with different requests, queue or log requests, and support undoable operations.
• Promote "invocation of a method on an object" to full object status
• An object-oriented callback

interpreter

A way to include language elements in a program

• Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
• Map a domain to a language, the language to a grammar, and the grammar to a hierarchical object-oriented design.

iterator

Sequentially access the elements of a collection

• Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
• The C++ and Java standard library abstraction that makes it possible to decouple collection classes and algorithms.
• Promote to "full object status" the traversal of a collection.
• Polymorphic traversal

mediator

Defines simplified communication between classes

• Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.
• Design an intermediary to decouple many peers.
• Promote the many-to-many relationships between interacting peers to "full object status".

memento

Capture and restore an object's internal state

• Without violating encapsulation, capture and externalize an object's internal state so that the object can be returned to this state later.
• A magic cookie that encapsulates a "check point" capability.
• Promote undo or rollback to full object status.

observer

A way of notifying change to a number of classes

• Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
• Encapsulate the core (or common or engine) components in a Subject abstraction, and the variable (or optional or user interface) components in an Observer hierarchy.
• The "View" part of Model-View-Controller.

state

Alter an object's behavior when its state changes

• Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
• An object-oriented state machine
• wrapper + polymorphic wrappee + collaboration

strategy

Encapsulates an algorithm inside a class

• Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from the clients that use it.
• Capture the abstraction in an interface, bury implementation details in derived classes.

template method

Defer the exact steps of an algorithm to a subclass

• Define the skeleton of an algorithm in an operation, deferring some steps to client subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.
• Base class declares algorithm 'placeholders', and derived classes implement the placeholders.

visitor

Defines a new operation to a class without change

• 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.
• The classic technique for recovering lost type information.
• Do the right thing based on the type of two objects.
• Double dispatch