CS3342 Lecture 8

Software Design Patterns - State Pattern, Singleton Pattern, Factory Method Pattern, Facade Pattern

Benefits of Design Patterns

Design patterns enable large-scale reuse of software architectures and also help document systems

Patterns explicitly capture expert knowledge and design tradeoffs and make it more widely available

Patterns help improve developer communication

Pattern names form a common vocabulary (Jargon)

Creational Patterns: a Catalog

Purpose: Deals with initialization of objects and selection among classes

Abstract Factory: Factory for building related objects

Builder: Factory for building complex objects incrementally

Factory Method: Method in a derived class creates associates

Prototype: Factory for cloning new instances from a prototype

Singleton: Factory for a singular (sole) instance.

Structural Patterns: a Catalog

Purpose: Describe ways to construct and compose objects to realize new functionality

Adapter: Translator adapts a server interface for a client

Bridge: Abstraction for binding one of many implementations

Composite: Structure for building recursive aggregations

Decorator: Decorator extends an object transparently

Facade: Simplifies the interface for a subsystem

Flyweight: Many fine-grained objects shared efficiently.

Proxy: One object approximates another

Behavioral Patterns: a Catalog

Purpose: Distribute the responsibility among objects and define the protocol for object and class interactions.

Chain of Responsibility: Request delegated to the responsible service provider

Command: Request or Action is first-class object, hence re-storable

Iterator: Aggregate and access elements sequentially

Interpreter: Language interpreter for a small grammar

Mediator: Coordinates interactions between its associates

Memento: Snapshot captures and restores object states privately

Observer: Dependents update automatically when subject changes

State: Object whose behavior depends on its state

Strategy: Abstraction for selecting one of many algorithms

Template Method: Algorithm with some steps supplied by a derived class

Visitor: Operations applied to elements of a heterogeneous object structure

Design Pattern: State and Strategy

Intend

Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

Motivation

An object’s behavior depends on its state
According to different classifications at run-time
Ability to change to another state with ease

Example – Membership Classification

Provides different behaviors/actions for different members
Customers can be treated differently due to memberships
Customers can be easily changed to another membership

State Interface

Implemented by concrete classes
Different Handle() functions

Context Class

Maintains a State
Aggregation link
Different Request() functions
state.handle()
Implemented by different ConcreteState
state object can be changed.

Example1 - Membership State:

Loyalty Customer has 3 Membership Types

Green (New Customer) - No discount (pays 100%) - Deposit (needs to pay 100% upfront)

Silver (After spending >$5,000, upgraded) - 20% discounts (pays 80%) - Deposit (only need to pay 50% upfront)

Gold (After spending >$10,000, upgraded) - 50% discounts (pays 50%) - Deposit (do not need to pay, 0% upfront)

=> =>

Example 2 – Library Members:

Members pay for a penalty for late-returns

Senior, Adult, Child

Problem

A Child will grow up
A Adult will grow old
Needs to change membership

Solution:

Summary:

　　Benefits

Consolidate
- Puts all behavior associated with a state into 1 object
Consistent
- Allows state transition logic into a state rather than many if/then switch statements for different logics
Allows state changes
- Avoids inconsistent states changes to occur and complexity involved

　　Drawback

The number of objects increased.
Might need to use with Singleton pattern to save the number of objects being created

Design Pattern: Strategy

(Similar to State Pattern, but it has only 1 function in each state.)

Intend

Define a family of algorithms, encapsulate each one, and make them interchangeable.
Provides more independence for clients to use it.

Motivation

An object’s behavior depends on its choice
Need different variants of an algorithm
Difference to State Pattern
Only 1 function in State

Example:

A class wants to decide at run-time what algorithm it should use to sort an array. Many different sort algorithms are readily available.

Encapsulate different sorting algorithms using the Strategy pattern.

Strategy has 1 (one) function in each state, sort().

State vs Strategy Pattern

Task Handling

Strategy Pattern handles a single and specific task (sort ())
State Pattern handles many tasks (discount(), deposit()..etc)

Object Handling

Strategies are passed to the context object (e.g..to SortArray as parameters)
States are created by the context object

Client Perspective

Both solutions are very similar.

Design Pattern: Singleton

Motivation/when to use

Need to have ONLY one(1) instance for a class.
Singleton instantiates itself, and only one(1) of such.

Intent

Ensure that only 1 object instance is ever created.
Provide a global point of access.
Save the number of duplicated objects being created. Such as in State Pattern.

Implementation

Variable instance (refers to itself, initialized before constructor is called).
Constructor > private(-) Constructor (non-accessible)
Method > getInstance() (returns the single instance)

class Singleton {
    public: 
        static Singleton* getInstance(); 
    protected: 
        Singleton(); 
        Singleton(const Singleton&); 
        Singleton& operator = (const Singleton&); 
    private:
        static Singleton* instance; 
}; 
Singleton *p2 = Singleton::getInstance();

Caller/Client Usage:

Singleton.getInstance().method_x();
Ensure that only one(1) instance is created.
Constructor is private, not accessible.
Only way of instantiating the class is through the method getInstance();
Provides a global point of access.

Example - President:

President Case study – Using Singleton Design Pattern

To enforce that: There is Only 1 President at any time.
There are many (n) Ministers need to contact the president.

Pointing to one (1), single object:

President p1 = President.getInstance (); //refer to the same object
President p2 = President.getInstance (); //refer to the same object

To validate that it only produce 1 single object

Design:

We have 1 president and many Minsters calling this 1 president object.
Use a counter to keep track how many the “same” one object being called.

Result:

Singleton vs Non_Singleton:

Summary:

Used to ensure that a class has only one instance. For example, one printer spooler object, one file system, one window manager, etc.
One may use a global variable to access an object but it does not prevent one from creating more than one instance, Singleton prevents creating more than 1 object instance.
Instead the class itself is made responsible for keeping track of its instance. It can thus ensure that no more than one instance is created. This is the singleton pattern.

Design Pattern: Factory Method

Motivation/when to use

Also known as “Virtual Constructor”
Defines an interface for creating an object
Leaves the “choice” of its type to the subclasses
Creation of object to be determined at run-time.

Intent

Defines an interface for creating object
Let subclasses to decide which class to create.
Provides the newly created object through a common interface

Implementation

Product (interface) ConcreteProduct implements the Product interface
Factory creates/generate Product object via factoryMethod()
ConcreteFactory implements and produces ConcreteProduct object.

Example - Document Application:

Document class that needs to be created is specific to the application at run-time by user. So it doesn’t know which class to use in advance.
Factory Method design pattern solves this problem

Participants and Communication

•Product (Document): Defines the interface of objects the factory method creates.

•ConcreteProduct (MyDocument, HtmlDocument, PdfDocument): Implements the Product interface.

•Creator (Application): Declares factory method which returns an object of type Product(Document). Also, may define the factory method to create a Product(Document) object.

•ConcreteCreator (DocumentFactory): Extends and overrides the factory method to return an object instance of ConcreteProduct (MyDocument, HtmlDocument, PdfDocument) upon request.