CS 210

Introduction to Design Patterns

September 28th, 2006

Head First Design Patterns

Chapter 6

Command Pattern

Encapsulating Invocation

Motivating problem description

Build a remote that will control variety of home devices

Sample devices: lights, stereo, TV, ceiling light, thermostat, sprinkler, hot tub, garden light, ceiling fan, garage door

Introducing the command pattern

CreateCommandObject

execute()

setCommand

execute()action1action2

creatCommandObject()

setCommand()

execute()

action_X()

Command Pattern for home automation

action()

execute(){ receiver.action()}

execute()

execute()

execute()

An encapsulated Request

Invoker

Command Pattern defined

The Command Pattern encapsulates a request as an object, thereby letting you parameterize other objects with different requests, queue or log requests, and support undoable operations.

Command Pattern Class Diagram

Client Invoker

setCommand()

<<Interface>>Command

execute()undo()

Receiver

action()

ConcreteCommand

execute()undo()

Command Pattern Class Diagram for Home automation

RemoteLoader

RemoteControl

onCommandsoffCommandssetCommand()onButtonPushed()offButtonPushed()

<<Interface>>Command

execute()undo()

Light

on()off()

LightOnCommand

execute()undo()

LightOffCommand

execute()undo()

Command pattern – Undo operation

Eclipse code review

Macro Commands – Party modepublic class MacroCommand implements Command {

Command[] commands; public MacroCommand(Command[] commands) { this.commands = commands;}

public void execute() {for (int i = 0; i < commands.length; i++) {

commands[i].execute();}

}

public void undo() {for (int i = 0; i < commands.length; i++) {

commands[i].undo();}

}}

Macro Command – Party mode

Eclipse code review

Page 228 – Head First Design Patterns

Summary so far.. OO Basics

• Abstraction• Encapsulation• Inheritance• Polymorphism

OO Principles• Encapsulate what varies• Favor composition over inheritance• Program to interfaces not to implementations• Strive for loosely coupled designs between objects that interact• Classes should be open for extension but closed for modification.• Depend on abstracts. Do not depend on concrete classes.

Summary so far… OO Patterns

• Strategy Pattern defines a family of algorithms, Encapsulates each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

• Observer Pattern defines a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically.

• Decorator Pattern – attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative for sub-classing for extending functionality

• Abstractor Factory – Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

• Factory Method – Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory method lets a class defer instantiation to the subclasses.

• Command Pattern – Encapsulates a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.