Command Pattern

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Hello ! I am Nelson Suwal

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Apprentice

Jyaasa Technologies

What is Command Pattern? The command pattern is a behavioral design pattern in which an object is used to encapsulate all information needed to perform an action or trigger an event at a later time.

Four terms always associated with the command pattern:

● Command● Receiver● Invoker● Client

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Diagram:

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Example:

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# Invokerclass Switch attr_reader :history

def execute(cmd) @history ||= [] @history << cmd.execute endend

# Command Interfaceclass Command attr_reader :light

def initialize(light) @light = light end def execute raise NotImplementedError endend

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# Command for turning onclass TurnOnCommand < Command def execute light.turn_on endend

# Command for turning offclass TurnOffCommand < Command def execute light.turn_off endend

# Receiverclass Light def turn_on 'the light is on' end

def turn_off 'the light is off' endend

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# Clientclass LightSwitchClient attr_reader :switch

def initialize @lamp = Light.new @switch = Switch.new end

def switch_for(cmd) case cmd when 'on' then @switch.execute(TurnOnCommand.new(@lamp)) when 'off' then @switch.execute(TurnOffCommand.new(@lamp)) else puts 'Sorry, I so sorry' end endend

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client = LightSwitchClient.newputs client.switch_for('on') puts client.switch_for('off')puts client.switch.history

#output :'the light is on'#output :'the light is off'#output :['the light is on',the light is off']

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Resources:

● https://en.wikipedia.org/wiki/Command_pattern● https://sourcemaking.com/design_patterns/command● http://www.oodesign.com/command-pattern.html

http://jyaasa.comCopyright 2016. Jyaasa Technologies.

http://jyaasa.comCopyright 2016. Jyaasa Technologies.

Thank you !