designpatterns-09.pdf

7/29/2019 designPatterns-09.pdf

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The Command Pattern

CSCI 3132 Summer 20111


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Example:RemoteControl

Givenremotecontrolwithsevenprogrammableslots.

Adifferentdevicecanbeputintoeachslot. ThereisanOnandOffswitchforeachdeviceslot. GlobalUndobuBonundoesthelastbuBonpressed. AlsogivenaCDwithdifferentvendorclassesthat

havealreadybeenwriBen(forthedifferentdevices,

TV,Light,Sprinkler,etc)

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FirstThoughts

Weknowthattherearesevenprogrammableslotsfordifferentdevicessoeachdevicemaypossibly

adheretosomecommoninterface.

Weknowthatweneedtoturneachdeviceonoroff..sothatneedstobecommonlydoneforany

device.

Undoneedstoworkforanydeviceaswell.

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WhatVaries?Whatstaysthesame?

WhatVariesTheactualdeviceassignedtoaslotTheinstruconforOnonaspecificdeviceTheinstruconforOffonaspecificdevice

WhatStaystheSameDevicewithsevenslotsCapabilitytoassignaslottoadeviceCapabilitytorequestthatadeviceturnOnorOffCapabilitytoundothelastaconrequestedagainst

thedevice

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TheVendorClasses

VendorclasseshavebeenprovidedtousviaaCD.CeilingLightTV

HoBub

WeknowthateachdeviceneedsanOnorOffstate.

SincethecapabilitytoturnadeviceOnorOffissomethingthatstaysthesame.

However,eachvendorclasshastheirownuniquewayofdoingOnorOff.

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Onepossiblesoluon

if (slot1 == Light)

light.on();

Else if (slot1 == Hottub) {

hottub.prepareJets();

hottub.jetsOn();

} Else if (slot1 == TV)

tv.on();

//

Problems:

The Remote needs to beaware of all the details aboutturning a device on (or off).

If device On/Off mechanism

changes, the Remote codewill need to be changed.

If a new device is added, this

code would need to bechanged.

Is this Open forExtension??

Alsowhat aboutundo????

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SeparaonofConcerns

TheVendorClass One(ormore)methodsthatdefineOn One(ormore)methodsthatdefineOff

TheCommand Sendsamessagetoadevice(OnorOff) Handletheundoofamessage Possiblefutureenhancements

LoggingrequestQueuerequest

TheRemotehandlesoneormoreCommands.TheRemotedoesntknowanythingabouttheactual

vendorclassspecifics.7


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TheCommandPaBern

Allowsyoutodecoupletherequestoroftheaconfromtheobjectthatperformstheacon.

ACommandobjectencapsulatesarequesttodosomething.

Note:ACommandobjecthandlesasinglerequest. TheCommandinterface(inthisexample)justdoes

onethingexecutesacommand.

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LightOnCommandAcommand

public class LightOnCommand implements Command {

Light light;

public LightOnCommand(Light light) {

this.light = light;

}

public void execute() {

light.on();}

}

The command is

composed of a vendorclass.

Constructor takes the

vendor class asparameter.

Here the command delegates

execution to the vendor class.Could be more steps.

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SimpleRemoteControlAninvoker

public class SimpleRemoteControl {

Command slot;

public SimpleRemoteControl() {}

public void setCommand(Command command) {

slot = command;

}

public void buttonWasPressed() {

slot.execute();

}

}

This version of the

remote just has oneslot.

setCommand assigns

a Command to a slot.

Tells the command to

execute. Note that anycommand would work

here. The remotedoesnt know anything

about the specific

vendor class.10


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Encapsulaon

action()

execute()

{ receiver.action

()

}

execute()

execute()

execute()

An encapsulated Request

Invoker

Command encapsulates a Receiver object, use delegation to thecorresponding device,Different commands can fit into a Remote Slot in the remote control.

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RemoteControlTestAclient

SimpleRemoteControl remote = new SimpleRemoteControl();

Light light = new Light();

GarageDoor garageDoor = new GarageDoor();

LightOnCommand lightOn =

new LightOnCommand(light);

GarageDoorOpenCommand garageOpen =

new GarageDoorOpenCommand(garageDoor);

remote.setCommand(lightOn);remote.buttonWasPressed();

remote.setCommand(garageOpen);

remote.buttonWasPressed();

Create two vendor

classes.

Create two commandsbased on these vendorclasses.

Set the command and press

button

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TheCommandPaBern

GoFIntent:Encapsulatesarequestasanobject,therebyle\ngyouparameterizeotherobjectswithdifferent

requests,queueorlogrequests,andsupportundoable

operaons.

Par(cipants: Client(RemoteControlTest)createscommandandassociatescommand

withreceiver.

Receiver(TV,HotTub,ec)knowshowtoperformthework. ConcreteCommand(LightOnCommand)-implementaonofCommand

interface

CommandInterfacedefinesinterfaceforallcommands. Invoker(RemoteControl)holdsreferencetoacommandandcalls

execute()method

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CommandPaBernClassDiagram

Client Invoker

setCommand()

Command

execute()undo()

Receiver

action()

ConcreteCommand

execute()undo()

ClientcreatesaConcreteCommandandbindsitwithaReceiver. ClienthandstheConcreteCommandovertotheInvokerwhichstores

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TheSequenceDiagram

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ClassDiagramforHomeautomaon

RemoteLoader

RemoteControl

onCommandsoffCommandssetCommand()onButtonPushed()offButtonPushed()

Command

execute()undo()

Light

on()

off()

LightOnCommand

execute()undo()LightOffCommand

execute()undo()

Creates command objects,binds with devices

Invokes execute() method ofthe button command object

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MacroCommandsPartymode

public class MacroCommand implements Command {

Command[] commands;

public MacroCommand(Command[] commands) {

this.commands = commands;

}


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();

}

}

}

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Easyforsomeobjects

public class LightOnCommand implements Command {

Light light;

public LightOnCOmmand(Light light) {

this.light = light;

}


light.on();

}

public void undo() {

light.off();}

}

AddingUndo

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Implemenngtheremotecontrolwithundo.public class RemoteControl {

Command[] on Commands;

Command[] offCommands;

Command[] undoCommands;

public RemoteControl() {onCommands = new Command[7];

offCommands = new Command[7];

Command noCommand = new NoCommand();

for (int i = 0; i < 7; i++) {

onCommands[i] = noCommand;

offCommands[i] = noCommand;}

undoCommand = noCommand;

}

public void setCommand(int slot, Command onCommand, CommandoffCommand) {

onCommand[slot] = onCommand;

offCommand[slot] = offCommand;

}

AddingUndo

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public void onButtonWasPushed(int slot) {

onCommands(slot].execute();

undoCommand = onCommands[slot];

}

public void offButtonWasPushed(int slot) {offCommands(slot].execute();

undoCommand = offCommands[slot];

}

public void undoButtonwWasPushed() {

undoCommand.undo();

}

//

}

AddingUndo

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public class CeilingFan {

public static final int HIGH = 3;public static final int MEDIUM = 2;

public static final int LOW= 1;

public static final int OFF= 0;

public CeilingFan(String location) {

this.location = location;

speed = OFF;}

public void high() {

speed = HIGH;

//code to set fan to high

}

public void medium() {

speed = MEDIUM;//code to set fan to medium

}

//

public void off() {

speed = OFF;

//code to turn fan off

}

AddingUndoForCeilingFan

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public class CeilingFanHighCommand implements Command {CeilingFan ceilingFan;

int prevSpeed;

public CeilingFanHighCommand(CeilingFan ceilingFan) {

this.ceilingFan = ceilingFan;

}

public void execute() {prevSpeed = ceilingFan.getSpeed();

ceilingFan.high();

}

public void undo() {

if (prevSpeed == CeilingFan.HIGH) {

celingFan.high();

} else if (prevSpeed = CeilingFan.MEDIUM) {

ceilingFan.medium();

} else if (prevSpeed = CeilingFan.LOW) {

ceilingFan.low();

} else if (prevSpeed = CeilingFan.OFF) {

ceilingFan.off();}}

AddingUndoForCeilingFan

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HistoryofUndoOperaons

IftheUndobuBonispressedmulplemes,wewanttoundoeachcommandthathadbeenpreviouslyapplied.

Whichobjectshouldweenhancetostoreahistoryofeachcommandapplied?Why?

Client(RemoteControlTest) Receiver(TV,DVD,etc) ConcreteCommand(LightOnCommand,LightOffCommand,

etc)

Invoker(RemoteControl) Weneedtobeabletoaddcommandstoalistandthenlater

getthemostrecentone.Whatkindofobjectcanweuse?

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HistoryofUndoOperaonspublic class RemoteControl {

Stack undoStack; //this gets initialized in

//constructor.

public void onButtonWasPushed(int slot) {

onCommands[slot].execute();

undoStack.push(onCommands[slot]);

}

public void offButtonWasPushed(int slot) {

offCommands[slot].execute();

undoStack.push(offCommands[slot]);

}

public void undoButtonWasPushed() {

Command c = undoStack.pop();

c.undo();

} 24


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SimpleLogging

WewanttoenhancetheRemoteControlagaintologeverymeaCommandisexecuted(onoroff).

Whichobjectshouldweenhance??

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SimpleLogging

ChangestoRemoteControl public void onButtonWasPushed(int slot) {

onCommands[slot].execute();

//Log here

}


offCommands[slot].execute();

//Log here

}

Advantage: We can add logging in the Invoker. Nochange is needed in any of the Command or Receiverobjects! 26


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ComplexLogging

Letssaywehadaspreadsheetapplicaonandweknowit

maycrashoen.

Forfailurerecovery,wecouldperiodicallystoreabackupof

thespreadsheetevery5minutes...orwecould

periodicallypersistthelistofcommandsexecutedonthespreadsheetsincethelastsavepoint.

Whenacrashoccurs,weloadthelastsaveddocumentand

re-applythepersistedcommands.

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public void onButtonWasPushed(int slot) {onCommands[slot].execute();

StoreToDisk(onCommands[slot]);

}


offCommands[slot].execute();StoreToDisk(offCommands[slot]);

}

public void SaveButtonPressed() {

//Delete stored commands from disk.

}

public void RestoreCommands() {

//load last saved state

Commands[] storedCommands = GetCommandsFromDisk();

//for each Command, call execute()

}

ComplexLogging

Once again,we can makethese changesin one place(the Invoker)

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Summarysofar..

OOBasics Abstracon Encapsulaon Inheritance Polymorphism

OOPrinciples Encapsulatewhatvaries Favorcomposionoverinheritance Programtointerfacesnottoimplementaons

Striveforlooselycoupleddesignsbetweenobjectsthatinteract

Classesshouldbeopenforextensionbutclosedformodificaon.

Dependonabstracts.Donotdependonconcreteclasses.29

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