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POLA-POLA

PERANCANGAN (PPP)

Behavioral pattern: Command

Bahan Kuliah PPP - Command pattern | Tri A. Kurniawan, S.T, M.T, Ph.D 2/20

Tujuan perkuliahan

• Memahami behavioral pattern: Command


Intent and Known As• Intent:

– Encapsulate a request as an object, thereby

letting you parameterize clients with different

requests, queue or log requests, and support

undoable operations

• Know As:

– Action, Transaction


Motivation

• Bad event-handling code what if there is

a new event?


Motivation (1)

• Common UI commands ensure the

code implementing these common

commands is not duplicated


Motivation (2) – remote control


Motivation (3) – goals

• Separate the remote’s ability to execute a

command from the variety of operations

and devices that can be supported

• What varies?

– Devices and operations

• What stays the same?

– The structure of the remote control


Motivation (4) – parts

• The actions that we want each button to

cause (Command)

• Someone to set up those actions (Client)

• The remote control to execute the actions

(Invoker)

• The device affected by the action

(Receiver)


Structure


Participants

• Command– declares an interface for executing an operation

• ConcreteCommand– defines a binding between a Receiver object and an action

– implements Execute by invoking the corresponding operation(s) on Receiver

• Invoker– asks the command to carry out the request

• Receiver– knows how to perform the operations associated with carrying

out a request

– Any class may serve as a Receiver


Collaborations

• The client creates a ConcreteCommand object and specifies its Receiver

• An Invoker object stores the ConcreteCommand object

• The Invoker issues a request by calling Execute() on the Command. When commands are undoable, ConcreteCommand stores state for undoing the command prior to invoking Execute().

• The ConcreteCommand object invokes operations on its Receiver to carry out the request


Consequences

• Command decouples the object that invokes the operation from the one that knows how to perform it

• Commands are first-class objects. They can be manipulated and extended like any other object.

• You can assemble commands into a composite command. In general, composite commands are an instance of the Composite pattern.

• It's easy to add new commands, because you don't have to change existing classes


Sample code


Sample code (1)


Sample code (2)


Sample code (3)


Sample code (4)


Sample code (1)


Sample code (6)


Sample code (7)

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