observer pattern

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Observer PatternAlbert Guo

junyuo@gmail.com

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Motivation Intent Implementation Applicability & Examples

Agenda

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Let's assume we have a stock system which provides data for several types of client. We want to have a client implemented as a web based application but in near future we need to add clients for mobile devices, Palm or Pocket PC, or to have a system to notify the users with sms alerts. Now it's simple to see what we need from the observer pattern: we need to separate the subject(stocks server) from it's observers(client applications) in such a way that adding new observer will be transparent for the server.

Motivation

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

Intent

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Implementation

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Class/Interface Description

Subject interface or abstract class defining the operations for attaching and de-attaching observers to the client.

ConcreteSubject It maintain the state of the object and when a change in the state occurs it notifies the attached Observers.

Observer interface or abstract class defining the operations to be used to notify this object.

ConcreteObserver concrete Observer implementations.

Implementation – cont.

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Applicability & Examples

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Applicability & Examples – cont.

Subject interface or abstract class defining the operations for attaching and de-attaching observers to the client.

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Applicability & Examples – cont.

ConcreteSubject

It maintain the state of the object and when a change in the state occurs it notifies the attached Observers.

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Applicability & Examples – cont.

Observer interface or abstract class defining the operations to be used to notify this object.

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Applicability & Examples – cont.

ConcreteObserver

concrete Observer implementations.

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Applicability & Examples – cont.

ConcreteObserver concrete Observer implementations.

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Applicability & Examples – cont.

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