45 lines
1.7 KiB
Markdown
45 lines
1.7 KiB
Markdown
---
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layout: pattern
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title: Observer
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folder: observer
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permalink: /patterns/observer/
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categories: Behavioral
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tags:
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- Gang Of Four
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- Reactive
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---
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## Also known as
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Dependents, Publish-Subscribe
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## Intent
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Define a one-to-many dependency between objects so that when one
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object changes state, all its dependents are notified and updated
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automatically.
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## Class diagram
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## Applicability
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Use the Observer pattern in any of the following situations
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* When an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently
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* When a change to one object requires changing others, and you don't know how many objects need to be changed
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* When an object should be able to notify other objects without making assumptions about who these objects are. In other words, you don't want these objects tightly coupled
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## Typical Use Case
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* Changing in one object leads to a change in other objects
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## Real world examples
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* [java.util.Observer](http://docs.oracle.com/javase/8/docs/api/java/util/Observer.html)
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* [java.util.EventListener](http://docs.oracle.com/javase/8/docs/api/java/util/EventListener.html)
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* [javax.servlet.http.HttpSessionBindingListener](http://docs.oracle.com/javaee/7/api/javax/servlet/http/HttpSessionBindingListener.html)
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* [RxJava](https://github.com/ReactiveX/RxJava)
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## Credits
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* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
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* [Java Generics and Collections](http://www.amazon.com/Java-Generics-Collections-Maurice-Naftalin/dp/0596527756/)
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