27 lines
1.0 KiB
Markdown
27 lines
1.0 KiB
Markdown
|
---
|
||
|
|
layout: pattern
|
||
|
|
title: Observer
|
||
|
|
folder: observer
|
||
|
|
categories: pattern_cat
|
||
|
|
tags: pattern_tag
|
||
|
|
---
|
||
|
|
|
||
|
|
**Intent:** Define a one-to-many dependency between objects so that when one
|
||
|
|
object changes state, all its dependents are notified and updated
|
||
|
|
automatically.
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
**Applicability:** Use the Observer pattern in any of the following situations
|
||
|
|
|
||
|
|
* when an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently
|
||
|
|
* when a change to one object requires changing others, and you don't know how many objects need to be changed
|
||
|
|
* 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
|
||
|
|
|
||
|
|
**Typical Use Case:**
|
||
|
|
|
||
|
|
* changing in one object leads to a change in other objects
|
||
|
|
|
||
|
|
**Real world examples:**
|
||
|
|
|
||
|
|
* [java.util.Observer](http://docs.oracle.com/javase/8/docs/api/java/util/Observer.html)
|