5cf2fe009b
* Use lombok, reformat, and optimize the code * Fix merge conflicts and some sonar issues Co-authored-by: va1m <va1m@email.com>
157 lines
5.1 KiB
Markdown
157 lines
5.1 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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## 又被称为
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家属，发布订阅模式
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## 目的
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定义一种一对多的对象依赖关系这样当一个对象改变状态时，所有依赖它的对象都将自动通知或更新。
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## 解释
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真实世界例子
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> 在遥远的土地上生活着霍比特人和兽人的种族。他们都是户外生活的人所以他们密切关注天气的变化。可以说他们不断地关注着天气。
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通俗的说
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> 注册成为一个观察者以接收对象状态的改变。
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维基百科说
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> 观察者模式是这样的一种软件设计模式：它有一个被称为主题的对象，维护着一个所有依赖于它的依赖者清单，也就是观察者清单，当主题的状态发生改变时，主题通常会调用观察者的方法来自动通知观察者们。
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**编程示例**
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让我们先来介绍天气观察者的接口以及我们的种族，兽人和霍比特人。
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```java
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public interface WeatherObserver {
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void update(WeatherType currentWeather);
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}
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@Slf4j
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public class Orcs implements WeatherObserver {
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@Override
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public void update(WeatherType currentWeather) {
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LOGGER.info("The orcs are facing " + currentWeather.getDescription() + " weather now");
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}
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}
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@Slf4j
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public class Hobbits implements WeatherObserver {
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@Override
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public void update(WeatherType currentWeather) {
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switch (currentWeather) {
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LOGGER.info("The hobbits are facing " + currentWeather.getDescription() + " weather now");
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}
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}
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```
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然后这里是不断变化的天气。
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```java
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@Slf4j
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public class Weather {
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private WeatherType currentWeather;
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private final List<WeatherObserver> observers;
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public Weather() {
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observers = new ArrayList<>();
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currentWeather = WeatherType.SUNNY;
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}
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public void addObserver(WeatherObserver obs) {
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observers.add(obs);
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}
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public void removeObserver(WeatherObserver obs) {
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observers.remove(obs);
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}
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/**
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* Makes time pass for weather.
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*/
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public void timePasses() {
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var enumValues = WeatherType.values();
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currentWeather = enumValues[(currentWeather.ordinal() + 1) % enumValues.length];
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LOGGER.info("The weather changed to {}.", currentWeather);
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notifyObservers();
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}
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private void notifyObservers() {
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for (var obs : observers) {
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obs.update(currentWeather);
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}
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}
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}
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```
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这是完整的示例。
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```java
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var weather = new Weather();
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weather.addObserver(new Orcs());
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weather.addObserver(new Hobbits());
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weather.timePasses();
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// The weather changed to rainy.
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// The orcs are facing rainy weather now
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// The hobbits are facing rainy weather now
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weather.timePasses();
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// The weather changed to windy.
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// The orcs are facing windy weather now
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// The hobbits are facing windy weather now
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weather.timePasses();
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// The weather changed to cold.
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// The orcs are facing cold weather now
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// The hobbits are facing cold weather now
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weather.timePasses();
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// The weather changed to sunny.
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// The orcs are facing sunny weather now
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// The hobbits are facing sunny weather now
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```
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## Class diagram
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## 应用
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在下面任何一种情况下都可以使用观察者模式
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* 当抽象具有两个方面时，一个方面依赖于另一个方面。将这些方面封装在单独的对象中，可以使你分别进行更改和重用
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* 当一个对象的改变的同时需要改变其他对象，同时你又不知道有多少对象需要改变时
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* 当一个对象可以通知其他对象而无需假设这些对象是谁时。换句话说，你不想让这些对象紧耦合。
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## 典型用例
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* 一个对象的改变导致其他对象的改变
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## Java中的例子
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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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## 鸣谢
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* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
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* [Java Generics and Collections](https://www.amazon.com/gp/product/0596527756/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596527756&linkCode=as2&tag=javadesignpat-20&linkId=246e5e2c26fe1c3ada6a70b15afcb195)
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* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
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* [Refactoring to Patterns](https://www.amazon.com/gp/product/0321213351/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321213351&linkCode=as2&tag=javadesignpat-20&linkId=2a76fcb387234bc71b1c61150b3cc3a7)
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