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---
layout: pattern
title: Observer
folder: observer
permalink: /patterns/observer/
categories: Behavioral
tags:
- Gang Of Four
- Reactive
---
## Also known as
## 又被称为
家属，发布订阅模式
## 目的
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。
## 解释
真实世界例子
> 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。
通俗的说
> 注册成为一个观察者以接收对象状态的改变。
维基百科说
> 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。
**编程示例**
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。
```java
public interface WeatherObserver {
void update(WeatherType currentWeather);
}
public class Orcs implements WeatherObserver {
private static final Logger LOGGER = LoggerFactory.getLogger(Orcs.class);
@Override
public void update(WeatherType currentWeather) {
LOGGER.info("The orcs are facing " + currentWeather.getDescription() + " weather now");
}
}
public class Hobbits implements WeatherObserver {
private static final Logger LOGGER = LoggerFactory.getLogger(Hobbits.class);
@Override
public void update(WeatherType currentWeather) {
switch (currentWeather) {
LOGGER.info("The hobbits are facing " + currentWeather.getDescription() + " weather now");
}
}
```
然后这里是不断变化的天气。
```java
public class Weather {
private static final Logger LOGGER = LoggerFactory.getLogger(Weather.class);
private WeatherType currentWeather;
private final List<WeatherObserver> observers;
public Weather() {
observers = new ArrayList<>();
currentWeather = WeatherType.SUNNY;
}
public void addObserver(WeatherObserver obs) {
observers.add(obs);
}
public void removeObserver(WeatherObserver obs) {
observers.remove(obs);
}
/**
* Makes time pass for weather.
*/
public void timePasses() {
var enumValues = WeatherType.values();
currentWeather = enumValues[(currentWeather.ordinal() + 1) % enumValues.length];
LOGGER.info("The weather changed to {}.", currentWeather);
notifyObservers();
}
private void notifyObservers() {
for (var obs : observers) {
obs.update(currentWeather);
}
}
}
```
这是完整的示例。
```java
var weather = new Weather();
weather.addObserver(new Orcs());
weather.addObserver(new Hobbits());
weather.timePasses();
// The weather changed to rainy.
// The orcs are facing rainy weather now
// The hobbits are facing rainy weather now
weather.timePasses();
// The weather changed to windy.
// The orcs are facing windy weather now
// The hobbits are facing windy weather now
weather.timePasses();
// The weather changed to cold.
// The orcs are facing cold weather now
// The hobbits are facing cold weather now
weather.timePasses();
// The weather changed to sunny.
// The orcs are facing sunny weather now
// The hobbits are facing sunny weather now
```
## Class diagram
![alt text](../../observer/etc/observer.png "Observer")
## 应用
在下面任何一种情况下都可以使用观察者模式
* 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
* 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
* 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。
## 典型用例
* 一个对象的改变导致其他对象的改变
## Java中的例子
* [java.util.Observer](http://docs.oracle.com/javase/8/docs/api/java/util/Observer.html)
* [java.util.EventListener](http://docs.oracle.com/javase/8/docs/api/java/util/EventListener.html)
* [javax.servlet.http.HttpSessionBindingListener](http://docs.oracle.com/javaee/7/api/javax/servlet/http/HttpSessionBindingListener.html)
* [RxJava](https://github.com/ReactiveX/RxJava)
## 鸣谢
* [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)
* [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)
* [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)
* [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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---
layout: pattern
title: Strategy
folder: strategy
permalink: /patterns/strategy/
categories: Behavioral
tags:
- Gang of Four
---
## 又被称为
政策（方针）模式
## 目的
定义一个家族算法，并封装好其中每一个，使它们可以互相替换。策略模式使算法的变化独立于使用它的客户。
## 解释
现实世界例子
> 屠龙是一项危险的职业。有经验将会使它变得简单。经验丰富的屠龙者对不同类型的龙有不同的战斗策略。
直白点说
> 策略模式允许在运行时选择最匹配的算法。
维基百科上说
> 在程序编程领域，策略模式（又叫政策模式）是一种启用在运行时选择算法的行为型软件设计模式。
**编程实例**
让我们先介绍屠龙的策略模式接口和它的实现。
```java
@FunctionalInterface
public interface DragonSlayingStrategy {
void execute();
}
public class MeleeStrategy implements DragonSlayingStrategy {
private static final Logger LOGGER = LoggerFactory.getLogger(MeleeStrategy.class);
@Override
public void execute() {
LOGGER.info("With your Excalibur you sever the dragon's head!");
}
}
public class ProjectileStrategy implements DragonSlayingStrategy {
private static final Logger LOGGER = LoggerFactory.getLogger(ProjectileStrategy.class);
@Override
public void execute() {
LOGGER.info("You shoot the dragon with the magical crossbow and it falls dead on the ground!");
}
}
public class SpellStrategy implements DragonSlayingStrategy {
private static final Logger LOGGER = LoggerFactory.getLogger(SpellStrategy.class);
@Override
public void execute() {
LOGGER.info("You cast the spell of disintegration and the dragon vaporizes in a pile of dust!");
}
}
```
现在有一个强力的屠龙者要基于上面的组件来选择他的战斗策略。
```java
public class DragonSlayer {
private DragonSlayingStrategy strategy;
public DragonSlayer(DragonSlayingStrategy strategy) {
this.strategy = strategy;
}
public void changeStrategy(DragonSlayingStrategy strategy) {
this.strategy = strategy;
}
public void goToBattle() {
strategy.execute();
}
}
```
最后是屠龙者的行动。
```java
LOGGER.info("Green dragon spotted ahead!");
var dragonSlayer = new DragonSlayer(new MeleeStrategy());
dragonSlayer.goToBattle();
LOGGER.info("Red dragon emerges.");
dragonSlayer.changeStrategy(new ProjectileStrategy());
dragonSlayer.goToBattle();
LOGGER.info("Black dragon lands before you.");
dragonSlayer.changeStrategy(new SpellStrategy());
dragonSlayer.goToBattle();
// Green dragon spotted ahead!
// With your Excalibur you sever the dragon's head!
// Red dragon emerges.
// You shoot the dragon with the magical crossbow and it falls dead on the ground!
// Black dragon lands before you.
// You cast the spell of disintegration and the dragon vaporizes in a pile of dust!
```
## 类图
![alt text](../../strategy/etc/strategy_1.png "Strategy")
## 应用
使用策略模式当
* 许多相关的类只是行为不同。策略模式提供了一种为一种类配置多种行为的能力。
* 你需要一种算法的不同变体。比如，你可能定义反应不用时间空间权衡的算法。当这些算法的变体使用类的层次结构来实现时就可以使用策略模式。
* 一个算法使用的数据客户不应该对其知晓。使用策略模式来避免暴露复杂的，特定于算法的数据结构。
* 一个类定义了许多行为，这些行为在其操作中展现为多个条件语句。移动相关的条件分支到它们分别的策略类中来代替这些条件语句。
## 教学
* [Strategy Pattern Tutorial](https://www.journaldev.com/1754/strategy-design-pattern-in-java-example-tutorial)
## 鸣谢
* [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)
* [Functional Programming in Java: Harnessing the Power of Java 8 Lambda Expressions](https://www.amazon.com/gp/product/1937785467/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1937785467&linkCode=as2&tag=javadesignpat-20&linkId=7e4e2fb7a141631491534255252fd08b)
* [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)
* [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)