添加观察者模式

23种设计模式/观察者模式的概念及Java实现.md

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+​		定义对象之间的一对多依赖，当一个对象状态改变时，它的所有依赖都会收到通知并且自动更新状态。
+
+​		主题（Subject）是被观察的对象，而其所有依赖者（Observer）称为观察者。
+
+**优点：**
+
+​		在观察者与被观察者之间建立了一套时间触发机制，以降低二者之间的耦合度。
+
+<img src="https://gitee.com/venture_git/PicGo-CloudImg/raw/master/img/20210710185334.jpeg" alt="img" style="zoom:67%;" />
+
+​		主题（Subject）具有注册和移除观察者、并通知所有观察者的功能，主题是通过维护一张观察者列表来实现这些操作的。
+
+​		观察者（Observer）的注册功能需要调用主题的 registerObserver() 方法。
+
+![img](https://gitee.com/venture_git/PicGo-CloudImg/raw/master/img/20210710185424.png)
+
+天气数据布告板会在天气信息发生改变时更新其内容，布告板有多个，并且在将来会继续增加。
+
+![img](https://gitee.com/venture_git/PicGo-CloudImg/raw/master/img/20210710185507.jpeg)
+
+
+
+```java
+public interface Subject {
+    void registerObserver(Observer o);
+
+    void removeObserver(Observer o);
+
+    void notifyObserver();
+}
+```
+
+```java
+public class WeatherData implements Subject {
+    private List<Observer> observers;
+    private float temperature;
+    private float humidity;
+    private float pressure;
+
+    public WeatherData() {
+        observers = new ArrayList<>();
+    }
+
+    public void setMeasurements(float temperature, float humidity, float pressure) {
+        this.temperature = temperature;
+        this.humidity = humidity;
+        this.pressure = pressure;
+        notifyObserver();
+    }
+
+    @Override
+    public void registerObserver(Observer o) {
+        observers.add(o);
+    }
+
+    @Override
+    public void removeObserver(Observer o) {
+        int i = observers.indexOf(o);
+        if (i >= 0) {
+            observers.remove(i);
+        }
+    }
+
+    @Override
+    public void notifyObserver() {
+        for (Observer o : observers) {
+            o.update(temperature, humidity, pressure);
+        }
+    }
+}
+```
+
+```java
+public interface Observer {
+    void update(float temp, float humidity, float pressure);
+}
+```
+
+```java
+public class StatisticsDisplay implements Observer {
+
+    public StatisticsDisplay(Subject weatherData) {
+        weatherData.registerObserver(this);
+    }
+
+    @Override
+    public void update(float temp, float humidity, float pressure) {
+        System.out.println("StatisticsDisplay.update: " + temp + " " + humidity + " " + pressure);
+    }
+}
+```
+
+```java
+public class CurrentConditionsDisplay implements Observer {
+
+    public CurrentConditionsDisplay(Subject weatherData) {
+        weatherData.registerObserver(this);
+    }
+
+    @Override
+    public void update(float temp, float humidity, float pressure) {
+        System.out.println("CurrentConditionsDisplay.update: " + temp + " " + humidity + " " + pressure);
+    }
+}
+```
+
+```java
+public class WeatherStation {
+    public static void main(String[] args) {
+        WeatherData weatherData = new WeatherData();
+        CurrentConditionsDisplay currentConditionsDisplay = new CurrentConditionsDisplay(weatherData);
+        StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);
+
+        weatherData.setMeasurements(0, 0, 0);
+        weatherData.setMeasurements(1, 1, 1);
+    }
+}
+```
+
+```html
+CurrentConditionsDisplay.update: 0.0 0.0 0.0
+StatisticsDisplay.update: 0.0 0.0 0.0
+CurrentConditionsDisplay.update: 1.0 1.0 1.0
+StatisticsDisplay.update: 1.0 1.0 1.0
+```
+
+### JDK
+
+- [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)
+