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notes/设计模式.md

@@ -610,6 +610,131 @@ public class Client {
 
 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。
+
+<div align="center"> <img src="../pics//7a3c6a30-c735-4edb-8115-337288a4f0f2.jpg" width="600"/> </div><br>
+
+### 类图
+
+主题（Subject）具有注册和移除观察者、并通知所有注册者的功能，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。
+
+<div align="center"> <img src="../pics//48a934ff-a29b-434c-8e1d-8c8ec20cb91d.png"/> </div><br>
+
+### 实现
+
+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。
+
+<div align="center"> <img src="../pics//b1df9732-86ce-4d69-9f06-fba1db7b3b5a.jpg"/> </div><br>
+
+```java
+public interface Subject {
+    void resisterObserver(Observer o);
+
+    void removeObserver(Observer o);
+
+    void notifyObserver();
+}
+```
+
+```java
+import java.util.ArrayList;
+import java.util.List;
+
+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 resisterObserver(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.resisterObserver(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.resisterObserver(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)