## 7. 观察者（Observer）
### Intent
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。
### Class Diagram
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。
### Implementation
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。
```java
public interface Subject {
void registerObserver(Observer o);
void removeObserver(Observer o);
void notifyObserver();
}
```
```java
public class WeatherData implements Subject {
private List 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)