135 lines
3.9 KiB
Markdown
135 lines
3.9 KiB
Markdown
## 7. 观察者（Observer）
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### Intent
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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。
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主题（Subject）是被观察的对象，而其所有依赖者（Observer）称为观察者。
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<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/7a3c6a30-c735-4edb-8115-337288a4f0f2.jpg" width="600"/> </div><br>
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### Class Diagram
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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。
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观察者（Observer）的注册功能需要调用主题的 registerObserver() 方法。
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<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/a8c8f894-a712-447c-9906-5caef6a016e3.png"/> </div><br>
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### Implementation
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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。
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<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/b1df9732-86ce-4d69-9f06-fba1db7b3b5a.jpg"/> </div><br>
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```java
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public interface Subject {
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void registerObserver(Observer o);
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void removeObserver(Observer o);
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void notifyObserver();
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}
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```
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```java
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public class WeatherData implements Subject {
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private List<Observer> observers;
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private float temperature;
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private float humidity;
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private float pressure;
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public WeatherData() {
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observers = new ArrayList<>();
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}
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public void setMeasurements(float temperature, float humidity, float pressure) {
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this.temperature = temperature;
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this.humidity = humidity;
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this.pressure = pressure;
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notifyObserver();
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}
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@Override
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public void registerObserver(Observer o) {
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observers.add(o);
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}
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@Override
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public void removeObserver(Observer o) {
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int i = observers.indexOf(o);
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if (i >= 0) {
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observers.remove(i);
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}
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}
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@Override
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public void notifyObserver() {
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for (Observer o : observers) {
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o.update(temperature, humidity, pressure);
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}
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}
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}
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```
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```java
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public interface Observer {
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void update(float temp, float humidity, float pressure);
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}
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```
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```java
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public class StatisticsDisplay implements Observer {
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public StatisticsDisplay(Subject weatherData) {
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weatherData.registerObserver(this);
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}
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@Override
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public void update(float temp, float humidity, float pressure) {
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System.out.println("StatisticsDisplay.update: " + temp + " " + humidity + " " + pressure);
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}
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}
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```
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```java
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public class CurrentConditionsDisplay implements Observer {
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public CurrentConditionsDisplay(Subject weatherData) {
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weatherData.registerObserver(this);
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}
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@Override
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public void update(float temp, float humidity, float pressure) {
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System.out.println("CurrentConditionsDisplay.update: " + temp + " " + humidity + " " + pressure);
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}
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}
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```
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```java
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public class WeatherStation {
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public static void main(String[] args) {
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WeatherData weatherData = new WeatherData();
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CurrentConditionsDisplay currentConditionsDisplay = new CurrentConditionsDisplay(weatherData);
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StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);
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weatherData.setMeasurements(0, 0, 0);
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weatherData.setMeasurements(1, 1, 1);
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}
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}
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```
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```html
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CurrentConditionsDisplay.update: 0.0 0.0 0.0
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StatisticsDisplay.update: 0.0 0.0 0.0
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CurrentConditionsDisplay.update: 1.0 1.0 1.0
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StatisticsDisplay.update: 1.0 1.0 1.0
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```
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### JDK
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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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