题目
介绍下观察者模式
答案
观察者模式（Observer Pattern）定义对象间的一种一对多依赖关系，当“被观察者”状态改变时，所有“观察者”都会收到通知并自动更新。
适用场景:
- 实现事件订阅/广播机制。
- 实时更新界面、通知机制、消息推送等。
Java 示例：天气预报
```java
// 观察者接口
interface Observer {
void update(float temperature);
}
// 主题接口
interface Subject {
void register(Observer o);
void remove(Observer o);
void notifyObservers();
}
// 具体主题
class WeatherData implements Subject {
private List<Observer> observers = new ArrayList<>();
private float temperature;
public void register(Observer o) { observers.add(o); }
public void remove(Observer o) { observers.remove(o); }
public void setTemperature(float temperature) {
this.temperature = temperature;
notifyObservers();
}
public void notifyObservers() {
for (Observer o : observers) {
o.update(temperature);
}
}
}
// 具体观察者
class PhoneDisplay implements Observer {
private String name;
public PhoneDisplay(String name) {
this.name = name;
}
public void update(float temperature) {
System.out.println(name + " receives temperature update: " + temperature);
}
}
// 测试
public class ObserverTest {
public static void main(String[] args) {
WeatherData weather = new WeatherData();
Observer phone1 = new PhoneDisplay("Phone1");
Observer phone2 = new PhoneDisplay("Phone2");
weather.register(phone1);
weather.register(phone2);
weather.setTemperature(25.0f);
weather.setTemperature(30.0f);
}
}
```