观察者模式

+------------------+      1       +------------------+
|     Subject      |--------------|     Observer     |
+------------------+  depends on  +------------------+
| + attach(o)      |              | + update()       |
| + detach(o)      |              |                  |
| + notify()       |              |                  |
+------------------+              +------------------+
         ^                                ^
         |                                |
         |                                |
+------------------+           +-----------------------+
| ConcreteSubject  |           |  ConcreteObserver     |
+------------------+           +-----------------------+
| + state          |           | + update()            |
| + getState()     |           | + show updated data   |
| + setState()     |           +-----------------------+
+------------------+

// 被观察对象（Subject）
class NewsPublisher {
    // 保存所有订阅者
    private val subscribers = mutableListOf<Subscriber>()
 
    // 添加订阅者
    fun subscribe(subscriber: Subscriber) {
        subscribers.add(subscriber)
    }
 
    // 移除订阅者
    fun unsubscribe(subscriber: Subscriber) {
        subscribers.remove(subscriber)
    }
 
    // 发布新闻，通知所有订阅者
    fun publish(news: String) {
        for (subscriber in subscribers) {
            subscriber.update(news) // 通知订阅者
        }
    }
}
 
// 观察者（Subscriber 接口）
interface Subscriber {
    fun update(news: String) // 当有新闻时，会触发该方法
}
 
// 具体的观察者 A
class EmailSubscriber(private val name: String) : Subscriber {
    override fun update(news: String) {
        println("$name 收到了新闻：$news")
    }
}
 
// 具体的观察者 B
class SmsSubscriber(private val phoneNumber: String) : Subscriber {
    override fun update(news: String) {
        println("短信 $phoneNumber 收到了新闻：$news")
    }
}
 
// 主函数模拟
fun main() {
    // 创建新闻发布者
    val newsPublisher = NewsPublisher()
 
    // 创建订阅者
    val emailSubscriber = EmailSubscriber("Alice")
    val smsSubscriber = SmsSubscriber("1234567890")
 
    // 订阅新闻
    newsPublisher.subscribe(emailSubscriber)
    newsPublisher.subscribe(smsSubscriber)
 
    // 发布新闻
    newsPublisher.publish("《观察者模式入门》已经上线啦！")
    newsPublisher.publish("第二条新闻：Kotlin 是不是很好用？")
 
    // 取消订阅
    newsPublisher.unsubscribe(emailSubscriber)
 
    // 再次发布新闻
    newsPublisher.publish("只有短信订阅者会收到这条新闻！")
}