观察者模式

• 定义：又叫发布订阅模式，定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态发生变化时，会通知所有观察者对象，使它们能够自动更新自己

// 定义发布者基类
interface Subject {
    // Attach an observer to the subject.
    attach(observer: Observer): void;

    // Detach an observer from the subject.
    detach(observer: Observer): void;

    // Notify all observers about an event.
    notify(): void;
}
// 实现发布者
class ConcreteSubject implements Subject {
    /**
     * @type {number} For the sake of simplicity, the Subject's state, essential
     * to all subscribers, is stored in this variable.
     */
    public state: number;

    /**
     * @type {Observer[]} List of subscribers. In real life, the list of
     * subscribers can be stored more comprehensively (categorized by event
     * type, etc.).
     */
    private observers: Observer[] = [];

    /**
     * The subscription management methods.
     */
    public attach(observer: Observer): void {
        const isExist = this.observers.includes(observer);
        if (isExist) {
            return console.log('Subject: Observer has been attached already.');
        }

        console.log('Subject: Attached an observer.');
        this.observers.push(observer);
    }

    public detach(observer: Observer): void {
        const observerIndex = this.observers.indexOf(observer);
        if (observerIndex === -1) {
            return console.log('Subject: Nonexistent observer.');
        }

        this.observers.splice(observerIndex, 1);
        console.log('Subject: Detached an observer.');
    }
    public notify(): void {
        console.log('Subject: Notifying observers...');
        for (const observer of this.observers) {
            observer.update(this);
        }
    }

    /**
     * Usually, the subscription logic is only a fraction of what a Subject can
     * really do. Subjects commonly hold some important business logic, that
     * triggers a notification method whenever something important is about to
     * happen (or after it).
     */
    public someBusinessLogic(): void {
        console.log('\nSubject: I\'m doing something important.');
        this.state = Math.floor(Math.random() * (10 + 1));

        console.log(`Subject: My state has just changed to: ${this.state}`);
        this.notify();
    }
}
// 订阅者基类
interface Observer {
    // Receive update from subject.
    update(subject: Subject): void;
}
// 订阅者实现
class ConcreteObserverA implements Observer {
    public update(subject: Subject): void {
        if (subject instanceof ConcreteSubject && subject.state < 3) {
            console.log('ConcreteObserverA: Reacted to the event.');
        }
    }
}

class ConcreteObserverB implements Observer {
    public update(subject: Subject): void {
        if (subject instanceof ConcreteSubject && (subject.state === 0 || subject.state >= 2)) {
            console.log('ConcreteObserverB: Reacted to the event.');
        }
    }
}

//使用
// 发布者
const subject = new ConcreteSubject();
// 发布者登记观察者
const observer1 = new ConcreteObserverA();
subject.attach(observer1);

const observer2 = new ConcreteObserverB();
subject.attach(observer2);
// 发布者出发变更，通知观察者
subject.someBusinessLogic();
subject.someBusinessLogic();

subject.detach(observer2);

subject.someBusinessLogic();

• 缺点：通知顺序无法保证