如何从AsyncSubject(消费者模式)订阅一个元素

Question

在rxjs5,我有一个AsyncSubject,并希望多次订阅它,但只有一个订阅者应该收到该next()事件.所有其他人(如果他们还没有取消订阅)应立即获得该complete()活动next().

例:

let fired = false;
let as = new AsyncSubject();

const setFired = () => {
    if (fired == true) throw new Error("Multiple subscriptions executed");
    fired = true;
}

let subscription1 = as.subscribe(setFired);
let subscription2 = as.subscribe(setFired);

// note that subscription1/2 could be unsubscribed from in the future
// and still only a single subscriber should be triggered

setTimeout(() => {
    as.next(undefined);
    as.complete();
}, 500);

Answer 1

最简单的方法是将 AsyncSubject 包装在另一个对象中，该对象仅处理调用 1 个订阅者的逻辑。假设您只想调用第一个订阅者，下面的代码应该是一个很好的起点

let as = new AsyncSubject();

const createSingleSubscriberAsyncSubject = as => {
    // define empty array for subscribers
    let subscribers = [];

    const subscribe = callback => {
        if (typeof callback !== 'function') throw new Error('callback provided is not a function');

        subscribers.push(callback);

        // return a function to unsubscribe
        const unsubscribe = () => { subscribers = subscribers.filter(cb => cb !== callback); };
        return unsubscribe;
    };

    // the main subscriber that will listen to the original AS
    const mainSubscriber = (...args) => {
        // you can change this logic to invoke the last subscriber
        if (subscribers[0]) {
            subscribers[0](...args);
        }
    };

    as.subscribe(mainSubscriber);

    return {
        subscribe,
        // expose original AS methods as needed
        next: as.next.bind(as),
        complete: as.complete.bind(as),
    };
};

// use

const singleSub = createSingleSubscriberAsyncSubject(as);

// add 3 subscribers
const unsub1 = singleSub.subscribe(() => console.log('subscriber 1'));
const unsub2 = singleSub.subscribe(() => console.log('subscriber 2'));
const unsub3 = singleSub.subscribe(() => console.log('subscriber 3'));

// remove first subscriber
unsub1();

setTimeout(() => {
    as.next(undefined);
    as.complete();
    // only 'subscriber 2' is printed
}, 500);