如何 "multicast" 异步迭代？

Question

async 生成器能否以某种方式进行广播或多播，以便其所有迭代器（“消费者”？订阅者？）接收所有值？

考虑这个例子：

const fetchMock = () => "Example. Imagine real fetch";
async function* gen() {
  for (let i = 1; i <= 6; i++) {
    const res = await fetchMock();
    yield res.slice(0, 2) + i;
  }
}
const ait = gen();

(async() => {
  // first "consumer"
  for await (const e of ait) console.log('e', e);
})();
(async() => {
  // second...
  for await (const é of ait) console.log('é', é);
})();

迭代“消耗”一个值，因此只有一个或另一个获得它。如果可以以某种方式创建这样的生成器，我希望它们（以及以后的任何一个）都能获得每个 yielded 值。（类似于 Observable。）

Answer 1

你是这个意思？

async function* gen() {
  for (let i = 1; i <= 6; i++) yield i;
}
//const ait = gen();


(async() => {
  // first iteration
  const ait = gen()
  for await (const e of ait) console.log(1, e);
})();
(async() => {
  // second...
  const ait = gen()
  for await (const é of ait) console.log(2, é);
})();

Answer 2

这可不容易。您将需要明确地 tee it. This is similar to ，只是稍微复杂一点：

const AsyncIteratorProto = Object.getPrototypeOf(Object.getPrototypeOf(async function*(){}.prototype));
function teeAsync(iterable) {
    const iterator = iterable[Symbol.asyncIterator]();
    const buffers = [[], []];
    function makeIterator(buffer, i) {
        return Object.assign(Object.create(AsyncIteratorProto), {
            next() {
                if (!buffer) return Promise.resolve({done: true, value: undefined});
                if (buffer.length) return buffer.shift();
                const res = iterator.next();
                if (buffers[i^1]) buffers[i^1].push(res);
                return res;
            },
            async return() {
                if (buffer) {
                    buffer = buffers[i] = null;
                    if (!buffers[i^1]) await iterator.return();
                }
                return {done: true, value: undefined};
            },
        });
    }
    return buffers.map(makeIterator);
}

您应该确保两个迭代器的消耗速度大致相同，这样缓冲区就不会变得太大。

Answer 3

这是一个使用 Highland 作为中介的解决方案。来自文档：

A stream forked to multiple consumers will pull values, one at a time, from its source as only fast as the slowest consumer can handle them.

import _ from 'lodash'
import H from 'highland'

export function fork<T>(generator: AsyncGenerator<T>): [
    AsyncGenerator<T>,
    AsyncGenerator<T>
] {
    const source = asyncGeneratorToHighlandStream(generator).map(x => _.cloneDeep(x));
    return [
        highlandStreamToAsyncGenerator<T>(source.fork()),
        highlandStreamToAsyncGenerator<T>(source.fork()),
    ];
}

async function* highlandStreamToAsyncGenerator<T>(
    stream: Highland.Stream<T>
): AsyncGenerator<T> {
    for await (const row of stream.toNodeStream({ objectMode: true })) {
        yield row as unknown as T;
    }
}

function asyncGeneratorToHighlandStream<T>(
    generator: AsyncGenerator<T>
): Highland.Stream<T> {
    return H(async (push, next) => {
        try {
            const result = await generator.next();
            if (result.done) return push(null, H.nil);
            push(null, result.value);
            next();
        } catch (error) {
            return push(error);
        }
    });
}

用法：

const [copy1, copy2] = fork(source);

在 Node 中工作，浏览器未经测试。

如何 "multicast" 异步迭代？

How to "multicast" an async iterable?

javascript

observable

async-await

ecmascript-2016