/
simplePubSub.js
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/
simplePubSub.js
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/**
* Create an AsyncIterator from an EventEmitter. Useful for mocking a
* PubSub system for tests.
*/
export class SimplePubSub<T> {
_subscribers: Set<(T) => void>;
constructor() {
this._subscribers = new Set();
}
emit(event: T): boolean {
for (const subscriber of this._subscribers) {
subscriber(event);
}
return this._subscribers.size > 0;
}
getSubscriber<R>(transform: (T) => R): AsyncGenerator<R, void, void> {
const pullQueue: Array<(result: IteratorResult<R, void>) => void> = [];
const pushQueue = [];
let listening = true;
this._subscribers.add(pushValue);
const emptyQueue = () => {
listening = false;
this._subscribers.delete(pushValue);
for (const resolve of pullQueue) {
resolve({ value: undefined, done: true });
}
pullQueue.length = 0;
pushQueue.length = 0;
};
return {
next() {
if (!listening) {
return Promise.resolve({ value: undefined, done: true });
}
if (pushQueue.length > 0) {
return Promise.resolve({ value: pushQueue.shift(), done: false });
}
return new Promise((resolve) => pullQueue.push(resolve));
},
return(): Promise<IteratorResult<R, void>> {
emptyQueue();
return Promise.resolve({ value: undefined, done: true });
},
throw(error: mixed) {
emptyQueue();
return Promise.reject(error);
},
[Symbol.asyncIterator]() {
return this;
},
};
function pushValue(event: T): void {
const value: R = transform(event);
if (pullQueue.length > 0) {
pullQueue.shift()({ value, done: false });
} else {
pushQueue.push(value);
}
}
}
}