event-as-promise

Handle continuous stream of events with Promise and generator function.

There are multiple alternatives, for example, p-event is a popular choice.

Instead of listen to event just once, event-as-promise chose an approach to allow listening to the same event continuously. And we use generator function to enable for(of) loop to handle event indefinitely.

How to use

Web server

This sample code is converted from Node about page.

import EventAsPromise from 'event-as-promise';
import http from 'http';
 
async function main(ready) {
  const server = http.createServer();
  const listeningPromises = new EventAsPromise();
  const requestPromises = new EventAsPromise({ array: true });
 
  server
    .once('listening', listeningPromises.eventListener)
    .on('request', requestPromises.eventListener)
    .listen(3000);
 
  // Wait for "listening"
  await listeningPromises.one();
 
  // Loop indefinitely, using generator
  for (let requestPromise of requestPromises) {
    // Wait for "request"
    const [req, res] = await requestPromise;
 
    res.statusCode = 200;
    res.setHeader('Content-Type', 'text/plain');
    res.end('Hello World\n');
  }
}
 
main();

Note: as multiple results is not supported by Promise, the array option will group multiple event arguments into an array. By default, it is false.

Redux Saga

Handling event in a Promise may not reduce complexity. But it will be beneficial for redux-saga when mixed with call effect.

In this example, when the user is connected via CONNECTED action, we will keep the user posted about file changes, until a DISCONNECTED is received.

saga.run(function* () {
  yield takeLatest('CONNECTED', function* (action) {
    const watcher = fs.watch(action.payload);
    const changePromises = new EventAsPromise({ array: true });
 
    watcher.on('change', changePromises.eventListener);
 
    for (;;) {
      const [changes] = yield race([
        call(changePromises.one),
        take('DISCONNECTED'),
      ]);
 
      if (changes) {
        yield put({ type: 'CHANGED', payload: changes });
      } else {
        break;
      }
    }
 
    watcher.close();
  });
});

Futures

You can retrieve multiple Promise objects before the event is emitted.

const emitter = new EventEmitter();
const countPromises = new EventAsPromise();
 
emitter.on('count', countPromises.eventListener);
 
// Retrieve multiple future Promise before the actual event is fired
const promise1 = countPromises.one();
const promise2 = countPromises.one();
const promise3 = countPromises.one();
 
emitter.emit('count', 1);
emitter.emit('count', 2);
emitter.emit('count', 3);
 
await expect(promise1).resolves.toBe(1);
await expect(promise2).resolves.toBe(2);
await expect(promise3).resolves.toBe(3);

Same as event listener, if one() is not called before the event is emitted, the event will be lost.

Upcomings

Instead of futures, you can use upcoming() to get the Promise for the upcoming event. Futures and upcoming Promises are independent of each other, as shown in the sample below.

const emitter = new EventEmitter();
const countPromises = new EventAsPromise();
 
emitter.on('count', countPromises.eventListener);
 
const promiseOne1 = countPromises.upcoming();
const promiseOne2 = countPromises.upcoming();
const promiseOne3 = countPromises.one();
const promiseTwo = countPromises.one();
 
emitter.emit('count', 'one');
emitter.emit('count', 'two');
 
await expect(promiseOne1).resolves.toBe('one');
await expect(promiseOne2).resolves.toBe('one');
await expect(promiseOne3).resolves.toBe('one');
await expect(promiseTwo).resolves.toBe('two');
 
const promiseThree = countPromises.upcoming();
 
emitter.emit('count', 'three');
 
await expect(promiseOne1).resolves.toBe('one');
await expect(promiseThree).resolves.toBe('three');

Note: after the current upcoming() has resolved, you will need to call upcoming() again to obtain a new Promise for the next upcoming event.

Contributions

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