Observable

import { Observable } from '@hullo/core'
In case this is your first contact with Observer pattern and Observables, be sure to check out  Intro page.
What is an Observable?
/hullo-core/intro/what-is-an-observable
Observable constructor can have either:
A simple producer function
import {
  Producer,
  Observer,
  Observable
} from '@hullo/core';
​
const producer: Producer<any> =
  (observer: Observer<any>) => { /* ... */ };
  
const subject = new Observable(producer);
A producer function returning another function called when cancel method on Subscription is called
import { Observable } from '@hullo/core';
​
const subject = new Observable<string>(
  observer => {
    const token = setTimeout(() => {
      observer.next('surprise!');
    }, 0);
    
    return function onCancel () {
      clearTimeout(token);
    }
);
​
const sub = subject({ next: console.log });
​
sub.cancel()
// since this is called immediately,
// and calls clearTimeout synchronously,
// a message is never emitted by console.log
Async function producer (returned Promise is ignored)
import { Observable } from '@hullo/core';
​
const subject = new Observable<number>(
  async observer => {
    await observer.next(10);
    await observer.next(9);
    await observer.next(8);
    // ...
  }
);
Complex producer
An Observable constructor can also have a producer object with only subscribe method, receiving an observer, required.
Independently of if the producer is a function or an object, a cancellation can also be a function or an object with only cancel method required.
import { Observable, Observer } from '@hullo/core';
​
/*
all of this should create an observable
that emits a random number every 300ms
*/
const subject =
  new Observable<number>(
    {
      time: 300,
      timer: { token: null },
      subscribe
    }
  );
​
function subscribe(
  this: { time: number, timer: { token: any },
  observer: Observer<number>
) {
  this.timer.token = setInterval(
    () => observer.next(Math.random()),
    this.time
  ); 
  
  return {
    timer: this.timer,
    cancel
  };
}
​
function cancel(
  this: { timer: { token: any },
) {
  clearInterval(this.timer.token);
}
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