observable.ts

type Subscriber<T> = (val: T) => void
type Unsubscriber = () => void

export default class Observable<T> {
  private subscribers: Subscriber<T>[] = []
  private unsubscriber: Unsubscriber
  private _value: T

  constructor(source?: (cb: Subscriber<T>) => Unsubscriber) {
    if (source) {
      this.unsubscriber = source(val => {
        this.value = val
      })
    }
  }

  set value(val: T) {
    this._value = val
    this.subscribers.map(f => f(val))
  }

  get value() : T {
    return this._value
  }

  subscribe(f: Subscriber<T>) : void {
    this.subscribers.push(f)
  }

  unsubscribe() : void {
    if (this.unsubscriber) {
      this.unsubscriber()
    }
    this.subscribers = []
  }

  map<U>(f: (val: T) => U) : Observable<U> {
    const o = new Observable<U>()

    this.subscribe(val => {
      o.value = f(val)
    })

    return o
  }

  filter(f: (val: T) => boolean) : Observable<T> {
    const o = new Observable<T>()

    this.subscribe(val => {
      if (f(val)) {
        o.value = val
      }
    })

    return o
  }

  buffer(ms: number) : Observable<T[]> {
    const o = new Observable<T[]>()

    let timer: number = null
    let buffer: T[] = []

    this.subscribe(val => {
      clearTimeout(timer)
      buffer.push(val)

      timer = setTimeout(() => {
        o.value = buffer
        buffer = []
      }, ms)
    })

    return o
  }
}