feat(cron): better API (#40)

@xen-orchestra/cron/README.md

@@ -13,17 +13,20 @@ Installation of the [npm package](https://npmjs.org/package/@xen-orchestra/cron)
 ## Usage
 
 ```js
-import * as Cron from '@xen-orchestra/cron'
+import { createSchedule } from '@xen-orchestra/cron'
 
-Cron.parse('* * * jan,mar *')
-// → { month: [ 1, 3 ] }
+const schedule = createSchedule('0 0 * * sun', 'America/New_York')
 
-Cron.next('* * * jan,mar *', 2, 'America/New_York')
-// → [ 2018-01-19T22:15:00.000Z, 2018-01-19T22:16:00.000Z ]
+schedule.next(2)
+// [ 2018-02-11T05:00:00.000Z, 2018-02-18T05:00:00.000Z ]
 
-const stop = Cron.schedule('@hourly', () => {
+const job = schedule.createJob(() => {
   console.log(new Date())
-}, 'UTC+05:30')
+})
+
+job.start()
+
+job.stop()
 ```
 
 ### Pattern syntax

@xen-orchestra/cron/src/index.js

@@ -1,40 +1,49 @@
 import { DateTime } from 'luxon'
 
-import nextDate from './next'
+import next from './next'
 import parse from './parse'
 
-const autoParse = pattern =>
-  typeof pattern === 'string' ? parse(pattern) : schedule
-
-export const next = (cronPattern, n = 1, zone = 'utc') => {
-  const schedule = autoParse(cronPattern)
-
-  let date = DateTime.fromObject({ zone })
-  const dates = []
-  for (let i = 0; i < n; ++i) {
-    dates.push((date = nextDate(schedule, date)).toJSDate())
-  }
-  return dates
-}
-
-export { parse }
-
-export const schedule = (cronPattern, fn, zone = 'utc') => {
-  const wrapper = () => {
-    fn()
-    scheduleNextRun()
+class Job {
+  constructor (schedule, fn) {
+    const wrapper = scheduledRun => {
+      if (scheduledRun) {
+        fn()
+      }
+      this._timeout = setTimeout(wrapper, schedule.next() - Date.now(), true)
+    }
+    this._fn = wrapper
+    this._timeout = undefined
   }
 
-  let handle
-  const schedule = autoParse(cronPattern)
-  const scheduleNextRun = () => {
-    const now = DateTime.fromObject({ zone })
-    const nextRun = next(schedule, now)
-    handle = setTimeout(wrapper, nextRun - now)
+  start () {
+    this.stop()
+    this._fn()
   }
 
-  scheduleNextRun()
-  return () => {
-    clearTimeout(handle)
+  stop () {
+    clearTimeout(this._timeout)
   }
 }
+
+class Schedule {
+  constructor (pattern, zone = 'utc') {
+    this._schedule = parse(pattern)
+    this._dateTimeOpts = { zone }
+  }
+
+  createJob (fn) {
+    return new Job(this, fn)
+  }
+
+  next (n) {
+    const dates = new Array(n)
+    const schedule = this._schedule
+    let date = DateTime.fromObject(this._dateTimeOpts)
+    for (let i = 0; i < n; ++i) {
+      dates[i] = (date = next(schedule, date)).toJSDate()
+    }
+    return dates
+  }
+}
+
+export const createSchedule = (...args) => new Schedule(...args)