start timer plugin

plugins/play/scratch.js

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+import spacetime from 'spacetime'
+import plugin from './src/index.js'
+
+spacetime.extend(plugin)
+
+let s = spacetime.now()
+s.play()

plugins/play/src/Ticker.js

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+/* global performance */
+
+// recursive setTimeOut - not perfect, but does not drift
+// https://stackoverflow.com/questions/29971898/how-to-create-an-accurate-timer-in-javascript
+// see benchmarks at https://github.com/dbkaplun/driftless
+class Ticker {
+  constructor(hertz, callback) {
+    this.target = performance.now() // target time for the next frame
+    this.interval = (1 / hertz) * 1000 // the milliseconds between ticks
+    this.callback = callback
+    this.stopped = false
+    this.frame = 0
+    this.tick(this)
+  }
+
+  tick(self) {
+    if (self.stopped) {
+      return
+    }
+    const currentTime = performance.now()
+    const currentTarget = self.target
+    const currentInterval = (self.target += self.interval) - currentTime
+
+    setTimeout(self.tick, currentInterval, self)
+    self.callback(self.frame++, currentTime, currentTarget, self)
+  }
+
+  stop() {
+    this.stopped = true
+    return this.frame
+  }
+}
+
+export default Ticker
+
+// let c = new Ticker(2, () => { console.log('tick') })

plugins/play/src/index.js

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+let methods = {
+  start: function () {
+    this.startEpoch = this.epoch
+    return this
+  },
+  stop: function () {
+    this.startEpoch = null
+    this.isRunning = false
+    return this
+  },
+  pause: function () {
+    this.isRunning = false
+    return this
+  },
+  elapsed: async function () {
+    let start = this._from(this.startEpoch, this.tz)
+    return this.diff(start)
+  }
+}
+methods.play = methods.start
+export default methods