compute-pressure.ts

// This module uses an experimental API that is only available on Chromium using
// a trial or by enabling a browser flag. It's unlikely that the API will get
// standardized and if it is it'll probably end up having a quite different form,
// that's to say that we should remove this once the trial is over :)
//
//  - Spec: https://oyiptong.github.io/compute-pressure/
//  - Status: https://chromestatus.com/feature/5597608644968448
//  - Trial: https://developer.chrome.com/origintrials/#/view_trial/1838594547874004993
//  - Chrome flag: Experimental Web Platform features
//
// NOTE: as of Aug'21 (Chrome 95) the only OS supported is Linux!
//

export type ComputePressureObserverOptions = {
  cpuUtilizationThresholds: number[]
  cpuSpeedThresholds: number[]
}

export type ComputePressureObserverUpdate = {
  cpuSpeed: number
  cpuUtilization: number
  options: ComputePressureObserverOptions
}

export type ComputePressureUpdateCallback = (
  update: ComputePressureObserverUpdate,
  observer: any // ComputePressureObserver (how to do a class forward decl?)
) => void

declare class ComputePressureObserver {
  constructor(callback: ComputePressureUpdateCallback, options?: ComputePressureObserverOptions)
  observe(): void
  unobserve(): void // this one is from the spec
  stop(): void // this one is from latest Chrome
}

export const COMPUTE_PRESSURE_SUPPORTED = typeof ComputePressureObserver === 'function'

// just a place to hold onto references to fight the GC
const hodor = new Set()

/**
 * Starts a compute pressure observer if supported by the browser.
 * The return is a canceller function to stop observing.
 */
export default (cb: ComputePressureUpdateCallback) => {
  if (!COMPUTE_PRESSURE_SUPPORTED) {
    console.warn('ComputePressure interface is not support in this browser')
    return () => {}
  }

  // chrome's prototype seems to be limited to 3 threshold only
  const observer = new ComputePressureObserver(cb, {
    cpuSpeedThresholds: [0.5, 0.75],
    cpuUtilizationThresholds: [0.35, 0.65],
  })
  observer.observe()

  // need to hold onto it otherwise it'll be automatically stopped by GC
  hodor.add(observer)

  return () => {
    hodor.delete(observer)

    if (typeof observer.unobserve === 'function') {
      observer.unobserve()
    } else if (typeof observer.stop === 'function') {
      observer.stop()
    } else {
      console.warn('neither unobserve nor close are defined :shrug:')
    }
  }
}