watchFiles.ts

import * as typescript from './typescript'
import { watchFile, unwatchFile, Stats } from 'fs'

export function watchFiles(
  rootFileNames: string[],
  originalServicePath: string,
  serverless: Serverless.Instance,
  cb: () => void
) {
  const tsConfig = typescript.getTypescriptConfig(originalServicePath, serverless)
  let watchedFiles = typescript.getSourceFiles(rootFileNames, tsConfig)

  watchedFiles.forEach(fileName => {
    watchFile(fileName, { persistent: true, interval: 250 }, watchCallback)
  })

  function watchCallback(curr: Stats, prev: Stats) {
    // Check timestamp
    if (+curr.mtime <= +prev.mtime) {
      return
    }

    cb()

    // use can reference not watched yet file or remove reference to already watched
    const newWatchFiles = typescript.getSourceFiles(rootFileNames, tsConfig)
    watchedFiles.forEach(fileName => {
      if (newWatchFiles.indexOf(fileName) < 0) {
        unwatchFile(fileName, watchCallback)
      }
    })

    newWatchFiles.forEach(fileName => {
      if (watchedFiles.indexOf(fileName) < 0) {
        watchFile(fileName, { persistent: true, interval: 250 }, watchCallback)
      }
    })

    watchedFiles = newWatchFiles
  }
}