Function createWatcher

src/apis/watch.ts:203–398

(
  vm: ComponentInstance,
  source: WatchSource | WatchSource[] | WatchEffect,
  cb: WatchCallback | null,
  options: WatchOptions
)

}

function createWatcher(
  vm: ComponentInstance,
  source: WatchSource | WatchSource[] | WatchEffect,
  cb: WatchCallback | null,
  options: WatchOptions
): () => void {
  if (__DEV__ && !cb) {
    if (options.immediate !== undefined) {
      warn(
        `watch() "immediate" option is only respected when using the ` +
          `watch(source, callback, options?) signature.`
      )
    }
    if (options.deep !== undefined) {
      warn(
        `watch() "deep" option is only respected when using the ` +
          `watch(source, callback, options?) signature.`
      )
    }
  }

  const flushMode = options.flush
  const isSync = flushMode === 'sync'
  let cleanup: (() => void) | null
  const registerCleanup: InvalidateCbRegistrator = (fn: () => void) => {
    cleanup = () => {
      try {
        fn()
      } catch (
        // FIXME: remove any
        error: any
      ) {
        logError(error, vm, 'onCleanup()')
      }
    }
  }
  // cleanup before running getter again
  const runCleanup = () => {
    if (cleanup) {
      cleanup()
      cleanup = null
    }
  }
  const createScheduler = <T extends Function>(fn: T): T => {
    if (
      isSync ||
      /* without a current active instance, ignore pre|post mode */ vm ===
        fallbackVM
    ) {
      return fn
    }
    return ((...args: any[]) =>
      queueFlushJob(
        vm,
        () => {
          fn(...args)
        },
        flushMode as 'pre' | 'post'