How to use the jupyterlab.process.WatchHelper function in jupyterlab

Returns
    -------
    A list of `WatchHelper` objects.
    """
    parent = pjoin(HERE, '..')

    if not osp.exists(pjoin(parent, 'node_modules')):
        yarn_proc = Process(['node', YARN_PATH], cwd=parent, logger=logger)
        yarn_proc.wait()

    logger = logger or logging.getLogger('jupyterlab')
    ts_dir = osp.realpath(osp.join(HERE, '..', 'packages', 'metapackage'))

    # Run typescript watch and wait for compilation.
    ts_regex = r'.* Compilation complete\. Watching for file changes\.'
    ts_proc = WatchHelper(['node', YARN_PATH, 'run', 'watch'],
        cwd=ts_dir, logger=logger, startup_regex=ts_regex)

    # Run the metapackage file watcher.
    tsf_regex = 'Watching the metapackage files...'
    tsf_proc = WatchHelper(['node', YARN_PATH, 'run', 'watch:files'],
        cwd=ts_dir, logger=logger, startup_regex=tsf_regex)

    # Run webpack watch and wait for compilation.
    wp_proc = WatchHelper(['node', YARN_PATH, 'run', 'watch'],
        cwd=DEV_DIR, logger=logger,
        startup_regex=WEBPACK_EXPECT)

    return [ts_proc, tsf_proc, wp_proc]

if not osp.exists(pjoin(parent, 'node_modules')):
        yarn_proc = Process(['node', YARN_PATH], cwd=parent, logger=logger)
        yarn_proc.wait()

    logger = logger or logging.getLogger('jupyterlab')
    ts_dir = osp.realpath(osp.join(HERE, '..', 'packages', 'metapackage'))

    # Run typescript watch and wait for compilation.
    ts_regex = r'.* Compilation complete\. Watching for file changes\.'
    ts_proc = WatchHelper(['node', YARN_PATH, 'run', 'watch'],
        cwd=ts_dir, logger=logger, startup_regex=ts_regex)

    # Run the metapackage file watcher.
    tsf_regex = 'Watching the metapackage files...'
    tsf_proc = WatchHelper(['node', YARN_PATH, 'run', 'watch:files'],
        cwd=ts_dir, logger=logger, startup_regex=tsf_regex)

    # Run webpack watch and wait for compilation.
    wp_proc = WatchHelper(['node', YARN_PATH, 'run', 'watch'],
        cwd=DEV_DIR, logger=logger,
        startup_regex=WEBPACK_EXPECT)

    return [ts_proc, tsf_proc, wp_proc]

logger = logger or logging.getLogger('jupyterlab')
    ts_dir = osp.realpath(osp.join(HERE, '..', 'packages', 'metapackage'))

    # Run typescript watch and wait for compilation.
    ts_regex = r'.* Compilation complete\. Watching for file changes\.'
    ts_proc = WatchHelper(['node', YARN_PATH, 'run', 'watch'],
        cwd=ts_dir, logger=logger, startup_regex=ts_regex)

    # Run the metapackage file watcher.
    tsf_regex = 'Watching the metapackage files...'
    tsf_proc = WatchHelper(['node', YARN_PATH, 'run', 'watch:files'],
        cwd=ts_dir, logger=logger, startup_regex=tsf_regex)

    # Run webpack watch and wait for compilation.
    wp_proc = WatchHelper(['node', YARN_PATH, 'run', 'watch'],
        cwd=DEV_DIR, logger=logger,
        startup_regex=WEBPACK_EXPECT)

    return [ts_proc, tsf_proc, wp_proc]

Parameters
        ----------
        cmd: list
            The command to run.
        startup_regex: string
            The regex to wait for at startup.
        logger: :class:`~logger.Logger`, optional
            The logger instance.
        cwd: string, optional
            The cwd of the process.
        env: dict, optional
            The environment for the process.
        kill_event: callable, optional
            A function to call to check if we should abort.
        """
        super(WatchHelper, self).__init__(cmd, logger=logger,
            cwd=cwd, kill_event=kill_event, env=env)

        if not pty:
            self._stdout = self.proc.stdout

        while 1:
            line = self._stdout.readline().decode('utf-8')
            if not line:
                raise RuntimeError('Process ended improperly')
            print(line.rstrip())
            if re.match(startup_regex, line):
                break

        self._read_thread = threading.Thread(target=self._read_incoming)
        self._read_thread.setDaemon(True)
        self._read_thread.start()

def watch(self):
        """Start the application watcher and then run the watch in
        the background.
        """
        staging = pjoin(self.app_dir, 'staging')

        self._populate_staging()

        # Make sure packages are installed.
        self._run(['node', YARN_PATH, 'install'], cwd=staging)

        proc = WatchHelper(['node', YARN_PATH, 'run', 'watch'],
            cwd=pjoin(self.app_dir, 'staging'),
            startup_regex=WEBPACK_EXPECT,
            logger=self.logger)
        return [proc]