How to use the jupyterlab.jupyterlab.commands._AppHandler function in jupyterlab

def _ensure_compat_errors(info, app_dir, logger):
    """Ensure that the app info has compat_errors field"""
    handler = _AppHandler(app_dir, logger)
    info['compat_errors'] = handler._get_extension_compat()

def uninstall_extension(name, app_dir=None, logger=None):
    """Uninstall an extension by name or path.

    Returns `True` if a rebuild is recommended, `False` otherwise.
    """
    logger = _ensure_logger(logger)
    _node_check(logger)
    handler = _AppHandler(app_dir, logger)
    return handler.uninstall_extension(name)

def list_extensions(app_dir=None, logger=None):
    """List the extensions.
    """
    handler = _AppHandler(app_dir, logger)
    return handler.list_extensions()

def update_extension(name=None, all_=False, app_dir=None, logger=None):
    """Update an extension by name, or all extensions.

    Either `name` must be given as a string, or `all_` must be `True`.
    If `all_` is `True`, the value of `name` is ignored.

    Returns `True` if a rebuild is recommended, `False` otherwise.
    """
    logger = _ensure_logger(logger)
    _node_check(logger)
    handler = _AppHandler(app_dir, logger)
    if all_ is True:
        return handler.update_all_extensions()
    return handler.update_extension(name)

def get_app_info(app_dir=None, logger=None):
    """Get a dictionary of information about the app.
    """
    handler = _AppHandler(app_dir, logger)
    return handler.info

def check_extension(extension, app_dir=None, installed=False, logger=None):
    """Check if a JupyterLab extension is enabled or disabled.
    """
    handler = _AppHandler(app_dir, logger)
    return handler.check_extension(extension, installed)

def get_app_version(app_dir=None):
    """Get the application version."""
    app_dir = app_dir or get_app_dir()
    handler = _AppHandler(app_dir)
    return handler.info['version']

def _build_check_info(app_dir, logger):
    """Get info about packages scheduled for (un)install/update"""
    handler = _AppHandler(app_dir, logger)
    messages = handler.build_check(fast=True)
    # Decode the messages into a dict:
    status = {'install': [], 'uninstall': [], 'update': []}
    for msg in messages:
        for key, pattern in _message_map.items():
            match = pattern.match(msg)
            if match:
                status[key].append(match.group('name'))
    return status

"""Watch the application.

    Parameters
    ----------
    app_dir: string, optional
        The application directory.
    logger: :class:`~logger.Logger`, optional
        The logger instance.

    Returns
    -------
    A list of processes to run asynchronously.
    """
    logger = _ensure_logger(logger)
    _node_check(logger)
    handler = _AppHandler(app_dir, logger)
    return handler.watch()