Source code for hbutils.design.observer

"""
Overview:
    Implementation of observer pattern.
    See `Observer Pattern - Wikipedia <https://en.wikipedia.org/wiki/Observer_pattern>`_.
"""
from enum import Enum
from typing import Union, Type, TypeVar, Dict, Callable, Tuple, Set, Any

__all__ = [
    'Observable'
]

_EventSetType = Union[Type[Enum], list, tuple]


def _auto_members(events: _EventSetType):
    if isinstance(events, type) and issubclass(events, Enum):
        return list(events.__members__.values())
    elif isinstance(events, (list, tuple)):
        return list(events)
    else:
        raise TypeError(f'Invalid event set - {repr(events)}.')


def _get_object_id(obj) -> Tuple[str, int]:
    try:
        return 'hash', hash(obj)
    except TypeError:
        return 'id', id(obj)


_EventType = TypeVar('_EventType')
_SubscriberType = TypeVar('_SubscriberType')
_CallbackType = TypeVar('_CallbackType', bound=Callable[..., Any])


class Observable:
    """
    Overview:
        Observable object.

        * :meth:`subscribe` can be used for subscribing on specific event.
        * :meth:`unsubscribe` can be used for unsubscribing from specific event.
        * :meth:`dispatch` can be used for broadcasting a specific event, and all the subscribed callback will be \
            triggered.

    Examples::
        >>> from enum import IntEnum, unique
        >>> from hbutils.design import Observable
        >>>
        >>> @unique
        ... class MyIntEnum(IntEnum):
        ...     A = 1
        ...     B = 2
        >>>
        >>> o = Observable(MyIntEnum)
        >>> list_a, list_b = [], []
        >>> o.subscribe(MyIntEnum.A, list_a, 'append')  # use list_a.append
        >>> o.subscribe(MyIntEnum.B, list_a, lambda v: list_a.append(v))  # custom function.
        >>> o.subscribe(MyIntEnum.A, list_b, 'append')  # use list_b.append
        >>>
        >>> list_a, list_b
        ([], [])
        >>> o.dispatch(MyIntEnum.A)
        >>> list_a, list_b
        ([<MyIntEnum.A: 1>], [<MyIntEnum.A: 1>])
        >>> o.dispatch(MyIntEnum.B)
        >>> list_a, list_b
        ([<MyIntEnum.A: 1>, <MyIntEnum.B: 2>], [<MyIntEnum.A: 1>])
        >>>
        >>> o.unsubscribe(MyIntEnum.A, list_a)
        >>> o.dispatch(MyIntEnum.A)
        >>> list_a, list_b
        ([<MyIntEnum.A: 1>, <MyIntEnum.B: 2>], [<MyIntEnum.A: 1>, <MyIntEnum.A: 1>])
        >>> o.dispatch(MyIntEnum.B)
        >>> list_a, list_b
        ([<MyIntEnum.A: 1>, <MyIntEnum.B: 2>, <MyIntEnum.B: 2>], [<MyIntEnum.A: 1>, <MyIntEnum.A: 1>])
    """

    def __init__(self, events: _EventSetType):
        """
        Constructor of :class:`Observable`.

        :param events: Set of events, can be a list, tuple or an enum class.

        .. note::
            When enum is used, its values will be used as events. For example:

            >>> from enum import IntEnum
            >>> from hbutils.design import Observable
            >>>
            >>> class MyIntEnum(IntEnum):
            ...     A = 1
            ...     B = 2
            >>>
            >>> # equals to `Observable([MyIntEnum.A, MyIntEnum.B])`
            ... o = Observable(MyIntEnum)
            >>> o._events  # just for explanation, do not do this on actual use
            {<MyIntEnum.A: 1>: {}, <MyIntEnum.B: 2>: {}}
        """
        events = _auto_members(events)
        self._event_set: Set[_EventType] = set(events)
        self._events: Dict[_EventType, Dict[_SubscriberType, _CallbackType]] = {e: {} for e in events}

    def get_subscribers(self, event: _EventType) -> Dict[_SubscriberType, _CallbackType]:
        """
        Get subscribers of the given ``event``.
        :param event: Event for querying.
        :return: A dictionary with subscribers and their callbacks.
        """
        return self._events[event]

    def subscribe(self, event: _EventType, subscriber: _SubscriberType,
                  callback: Union[_CallbackType, str, None] = None):
        """
        Subscribe to the given ``event``.

        :param event: Event to be subscribed.
        :param subscriber: Subscriber of this subscription.
        :param callback: Callback function. If ``str`` is given, method with this name on ``subscriber`` will be used. \
            Default is ``None`` which means the ``update`` method on ``subscriber`` will be used.

        .. note::
            Callback function should have no more than 2 positional arguments. For example:

            >>> o.subscribe(MyIntEnum.A, 'user1', lambda: 2)  # ok
            >>> o.subscribe(MyIntEnum.A, 'user2', lambda event: 2)  # ok
            >>> o.subscribe(MyIntEnum.A, 'user3', lambda event, obs: 2)  # ok
            >>> o.subscribe(MyIntEnum.A, 'user4', lambda x, y, z: 2)  # X
        """
        if callback is None:
            callback = getattr(subscriber, 'update')
        elif isinstance(callback, str):
            callback = getattr(subscriber, callback)
        if not callable(callback):
            raise TypeError(f'Callback should be callable, but {repr(callback)} found.')

        from ..reflection import dynamic_call, sigsupply
        self.get_subscribers(event)[_get_object_id(subscriber)] = \
            dynamic_call(sigsupply(callback, lambda x: None))

    def unsubscribe(self, event: _EventType, subscriber: _SubscriberType):
        """
        Unsubscribe from the given ``event``.

        :param event: Event to be unsubscribed.
        :param subscriber: Subscriber of this unsubscription.
        """
        subscriber_id = _get_object_id(subscriber)
        try:
            del self.get_subscribers(event)[subscriber_id]
        except KeyError:
            raise KeyError(subscriber)

    def dispatch(self, event: _EventType):
        """
        Dispatch event.

        :param event: Event to be dispatched.
        """
        for _, callback in self.get_subscribers(event).items():
            callback(event, self)