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"""Typical subject/observers pattern implementation. You can see this pattern |
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mentioned also as event/notification or broadcast/listeners. |
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Provides the Observer class, serving as the interface that needs to be implemented by concrete classes; the update |
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method needs to be overrode. Concrete Observers react to the notifications/updates issued by the Subject they had been |
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attached to. |
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Provides the Subject class, serving with mechanisms to subscribe/unsubscribe (attach/detach) observers and also with a |
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method to "notify" all subscribers about events. |
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""" |
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from abc import ABC, abstractmethod |
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from typing import List |
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__all__ = ['Subject', 'Observer'] |
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class ObserverInterface(ABC): |
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"""The Observer interface declares the update method, used by subjects. |
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Enables objects to act as "event" listeners; react to "notifications" |
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by executing specific handling logic. |
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""" |
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@abstractmethod |
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def update(self, *args, **kwargs) -> None: |
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"""Receive an update (from a subject); handle an event notification.""" |
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raise NotImplementedError |
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class SubjectInterface(ABC): |
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"""The Subject interface declares a set of methods for managing subscribers. |
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Enables objects to act as "subjects of observations"; notify the subscribed observers/listeners. |
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""" |
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@abstractmethod |
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def attach(self, observer: ObserverInterface) -> None: |
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"""Attach an observer to the subject; subscribe the observer.""" |
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raise NotImplementedError |
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@abstractmethod |
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def detach(self, observer: ObserverInterface) -> None: |
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"""Detach an observer from the subject; unsubscribe the observer.""" |
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raise NotImplementedError |
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@abstractmethod |
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def notify(self) -> None: |
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"""Notify all observers about an event.""" |
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raise NotImplementedError |
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class Observer(ObserverInterface, ABC): |
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pass |
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class Subject(SubjectInterface): |
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"""The Subject owns some important state and can notify observers. |
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Both the _state and _observers attributes have a simple implementation, |
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but can be overrode to accommodate for more complex scenarios. |
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The observers/subscribers are implemented as a python list. |
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In more complex scenarios, the list of subscribers can |
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be stored more comprehensively (categorized by event type, etc.). |
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The subscription management methods provided are 'attach' and 'detach' |
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to add or remove a subscriber respectively |
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""" |
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def __init__(self, *args, **kwargs): |
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self._observers: List[Observer] = [] |
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self._state = None |
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def add(self, *observers): |
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"""Subscribe multiple observers at once.""" |
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self._observers.extend(list(observers)) |
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@property |
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def state(self): |
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return self._state |
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@state.setter |
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def state(self, state): |
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self._state = state |
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def attach(self, observer: Observer) -> None: |
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self._observers.append(observer) |
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def detach(self, observer: Observer) -> None: |
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self._observers.remove(observer) |
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def notify(self) -> None: |
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"""Trigger an update in each subscriber/observer.""" |
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for observer in self._observers: |
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observer.update(self) |
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boromir674 /
so-magic
