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"""

http://code.activestate.com/recipes/131499-observer-pattern/

*TL;DR

Maintains a list of dependents and notifies them of any state changes.

*Examples in Python ecosystem:

Django Signals: https://docs.djangoproject.com/en/3.1/topics/signals/

Flask Signals: https://flask.palletsprojects.com/en/1.1.x/signals/

"""

from __future__ import annotations

from contextlib import suppress

from typing import Protocol

# define a generic observer type

class Observer(Protocol):

def update(self, subject: Subject) -> None:

pass

class Subject:

def __init__(self) -> None:

self._observers: list[Observer] = []

def attach(self, observer: Observer) -> None:

if observer not in self._observers:

self._observers.append(observer)

def detach(self, observer: Observer) -> None:

with suppress(ValueError):

self._observers.remove(observer)

def notify(self, modifier: Observer | None = None) -> None:

for observer in self._observers:

if modifier != observer:

observer.update(self)

class Data(Subject):

def __init__(self, name: str = "") -> None:

super().__init__()

self.name = name

self._data = 0

@property

def data(self) -> int:

return self._data

@data.setter

def data(self, value: int) -> None:

self._data = value

self.notify()

class HexViewer:

def update(self, subject: Data) -> None:

print(f"HexViewer: Subject {subject.name} has data 0x{subject.data:x}")

class DecimalViewer:

def update(self, subject: Data) -> None:

print(f"DecimalViewer: Subject {subject.name} has data {subject.data}")

def main():

"""

>>> data1 = Data('Data 1')

>>> data2 = Data('Data 2')

>>> view1 = DecimalViewer()

>>> view2 = HexViewer()

>>> data1.attach(view1)

>>> data1.attach(view2)

>>> data2.attach(view2)

>>> data2.attach(view1)

>>> data1.data = 10

DecimalViewer: Subject Data 1 has data 10

HexViewer: Subject Data 1 has data 0xa

>>> data2.data = 15

HexViewer: Subject Data 2 has data 0xf

DecimalViewer: Subject Data 2 has data 15

>>> data1.data = 3

DecimalViewer: Subject Data 1 has data 3

HexViewer: Subject Data 1 has data 0x3

>>> data2.data = 5

HexViewer: Subject Data 2 has data 0x5

DecimalViewer: Subject Data 2 has data 5

# Detach HexViewer from data1 and data2

>>> data1.detach(view2)

>>> data2.detach(view2)

>>> data1.data = 10

DecimalViewer: Subject Data 1 has data 10

>>> data2.data = 15

DecimalViewer: Subject Data 2 has data 15

"""

if __name__ == "__main__":

import doctest

doctest.testmod()