Observer Patterns

Author: MilovanTomasevic

src/1_patterns/4_BehavioralPatterns/7_Observer/1_events.py

@@ -0,0 +1,31 @@
+class Event(list):
+    def __call__(self, *args, **kwargs):
+        for item in self:
+            item(*args, **kwargs)
+
+
+class Person:
+    def __init__(self, name, address):
+        self.name = name
+        self.address = address
+        self.falls_ill = Event()
+
+    def catch_a_cold(self):
+        self.falls_ill(self.name, self.address)
+
+
+def call_doctor(name, address):
+    print(f'A doctor has been called to {address}')
+
+if __name__ == '__main__':
+    person = Person('Sherlock', '221B Baker St')
+
+    person.falls_ill.append(lambda name, addr: print(f'{name} is ill'))
+
+    person.falls_ill.append(call_doctor)
+
+    person.catch_a_cold()
+
+    # and you can remove subscriptions too
+    person.falls_ill.remove(call_doctor)
+    person.catch_a_cold()

src/1_patterns/4_BehavioralPatterns/7_Observer/2_property_observers.py

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+class Event(list):
+  def __call__(self, *args, **kwargs):
+    for item in self:
+      item(*args, **kwargs)
+
+
+class PropertyObservable:
+  def __init__(self):
+    self.property_changed = Event()
+
+
+class Person(PropertyObservable):
+  def __init__(self, age=0):
+    super().__init__()
+    self._age = age
+
+  @property
+  def age(self):
+    return self._age
+
+  @age.setter
+  def age(self, value):
+    if self._age == value:
+      return
+    self._age = value
+    self.property_changed('age', value)
+
+
+class TrafficAuthority:
+  def __init__(self, person):
+    self.person = person
+    person.property_changed.append(self.person_changed)
+
+  def person_changed(self, name, value):
+    if name == 'age':
+      if value < 16:
+        print('Sorry, you still cannot drive')
+      else:
+        print('Okay, you can drive now')
+        self.person.property_changed.remove(
+          self.person_changed
+        )
+
+
+if __name__ == '__main__':
+  p = Person()
+  ta = TrafficAuthority(p)
+  for age in range(14, 20):
+    print(f'Setting age to {age}')
+    p.age = age

src/1_patterns/4_BehavioralPatterns/7_Observer/3_property_dependencies.py

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+class Event(list):
+  def __call__(self, *args, **kwargs):
+    for item in self:
+      item(*args, **kwargs)
+
+
+class PropertyObservable:
+  def __init__(self):
+    self.property_changed = Event()
+
+
+class Person(PropertyObservable):
+  def __init__(self, age=0):
+    super().__init__()
+    self._age = age
+
+  @property
+  def can_vote(self):
+    return self._age >= 18
+
+  @property
+  def age(self):
+    return self._age
+
+  # @age.setter
+  # def age(self, value):
+  #   if self._age == value:
+  #     return
+  #   self._age = value
+  #   self.property_changed('age', value)
+
+  @age.setter
+  def age(self, value):
+    if self._age == value:
+      return
+
+    old_can_vote = self.can_vote
+
+    self._age = value
+    self.property_changed('age', value)
+
+    if old_can_vote != self.can_vote:
+      self.property_changed('can_vote', self.can_vote)
+
+
+if __name__ == '__main__':
+  def person_changed(name, value):
+    if name == 'can_vote':
+      print(f'Voting status changed to {value}')
+
+  p = Person()
+  p.property_changed.append(
+    person_changed
+  )
+
+  for age in range(16, 21):
+    print(f'Changing age to {age}')
+    p.age = age