Observer Pattern
Video Lecture
Overview
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Terminology
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Observer UML Diagram
Source Code
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./observer/observer_concept.py
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65 | # pylint: disable=too-few-public-methods
"Observer Design Pattern Concept"
from abc import ABCMeta, abstractmethod
class IObservable(metaclass=ABCMeta):
"The Subject Interface"
@staticmethod
@abstractmethod
def subscribe(observer):
"The subscribe method"
@staticmethod
@abstractmethod
def unsubscribe(observer):
"The unsubscribe method"
@staticmethod
@abstractmethod
def notify(observer):
"The notify method"
class Subject(IObservable):
"The Subject (Observable)"
def __init__(self):
self._observers = set()
def subscribe(self, observer):
self._observers.add(observer)
def unsubscribe(self, observer):
self._observers.remove(observer)
def notify(self, *args):
for observer in self._observers:
observer.notify(*args)
class IObserver(metaclass=ABCMeta):
"A method for the Observer to implement"
@staticmethod
@abstractmethod
def notify(observable, *args):
"Receive notifications"
class Observer(IObserver):
"The concrete observer"
def __init__(self, observable):
observable.subscribe(self)
def notify(self, *args):
print(f"Observer id:{id(self)} received {args}")
# The Client
SUBJECT = Subject()
OBSERVER_A = Observer(SUBJECT)
OBSERVER_B = Observer(SUBJECT)
SUBJECT.notify("First Notification", [1, 2, 3])
SUBJECT.unsubscribe(OBSERVER_B)
SUBJECT.notify("Second Notification", {"A": 1, "B": 2, "C": 3})
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Output
| python ./observer/observer_concept.py
Observer id:2084220160272 received ('First Notification', [1, 2, 3])
Observer id:2084220160224 received ('First Notification', [1, 2, 3])
Observer id:2084220160272 received ('Second Notification', {'A': 1, 'B': 2, 'C': 3})
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Example Use Case
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Example UML Diagram
Source Code
./observer/client.py
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27 | "Observer Design Pattern Concept"
from data_model import DataModel
from data_controller import DataController
from pie_graph_view import PieGraphView
from bar_graph_view import BarGraphView
from table_view import TableView
# A local data model that the hypothetical external controller updates
DATA_MODEL = DataModel()
# Add some visualisation that use the dataview
PIE_GRAPH_VIEW = PieGraphView(DATA_MODEL)
BAR_GRAPH_VIEW = BarGraphView(DATA_MODEL)
TABLE_VIEW = TableView(DATA_MODEL)
# A hypothetical data controller running in a different process
DATA_CONTROLLER = DataController()
# The hypothetical external data controller updates some data
DATA_CONTROLLER.notify([1, 2, 3])
# Client now removes a local BAR_GRAPH
BAR_GRAPH_VIEW.delete()
# The hypothetical external data controller updates the data again
DATA_CONTROLLER.notify([4, 5, 6])
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./observer/table_view.py
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19 | "An observer"
from interface_data_view import IDataView
class TableView(IDataView):
"The concrete observer"
def __init__(self, observable):
self._observable = observable
self._id = self._observable.subscribe(self)
def notify(self, data):
print(f"TableView, id:{self._id}")
self.draw(data)
def draw(self, data):
print(f"Drawing a Table view using data:{data}")
def delete(self):
self._observable.unsubscribe(self._id)
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./observer/bar_graph_view.py
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19 | "An observer"
from interface_data_view import IDataView
class BarGraphView(IDataView):
"The concrete observer"
def __init__(self, observable):
self._observable = observable
self._id = self._observable.subscribe(self)
def notify(self, data):
print(f"BarGraph, id:{self._id}")
self.draw(data)
def draw(self, data):
print(f"Drawing a Bar graph using data:{data}")
def delete(self):
self._observable.unsubscribe(self._id)
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./observer/pie_graph_view.py
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19 | "An observer"
from interface_data_view import IDataView
class PieGraphView(IDataView):
"The concrete observer"
def __init__(self, observable):
self._observable = observable
self._id = self._observable.subscribe(self)
def notify(self, data):
print(f"PieGraph, id:{self._id}")
self.draw(data)
def draw(self, data):
print(f"Drawing a Pie graph using data:{data}")
def delete(self):
self._observable.unsubscribe(self._id)
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./observer/interface_data_view.py
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20 | "The Data View interface"
from abc import ABCMeta, abstractmethod
class IDataView(metaclass=ABCMeta):
"A method for the Observer to implement"
@staticmethod
@abstractmethod
def notify(data):
"Receive notifications"
@staticmethod
@abstractmethod
def draw(data):
"Draw the view"
@staticmethod
@abstractmethod
def delete():
"a delete method to remove observer specific resources"
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./observer/data_model.py
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25 | "A Data Model that observes the Data Controller"
from interface_data_model import IDataModel
from data_controller import DataController
class DataModel(IDataModel):
"A Subject (Observable)"
def __init__(self):
self._observers = {}
self._counter = 0
# subscribing to an external hypothetical data controller
self._data_controller = DataController()
self._data_controller.subscribe(self)
def subscribe(self, observer):
self._counter = self._counter + 1
self._observers[self._counter] = observer
return self._counter
def unsubscribe(self, observer_id):
self._observers.pop(observer_id)
def notify(self, data):
for observer in self._observers:
self._observers[observer].notify(data)
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./observer/interface_data_model.py
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20 | "A Data Model Interface"
from abc import ABCMeta, abstractmethod
class IDataModel(metaclass=ABCMeta):
"A Subject Interface"
@staticmethod
@abstractmethod
def subscribe(observer):
"The subscribe method"
@staticmethod
@abstractmethod
def unsubscribe(observer_id):
"The unsubscribe method"
@staticmethod
@abstractmethod
def notify(data):
"The notify method"
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./observer/data_controller.py
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23 | "A Data Controller that is a Subject"
from interface_data_controller import IDataController
class DataController(IDataController):
"A Subject (Observable)"
_observers = set()
def __new__(cls):
return cls
@classmethod
def subscribe(cls, observer):
cls._observers.add(observer)
@classmethod
def unsubscribe(cls, observer):
cls._observers.remove(observer)
@classmethod
def notify(cls, *args):
for observer in cls._observers:
observer.notify(*args)
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./observer/interface_data_controller.py
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19 | "A Data Controller Interface"
from abc import ABCMeta, abstractmethod
class IDataController(metaclass=ABCMeta):
"A Subject Interface"
@staticmethod
@abstractmethod
def subscribe(observer):
"The subscribe method"
@staticmethod
@abstractmethod
def unsubscribe(observer):
"The unsubscribe method"
@staticmethod
@abstractmethod
def notify(observer):
"The notify method"
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Output
| python ./observer/client.py
PieGraph, id:1
Drawing a Pie graph using data:[1, 2, 3]
BarGraph, id:2
Drawing a Bar graph using data:[1, 2, 3]
TableView, id:3
Drawing a Table view using data:[1, 2, 3]
PieGraph, id:1
Drawing a Pie graph using data:[4, 5, 6]
TableView, id:3
Drawing a Table view using data:[4, 5, 6]
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New Coding Concepts
Python Set
A Python Set is similar to a List. Except that the items in the Set are guaranteed to be unique, even if you try to add a duplicate. A set is a good choice for keeping a collection of observables, since the problem of duplicate observables is automatically handled.
A Set can be instantiated pre-filled as {1, 2, 3} surrounded by curly braces or using the keyword set(), verses [] or list() for a List and () or tuple() for a Tuple. It is not the same as a Dictionary, that also uses {}, since the dictionary items are created as key:value pairs. ie {"a": 1, "b": 2, "c": 3}
| PS> python
>>> items = {"yankee", "doodle", "dandy", "doodle"}
>>> items
{'yankee', 'doodle', 'dandy'}
>>> items.add("grandy")
>>> items
{'grandy', 'yankee', 'doodle', 'dandy'}
>>> items.remove("doodle")
>>> items
{'grandy', 'yankee', 'dandy'}
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Note
If instantiating an empty Set then use my_object = Set() rather than my_object = {} to reduce ambiguity with creating an empty Dictionary.
Summary
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