"""
Flyweight Design Pattern
Intent: Lets you fit more objects into the available amount of RAM by sharing
common parts of state between multiple objects, instead of keeping all of the
data in each object.
"""
import json
from typing import Dict
class Flyweight():
"""
The Flyweight stores a common portion of the state (also called intrinsic
state) that belongs to multiple real business entities. The Flyweight
accepts the rest of the state (extrinsic state, unique for each entity) via
its method parameters.
"""
def __init__(self, shared_state: str) -> None:
self._shared_state = shared_state
def operation(self, unique_state: str) -> None:
s = json.dumps(self._shared_state)
u = json.dumps(unique_state)
print(f"Flyweight: Displaying shared ({s}) and unique ({u}) state.", end="")
class FlyweightFactory():
"""
The Flyweight Factory creates and manages the Flyweight objects. It ensures
that flyweights are shared correctly. When the client requests a flyweight,
the factory either returns an existing instance or creates a new one, if it
doesn't exist yet.
"""
_flyweights: Dict[str, Flyweight] = {}
def __init__(self, initial_flyweights: Dict) -> None:
for state in initial_flyweights:
self._flyweights[self.get_key(state)] = Flyweight(state)
def get_key(self, state: Dict) -> str:
"""
Returns a Flyweight's string hash for a given state.
"""
return "_".join(sorted(state))
def get_flyweight(self, shared_state: Dict) -> Flyweight:
"""
Returns an existing Flyweight with a given state or creates a new one.
"""
key = self.get_key(shared_state)
if not self._flyweights.get(key):
print("FlyweightFactory: Can't find a flyweight, creating new one.")
self._flyweights[key] = Flyweight(shared_state)
else:
print("FlyweightFactory: Reusing existing flyweight.")
return self._flyweights[key]
def list_flyweights(self) -> None:
count = len(self._flyweights)
print(f"FlyweightFactory: I have {count} flyweights:")
print("\n".join(map(str, self._flyweights.keys())), end="")
def add_car_to_police_database(
factory: FlyweightFactory, plates: str, owner: str,
brand: str, model: str, color: str
) -> None:
print("\n\nClient: Adding a car to database.")
flyweight = factory.get_flyweight([brand, model, color])
flyweight.operation([plates, owner])
if __name__ == "__main__":
"""
The client code usually creates a bunch of pre-populated flyweights in the
initialization stage of the application.
"""
factory = FlyweightFactory([
["Chevrolet", "Camaro2018", "pink"],
["Mercedes Benz", "C300", "black"],
["Mercedes Benz", "C500", "red"],
["BMW", "M5", "red"],
["BMW", "X6", "white"],
])
factory.list_flyweights()
add_car_to_police_database(
factory, "CL234IR", "James Doe", "BMW", "M5", "red")
add_car_to_police_database(
factory, "CL234IR", "James Doe", "BMW", "X1", "red")
print("\n")
factory.list_flyweights()