Легковаговик — це структурний патерн, який економить пам’ять завдяки розподілу спільного стану, винесеного в один об’єкт, між безліччю об’єктів.
Легковаговик дозволяє економити пам’ять, записуючи в кеш однакові дані, що використовуються різними об’єктами.
Складність:
Популярність:
Застосування: Сенс використання Легковаговика — це економія пам’яті. Тому, якщо в програмі немає такої проблеми, ви навряд чи знайдете там приклади Легковаговика.
Ознаки застосування патерна: Легковаговик можна визначити за створюваними методами класу, які повертають закешовані об’єкти, замість створення нових.
Концептуальний приклад
Цей приклад показує структуру патерна Легковаговик , а саме — з яких класів він складається, які ролі ці класи виконують і як вони взаємодіють один з одним.
main.py: Приклад структури патерна
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])
# The client code either stores or calculates extrinsic state and passes it
# to the flyweight's methods.
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()
Output.txt: Результат виконання
FlyweightFactory: I have 5 flyweights:
Camaro2018_Chevrolet_pink
C300_Mercedes Benz_black
C500_Mercedes Benz_red
BMW_M5_red
BMW_X6_white
Client: Adding a car to database.
FlyweightFactory: Reusing existing flyweight.
Flyweight: Displaying shared (["BMW", "M5", "red"]) and unique (["CL234IR", "James Doe"]) state.
Client: Adding a car to database.
FlyweightFactory: Can't find a flyweight, creating new one.
Flyweight: Displaying shared (["BMW", "X1", "red"]) and unique (["CL234IR", "James Doe"]) state.
FlyweightFactory: I have 6 flyweights:
Camaro2018_Chevrolet_pink
C300_Mercedes Benz_black
C500_Mercedes Benz_red
BMW_M5_red
BMW_X6_white
BMW_X1_red