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享元

享元在 Python 中的实现

享元是一种结构型设计模式 它允许你在消耗少量内存的情况下支持大量对象

模式通过共享多个对象的部分状态来实现上述功能 换句话来说 享元会将不同对象的相同数据进行缓存以节省内存

进一步了解享元模式

在 Python 中使用模式

复杂度

流行度

使用示例 享元模式只有一个目的 将内存消耗最小化 如果你的程序没有遇到内存容量不足的问题 则可以暂时忽略该模式

识别方法 享元可以通过构建方法来识别 它会返回缓存对象而不是创建新的对象

概念示例

本例说明了享元设计模式的结构并重点回答了下面的问题

  • 它由哪些类组成
  • 这些类扮演了哪些角色
  • 模式中的各个元素会以何种方式相互关联

main.py: 概念示例

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: 执行结果

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

享元在其他编程语言中的实现

享元在 Java 中的实现 享元在 C# 中的实现 享元在 PHP 中的实现 享元在 Ruby 中的实现 享元在 Swift 中的实现 享元在 TypeScript 中的实现