关于后端:结构型设计模式享元缓存Flyweight

享元模式更常见的叫法是缓存或 cache，一个对象只创立一次，之后始终复用这一对象。

跟单例不同在于，单例是全局只有惟一一个对象，所有人一起用同一个对象，且该对象是提前预设好的，不能定制。

而享元则能够依据参数不同创立出不同的实例，不过实例一旦创立就只保留一份，之后再次创立雷同参数的实例则返回该对象，供所有援用的中央应用，援用方之间共享同一份享元实例的数据。

过程有点像定制化的单例模式。

Flyweight 享元类

定制化的业务实体，依据不同须要可能定制不同的享元类，但一旦创立出一个享元类，当前都复用这一个类
FlyweightFactory 享元工厂类

用来创立和缓存享元实体的中央

class Flyweight
{
    private $sharedState;

    public function __construct($sharedState)
    {$this->sharedState = $sharedState;}

    public function operation($uniqueState): void
    {$s = json_encode($this->sharedState);
        $u = json_encode($uniqueState);
        echo "Flyweight: Displaying shared ($s) and unique ($u) state.\n";
    }
}

class FlyweightFactory
{private $flyweights = [];

    public function __construct(array $initialFlyweights)
    {foreach ($initialFlyweights as $state) {$this->flyweights[$this->getKey($state)] = new Flyweight($state);
        }
    }

    private function getKey(array $state): string
    {ksort($state);

        return implode("_", $state);
    }

    public function getFlyweight(array $sharedState): Flyweight
    {$key = $this->getKey($sharedState);

        if (!isset($this->flyweights[$key])) {
            echo "FlyweightFactory: Can't find a flyweight, creating new one.\n";
            $this->flyweights[$key] = new Flyweight($sharedState);
        } else {echo "FlyweightFactory: Reusing existing flyweight.\n";}

        return $this->flyweights[$key];
    }

    public function listFlyweights(): void
    {$count = count($this->flyweights);
        echo "\nFlyweightFactory: I have $count flyweights:\n";
        foreach ($this->flyweights as $key => $flyweight) {echo $key . "\n";}
    }
}

$factory = new FlyweightFactory([["Chevrolet", "Camaro2018", "pink"],
    ["Mercedes Benz", "C300", "black"],
    ["Mercedes Benz", "C500", "red"],
    ["BMW", "M5", "red"],
    ["BMW", "X6", "white"],
]);
$factory->listFlyweights();

function addCarToPoliceDatabase(
    FlyweightFactory $ff, $plates, $owner,
    $brand, $model, $color
) {
    echo "\nClient: Adding a car to database.\n";
    $flyweight = $ff->getFlyweight([$brand, $model, $color]);

    $flyweight->operation([$plates, $owner]);
}

addCarToPoliceDatabase($factory,
    "CL234IR",
    "James Doe",
    "BMW",
    "M5",
    "red"
);

addCarToPoliceDatabase($factory,
    "CL234IR",
    "James Doe",
    "BMW",
    "X1",
    "red"
);

$factory->listFlyweights();

output:

FlyweightFactory: I have 5 flyweights:
Chevrolet_Camaro2018_pink
Mercedes Benz_C300_black
Mercedes Benz_C500_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:
Chevrolet_Camaro2018_pink
Mercedes Benz_C300_black
Mercedes Benz_C500_red
BMW_M5_red
BMW_X6_white
BMW_X1_red

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关于后端:结构型设计模式享元缓存Flyweight

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