1.5.1. 目的

對(duì)象池模式是一種提前準(zhǔn)備了一組已經(jīng)初始化了的對(duì)象『池』而不是按需創(chuàng)建或者銷毀的創(chuàng)建型設(shè)計(jì)模式。對(duì)象池的客戶端會(huì)向?qū)ο蟪刂姓?qǐng)求一個(gè)對(duì)象透罢，然后使用這個(gè)返回的對(duì)象執(zhí)行相關(guān)操作。當(dāng)客戶端使用完畢季希，它將把這個(gè)特定類型的工廠對(duì)象返回給對(duì)象池蔬咬，而不是銷毀掉這個(gè)對(duì)象曹动。

在初始化實(shí)例成本高叹放，實(shí)例化率高饰恕，可用實(shí)例不足的情況下，對(duì)象池可以極大地提升性能井仰。在創(chuàng)建對(duì)象（尤其是通過(guò)網(wǎng)絡(luò)）時(shí)間花銷不確定的情況下懂盐，通過(guò)對(duì)象池在可期時(shí)間內(nèi)就可以獲得所需的對(duì)象。

無(wú)論如何糕档，對(duì)象池模式在需要耗時(shí)創(chuàng)建對(duì)象方面，例如創(chuàng)建數(shù)據(jù)庫(kù)連接拌喉，套接字連接速那，線程和大型圖形對(duì)象（比方字體或位圖等），使用起來(lái)都是大有裨益的尿背。在某些情況下端仰，簡(jiǎn)單的對(duì)象池（無(wú)外部資源，只占內(nèi)存）可能效率不高田藐，甚至?xí)袚p性能荔烧。

1.5.2. UML 類圖

1.5.3. 代碼

你可以在?GitHub?上找到這些例子

WorkerPool.php

<?php

namespace DesignPatterns\Creational\Pool;

class WorkerPool implements \Countable
{
    /**
    * @var StringReverseWorker[]
    */
    private $occupiedWorkers = [];

    /**
    * @var StringReverseWorker[]
    */
    private $freeWorkers = [];

    public function get(): StringReverseWorker
    {
        if (count($this->freeWorkers) == 0) {
            $worker = new StringReverseWorker();
        } else {
            $worker = array_pop($this->freeWorkers);
        }

        $this->occupiedWorkers[spl_object_hash($worker)] = $worker;

        return $worker;
    }

    public function dispose(StringReverseWorker $worker)
    {
        $key = spl_object_hash($worker);

        if (isset($this->occupiedWorkers[$key])) {
            unset($this->occupiedWorkers[$key]);
            $this->freeWorkers[$key] = $worker;
        }
    }

    public function count(): int
    {
        return count($this->occupiedWorkers) + count($this->freeWorkers);
    }
}

StringReverseWorker.php

<?php

namespace DesignPatterns\Creational\Pool;

class StringReverseWorker
{
    /**
    * @var \DateTime
    */
    private $createdAt;

    public function __construct()
    {
        $this->createdAt = new \DateTime();
    }

    public function run(string $text)
    {
        return strrev($text);
    }
}

1.5.4. 測(cè)試

Tests/PoolTest.php

<?php

namespace DesignPatterns\Creational\Pool\Tests;

use DesignPatterns\Creational\Pool\WorkerPool;
use PHPUnit\Framework\TestCase;

class PoolTest extends TestCase
{
    public function testCanGetNewInstancesWithGet()
    {
        $pool = new WorkerPool();
        $worker1 = $pool->get();
        $worker2 = $pool->get();

        $this->assertCount(2, $pool);
        $this->assertNotSame($worker1, $worker2);
    }

    public function testCanGetSameInstanceTwiceWhenDisposingItFirst()
    {
        $pool = new WorkerPool();
        $worker1 = $pool->get();
        $pool->dispose($worker1);
        $worker2 = $pool->get();

        $this->assertCount(1, $pool);
        $this->assertSame($worker1, $worker2);
    }
}