範例：計算機（簡單工廠模式)

2020-06-21

Example

Class Diagram: 計算機

需求一：客戶想要一個加法計算機

<?php

namespace App\FactoryPattern\Calculator;

class Program
{
    /**
     * @var int
     */
    private $firstNumber;

    /**
     * @var int
     */
    private $secondNumber;

    /**
     * @var string
     */
    private $operationString;

    public function __construct($firstNumber, $secondNumber)
    {
        $this->firstNumber = $firstNumber;
        $this->secondNumber = $secondNumber;
    }

    public function run()
    {
        return $this->firstNumber + $this->secondNumber;
    }
}

需求二：客戶想要在既有的計算機上，增加減法功能

我們思路如下：

首先改變建構式，讓參數有運算符號

public function __construct($firstNumber, $secondNumber, $operationString)
{
    $this->firstNumber = $firstNumber;
    $this->secondNumber = $secondNumber;
    $this->operationString = $operationString;
}

由運算符號，執行對應的方法

public function run()
{
    $operationString = $this->operationString;

    switch ($operationString) {
        case '+':
            return $this->firstNumber + $this->secondNumber;
            break;

        case '-':
            return $this->firstNumber - $this->secondNumber;
            break;
    }
}

需求三：客戶想要除法的功能

於是我們在函式中，再添加一個除法的功能

public function run()
{
    $operationString = $this->operationString;

    switch ($operationString) {
        case '+':
            return $this->firstNumber + $this->secondNumber;
            break;

        case '-':
            return $this->firstNumber - $this->secondNumber;
            break;

        case "/":
            if ($this->secondNumber === 0) {
                return 'Division by zero';
            }
            return $this->firstNumber / $this->secondNumber;
            break;
    }
}

這時候功能是完成了，但有沒有覺得哪裡怪怪的？

除法的case很醜，要額外判斷除數不為0，增加了嵌套層數。
每當客戶要新增/修改/刪除一種運算時，其餘的運算都會被修改到。
違反了開放封閉原則。

讓我們用簡單工廠改造它。

首先定義運算的介面

<?php

namespace App\FactoryPattern\Calculator\Contracts;

interface Operable
{
    public function execute();
}

實作加法運算

<?php

namespace App\FactoryPattern\Calculator\Operation;

use App\FactoryPattern\Calculator\Contracts\Operable;

class AddOperation implements Operable
{
    public $firstNumber;
    public $secondNumber;

    public function execute()
    {
        return $this->firstNumber + $this->secondNumber;
    }
}

實作減法運算

<?php

namespace App\FactoryPattern\Calculator\Operation;

use App\FactoryPattern\Calculator\Contracts\Operable;

class MinusOperation implements Operable
{
    public $firstNumber;
    public $secondNumber;

    public function execute()
    {
        return $this->firstNumber - $this->secondNumber;
    }
}

實作除法運算

<?php

namespace App\FactoryPattern\Calculator\Operation;

use App\FactoryPattern\Calculator\Contracts\Operable;

class DivideOperation implements Operable
{
    public $firstNumber;
    public $secondNumber;

    public function execute()
    {
        if ($this->secondNumber === 0) {
            return 'Division by zero';
        }
        return $this->firstNumber / $this->secondNumber;
    }
}

最後做一個創造運算的工廠

<?php

namespace App\FactoryPattern\Calculator;

use App\FactoryPattern\Calculator\Contracts\Operable;
use App\FactoryPattern\Calculator\Operation\AddOperation;
use App\FactoryPattern\Calculator\Operation\MinusOperation;
use App\FactoryPattern\Calculator\Operation\DivideOperation;

class SimpleOperationFactory
{
    /**
     * @param string $operationString
     * @return Operable
     */
    public function create($operationString): Operable
    {
        switch ($operationString) {
            case '+':
                return new AddOperation();
                break;

            case '-':
                return new MinusOperation();
                break;

            case '/':
                return new DivideOperation();
                break;
        }
    }
}

修改原本的產品程式碼，讓運算由工廠產生

<?php

namespace App\FactoryPattern\Calculator;

use App\FactoryPattern\Calculator\Contracts\Operable;
use App\FactoryPattern\Calculator\SimpleOperationFactory;

class Program
{
    /**
     * @var int
     */
    private $firstNumber;

    /**
     * @var int
     */
    private $secondNumber;

    /**
     * @var string
     */
    private $operationString;

    /**
     * @var Operable
     */
    private $operation;

    public function __construct($firstNumber, $secondNumber, $operationString)
    {
        $this->firstNumber = $firstNumber;
        $this->secondNumber = $secondNumber;
        $this->operationString = $operationString;
    }

    public function run()
    {
        $simpleOperationFactory = new SimpleOperationFactory();
        $this->operation = $simpleOperationFactory->create($this->operationString);

        return $this->execute();
    }

    /**
     * @return int|string
     */
    private function execute()
    {
        $this->operation->firstNumber = $this->firstNumber;
        $this->operation->secondNumber = $this->secondNumber;
        return $this->operation->execute();
    }
}

需求四：客戶想要乘法的功能

實作乘法運算

<?php

namespace App\FactoryPattern\Calculator\Operation;

use App\FactoryPattern\Calculator\Contracts\Operable;

class MultiplyOperation implements Operable
{
    public $firstNumber;
    public $secondNumber;

    public function execute()
    {
        return $this->firstNumber * $this->secondNumber;
    }
}

修改運算工廠

/**
  * @param string $operationString
  * @return Operable
  */
public function create($operationString): Operable
{
    switch ($operationString) {
        case '+':
            return new AddOperation();
            break;

        case '-':
            return new MinusOperation();
            break;

        case '/':
            return new DivideOperation();
            break;

        case '*':
            return new MultiplyOperation();
            break;
    }
}

[單一職責原則]
將創造運算的職責跟運算本身的職責分離，就是工廠模式的精神！

[開放封閉原則]
這下子，我們終於不會每次新增/修改/刪除一個運算時，
影響到其餘所有運算了。

ʕ •ᴥ•ʔ：函式出現switch時，很可能就是使用工廠模式的時機。

Clean Code

我們思路如下：