本文同步更新於blog

情境：平台有三種身份，分別是訪客 (guest)、會員 (member)及付費會員(premium)

1. 訪客藉由註冊 (register)，可成為會員。
2. 會員藉由訂閱 (subsribe)，可成為付費會員。
3. 付費會員藉由取消訂閱 (cancelSubscription)，可變回會員。
4. 會員及付費會員藉由刪除帳號 (deleteAccount)，可變回訪客。

<?php

namespace App\StatePattern\Youtube;

use Exception;

class Program
{
    /**
     * @var string
     */
    protected $license;

    public function __construct()
    {
        $this->setLicense('guest');
    }

    public function register()
    {
        if ($this->license == 'premium') {
            return;
        }

        $this->license = 'member';
    }

    public function getLicense()
    {
        return $this->license;
    }

    /**
     * @param string $license
     */
    public function setLicense($license)
    {
        $this->license = $license;
    }

    public function subscribe()
    {
        if ($this->license == 'premium') {
            return;
        }

        if ($this->license == 'member') {
            $this->license = 'premium';
            return;
        }

        throw new Exception('You need to be a member before subscribing.');
    }

    public function cancelSubscription()
    {
        if ($this->license == 'premium') {
            $this->license = 'member';
            return;
        }

        throw new Exception('Sorry, you have not subscribed.');
    }

    public function deleteAccount()
    {
        if ($this->license == 'member' || $this->license == 'premium') {
            $this->license = 'guest';
            return;
        }

        throw new Exception('You need to be a member before deleting account.');
    }
}

隨著功能越來越多，每次新增功能時，
我們都會有許多複雜的條件式（判斷當前用戶狀態）。

讓我們用狀態模式改善它。

需求一：切分出不同的用戶狀態

• 首先定義用戶狀態（抽象）

<?php

namespace App\StatePattern\Youtube\State;

abstract class UserState
{
    const LICENCE = 'undefined user';

    public function getLicense()
    {
        return $this::LICENCE;
    }

    abstract function register();

    abstract function subscribe();

    abstract function cancelSubscription();

    abstract function deleteAccount();
}

• 訪客狀態

<?php

namespace App\StatePattern\Youtube\State;

use App\StatePattern\Youtube\Program;
use Exception;
use App\StatePattern\Youtube\State\UserState;

class GuestState extends UserState
{
    /**
     * @var Program
     */
    protected $program;

    /**
     * @var string
     */
    const LICENCE = 'guest';

    /**
     * @param Program $program
     */
    public function __construct(Program $program)
    {
        $this->program = $program;
    }

    public function register()
    {
        $this->program->setMemberState();
    }

    public function subscribe()
    {
        throw new Exception('You need to be a member before subscribing.');
    }

    public function cancelSubscription()
    {
        throw new Exception('Sorry, you have not subscribed.');
    }

    public function deleteAccount()
    {
        throw new Exception('You need to be a member before deleting account.');
    }
}

• 會員狀態

<?php

namespace App\StatePattern\Youtube\State;

use App\StatePattern\Youtube\Program;
use Exception;
use App\StatePattern\Youtube\State\UserState;

class MemberState extends UserState
{
    /**
     * @var Program
     */
    protected $program;

    /**
     * @var string
     */
    const LICENCE = 'member';

    /**
     * @param Program $program
     */
    public function __construct(Program $program)
    {
        $this->program = $program;
    }

    public function register()
    {
        return;
    }

    public function subscribe()
    {
        $this->program->setPremiumState();
    }

    public function cancelSubscription()
    {
        throw new Exception('Sorry, you have not subscribed.');
    }

    public function deleteAccount()
    {
        $this->program->setGuestState();
    }
}

• 付費會員狀態

<?php

namespace App\StatePattern\Youtube\State;

use App\StatePattern\Youtube\Program;
use App\StatePattern\Youtube\State\UserState;

class PremiumState extends UserState
{
    /**
     * @var Program
     */
    protected $program;

    /**
     * @var string
     */
    const LICENCE = 'premium';

    /**
     * @param Program $program
     */
    public function __construct(Program $program)
    {
        $this->program = $program;
    }

    public function register()
    {
        return;
    }

    public function subscribe()
    {
        return;
    }

    public function cancelSubscription()
    {
        $this->program->setMemberState();
    }

    public function deleteAccount()
    {
        $this->program->setGuestState();
    }
}

• 最後修改原本的情境類別

<?php

namespace App\StatePattern\Youtube;

use App\StatePattern\Youtube\State\MemberState;
use App\StatePattern\Youtube\State\GuestState;
use App\StatePattern\Youtube\State\PremiumState;
use App\StatePattern\Youtube\State\UserState;

class Program
{
    /**
     * @var string
     */
    protected $license;

    /**
     * @var MemberState
     */
    protected $memberState;

    /**
     * @var GuestState
     */
    protected $guestState;

    /**
     * @var PremiumState
     */
    protected $premiumState;

    /**
     * @var UserState
     */
    protected $state;

    public function __construct()
    {
        $this->memberState = new MemberState($this);
        $this->guestState = new GuestState($this);
        $this->premiumState = new PremiumState($this);

        $this->setGuestState();
    }

    public function register()
    {
        $this->state->register();
    }

    public function getLicense()
    {
        return $this->state->getLicense();
    }

    public function subscribe()
    {
        $this->state->subscribe();
    }

    public function cancelSubscription()
    {
        $this->state->cancelSubscription();
    }

    public function setGuestState()
    {
        $this->state = $this->guestState;
    }

    public function setMemberState()
    {
        $this->state = $this->memberState;
    }

    public function setPremiumState()
    {
        $this->state = $this->premiumState;
    }

    public function getState()
    {
        return $this->state;
    }

    public function deleteAccount()
    {
        $this->state->deleteAccount();
    }
}

[單一職責原則]
我們將情境類別與狀態類別視作兩種不同的職責。
透過委派來實現不同狀態下的行為。

[開放封閉原則]
修改既有狀態類別的行為，不會影響到全部的既有狀態類別。
（新增狀態類別時，可能會影響到）

[介面隔離原則]
情境類別依賴於抽象的用戶狀態 (User State)。
不同的狀態類別實作抽象的用戶狀態 (User State)。

最後附上類別圖：

（註：若不熟悉 UML 類別圖，可參考UML類別圖說明。）

ʕ •ᴥ•ʔ：透過測試，重構這個範例時有遇到一些小困難。
大家也可以挑戰看看。