PHP 設計模式實戰：命令模式封裝請求與支援撤銷操作

前言

命令模式將「請求」封裝為物件，使請求可參數化、排隊、記錄、撤銷。適用於 GUI 按鈕、宏指令、交易回滾、工作佇列等場景。本文提供 PHP 介面導向實作，並示範撤銷功能。

角色定義

完整程式碼（含撤銷支援）

<?php
// 1. 命令介面
interface Command {
    public function execute();
    public function undo();
}

// 2. 接收者：實際執行業務邏輯
class LightReceiver {
    private $location;
    private $isOn = false;
    private $brightness = 0;

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

    public function turnOn() {
        $this->isOn = true;
        $this->brightness = 100;
        echo "[{$this->location} 燈] 開啟，亮度 {$this->brightness}%\n";
    }

    public function turnOff() {
        $this->isOn = false;
        $this->brightness = 0;
        echo "[{$this->location} 燈] 關閉\n";
    }

    public function dim(int $level) {
        $this->brightness = $level;
        echo "[{$this->location} 燈] 調光至 {$level}%\n";
    }

    public function getState(): array {
        return ['on' => $this->isOn, 'brightness' => $this->brightness];
    }
}

// 3. 具體命令：開燈
class LightOnCommand implements Command {
    private $light;
    private $prevState;

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

    public function execute() {
        $this->prevState = $this->light->getState(); // 記錄前狀態供撤銷
        $this->light->turnOn();
    }

    public function undo() {
        if ($this->prevState['on']) {
            $this->light->turnOn();
            $this->light->dim($this->prevState['brightness']);
        } else {
            $this->light->turnOff();
        }
        echo "[撤銷] 開燈命令 -> 恢復狀態\n";
    }
}

// 4. 具體命令：關燈
class LightOffCommand implements Command {
    private $light;
    private $prevState;

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

    public function execute() {
        $this->prevState = $this->light->getState();
        $this->light->turnOff();
    }

    public function undo() {
        if ($this->prevState['on']) {
            $this->light->turnOn();
            $this->light->dim($this->prevState['brightness']);
        } else {
            $this->light->turnOff();
        }
        echo "[撤銷] 關燈命令 -> 恢復狀態\n";
    }
}

// 5. 調用者：支援命令歷史與撤銷
class RemoteControlInvoker {
    /** @var Command[] */
    private $history = [];
    private $lastCommand = null;

    public function setCommand(Command $command) {
        $this->lastCommand = $command;
    }

    public function pressButton() {
        if ($this->lastCommand) {
            $this->lastCommand->execute();
            $this->history[] = $this->lastCommand;
        }
    }

    public function pressUndo() {
        if ($this->lastCommand) {
            $this->lastCommand->undo();
            array_pop($this->history);
            $this->lastCommand = end($this->history) ?: null;
        }
    }
}

// 6. 客戶端測試
$livingRoomLight = new LightReceiver('客廳');
$bedroomLight    = new LightReceiver('臥室');

$livingOn  = new LightOnCommand($livingRoomLight);
$livingOff = new LightOffCommand($livingRoomLight);
$bedOn     = new LightOnCommand($bedroomLight);

$remote = new RemoteControlInvoker();

echo "--- 開客廳燈 ---\n";
$remote->setCommand($livingOn);
$remote->pressButton();

echo "\n--- 開臥室燈 ---\n";
$remote->setCommand($bedOn);
$remote->pressButton();

echo "\n--- 關客廳燈 ---\n";
$remote->setCommand($livingOff);
$remote->pressButton();

echo "\n--- 撤銷最後操作 (關客廳燈) ---\n";
$remote->pressUndo();

echo "\n--- 再次撤銷 (開臥室燈) ---\n";
$remote->pressUndo();

輸出結果

--- 開客廳燈 ---
[客廳 燈] 開啟，亮度 100%

--- 開臥室燈 ---
[臥室 燈] 開啟，亮度 100%

--- 關客廳燈 ---
[客廳 燈] 關閉

--- 撤銷最後操作 (關客廳燈) ---
[撤銷] 關燈命令 -> 恢復狀態
[客廳 燈] 開啟，亮度 100%

--- 再次撤銷 (開臥室燈) ---
[撤銷] 開燈命令 -> 恢復狀態
[臥室 燈] 關閉

核心優勢

1. 請求物件化： 可儲存、傳遞、序列化、排隊（如工作佇列）。
2. 解耦調用者與接收者： Invoker 不知具體業務邏輯，只呼叫 execute()。
3. 支援撤銷/重做： 只要命令實作 undo() 並記錄前狀態即可。
4. 組合命令（宏命令）： 實作 MacroCommand 包含多個子命令，一次執行整個流程。
5. 日誌/審計： 命令物件天然包含操作語意，易於記錄、重放。

宏命令範例

class MacroCommand implements Command {
    /** @var Command[] */
    private $commands = [];

    public function add(Command $cmd) { $this->commands[] = $cmd; }

    public function execute() {
        foreach ($this->commands as $cmd) $cmd->execute();
    }

    public function undo() {
        // 撤銷順序相反
        for ($i = count($this->commands)-1; $i >= 0; $i--) {
            $this->commands[$i]->undo();
        }
    }
}

// 使用：一鍵「離家模式」= 關所有燈 + 鎖門 + 啟動警報
$leaveHome = new MacroCommand();
$leaveHome->add(new LightOffCommand($livingRoomLight));
$leaveHome->add(new LightOffCommand($bedroomLight));
$leaveHome->add(new LockDoorCommand($door));
$leaveHome->add(new AlarmOnCommand($alarm));

$remote->setCommand($leaveHome);
$remote->pressButton(); // 一次執行全部

結語

命令模式將「做什麼」與「誰做、何時做」分離，是實現可撤銷操作、工作佇列、巨集指令、交易回滾的基石。PHP 介面 + 型別提示讓結構清晰、易於測試與擴展。