@ryan.murray

The Command design pattern is a behavioral pattern that is used to encapsulate a request as an object, allowing the request to be parameterized with different arguments, queued, logged, or even undoable. In PHP, you can implement the Command pattern for undoable operations as follows:

Define an interface for the command:

interface Command {
    public function execute();
    public function undo();
}

Implement the interface in a concrete command class, which encapsulates the operation to be performed:

class ConcreteCommand implements Command {
    private $receiver;
    private $state;
    
    public function __construct(Receiver $receiver, $state) {
        $this->receiver = $receiver;
        $this->state = $state;
    }
    
    public function execute() {
        $this->receiver->performOperation($this->state);
    }
    
    public function undo() {
        $this->receiver->undoOperation($this->state);
    }
}

Define a receiver class, which is responsible for performing the operation:

class Receiver {
    private $state;
    
    public function performOperation($state) {
        // Perform the operation
        $this->state = $state;
    }
    
    public function undoOperation($state) {
        // Undo the operation
        $this->state = null;
    }
    
    public function getState() {
        return $this->state;
    }
}

Create a invoker class, which maintains a queue of commands and can execute or undo them:

class Invoker {
    private $history = [];
    
    public function execute(Command $command) {
        $command->execute();
        $this->history[] = $command;
    }
    
    public function undo() {
        $command = array_pop($this->history);
        if ($command) {
            $command->undo();
        }
    }
}

Finally, create an instance of the receiver and the invoker, and use them to execute and undo commands:

$receiver = new Receiver();
$invoker = new Invoker();

$command1 = new ConcreteCommand($receiver, 'state1');
$command2 = new ConcreteCommand($receiver, 'state2');

$invoker->execute($command1);
$invoker->execute($command2);

echo $receiver->getState(); // Outputs 'state2'

$invoker->undo();
echo $receiver->getState(); // Outputs 'state1'

$invoker->undo();
echo $receiver->getState(); // Outputs null

In the above example, we created a ConcreteCommand class that encapsulates an operation on a Receiver object. We also created an Invoker class that maintains a history of executed commands and can undo them. Finally, we created an instance of the Receiver and Invoker classes, and used them to execute and undo commands.

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