3.2. Command

3.2.1. Purpose

To encapsulate invocation and decoupling.

We have an Invoker and a Receiver. This pattern uses a “Command” to delegate the method call against the Receiver and presents the same method “execute”. Therefore, the Invoker just knows to call “execute” to process the Command of the client. The Receiver is decoupled from the Invoker.

The second aspect of this pattern is the undo(), which undoes the method execute(). Command can also be aggregated to combine more complex commands with minimum copy-paste and relying on composition over inheritance.

3.2.2. Examples

  • A text editor : all events are Command which can be undone, stacked and saved.
  • Symfony2: SF2 Commands that can be run from the CLI are built with just the Command pattern in mind
  • big CLI tools use subcommands to distribute various tasks and pack them in “modules”, each of these can be implemented with the Command pattern (e.g. vagrant)

3.2.3. UML Diagram

Alt Command UML Diagram

3.2.4. Code

You can also find these code on GitHub

CommandInterface.php

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<?php

namespace DesignPatterns\Behavioral\Command;

interface CommandInterface
{
    /**
     * this is the most important method in the Command pattern,
     * The Receiver goes in the constructor.
     */
    public function execute();
}

HelloCommand.php

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<?php

namespace DesignPatterns\Behavioral\Command;

/**
 * This concrete command calls "print" on the Receiver, but an external
 * invoker just knows that it can call "execute"
 */
class HelloCommand implements CommandInterface
{
    /**
     * @var Receiver
     */
    private $output;

    /**
     * Each concrete command is built with different receivers.
     * There can be one, many or completely no receivers, but there can be other commands in the parameters
     *
     * @param Receiver $console
     */
    public function __construct(Receiver $console)
    {
        $this->output = $console;
    }

    /**
     * execute and output "Hello World".
     */
    public function execute()
    {
        // sometimes, there is no receiver and this is the command which does all the work
        $this->output->write('Hello World');
    }
}

Receiver.php

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<?php

namespace DesignPatterns\Behavioral\Command;

/**
 * Receiver is specific service with its own contract and can be only concrete.
 */
class Receiver
{
    /**
     * @var bool
     */
    private $enableDate = false;

    /**
     * @var string[]
     */
    private $output = [];

    /**
     * @param string $str
     */
    public function write(string $str)
    {
        if ($this->enableDate) {
            $str .= ' ['.date('Y-m-d').']';
        }

        $this->output[] = $str;
    }

    public function getOutput(): string
    {
        return join("\n", $this->output);
    }

    /**
     * Enable receiver to display message date
     */
    public function enableDate()
    {
        $this->enableDate = true;
    }

    /**
     * Disable receiver to display message date
     */
    public function disableDate()
    {
        $this->enableDate = false;
    }
}

Invoker.php

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<?php

namespace DesignPatterns\Behavioral\Command;

/**
 * Invoker is using the command given to it.
 * Example : an Application in SF2.
 */
class Invoker
{
    /**
     * @var CommandInterface
     */
    private $command;

    /**
     * in the invoker we find this kind of method for subscribing the command
     * There can be also a stack, a list, a fixed set ...
     *
     * @param CommandInterface $cmd
     */
    public function setCommand(CommandInterface $cmd)
    {
        $this->command = $cmd;
    }

    /**
     * executes the command; the invoker is the same whatever is the command
     */
    public function run()
    {
        $this->command->execute();
    }
}

3.2.5. Test

Tests/CommandTest.php

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<?php

namespace DesignPatterns\Behavioral\Command\Tests;

use DesignPatterns\Behavioral\Command\HelloCommand;
use DesignPatterns\Behavioral\Command\Invoker;
use DesignPatterns\Behavioral\Command\Receiver;

class CommandTest extends \PHPUnit_Framework_TestCase
{
    public function testInvocation()
    {
        $invoker = new Invoker();
        $receiver = new Receiver();

        $invoker->setCommand(new HelloCommand($receiver));
        $invoker->run();
        $this->assertEquals($receiver->getOutput(), 'Hello World');
    }
}