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Command

Command em PHP

O Command é um padrão de projeto comportamental que converte solicitações ou operações simples em objetos.

A conversão permite a execução adiada ou remota de comandos, armazenamento do histórico de comandos, etc.

Navegação
 Introdução
 Exemplo conceitual
 index
 Output
 Exemplo do mundo real
 index
 Output

Complexidade:

Popularidade:

Exemplos de uso: O padrão Command é bastante comum no código PHP. É usado para enfileirar tarefas, rastrear o histórico de tarefas executadas e executar o “desfazer”.

Identificação: O padrão Command é reconhecível por métodos comportamentais em um tipo abstrato/interface (remetente) que chama um método em uma implementação de um tipo abstrato/interface diferente (destinatário) que foi encapsulado pela implementação do comando durante a sua criação. As classes do Command geralmente são limitadas a ações específicas.

Exemplo conceitual

Este exemplo ilustra a estrutura do padrão de projeto Command. Ele se concentra em responder a estas perguntas:

  • De quais classes ele consiste?
  • Quais papéis essas classes desempenham?
  • De que maneira os elementos do padrão estão relacionados?

Depois de aprender sobre a estrutura do padrão, será mais fácil entender o exemplo a seguir, com base em um caso de uso PHP do mundo real.

index.php: Exemplo conceitual

<?php

namespace RefactoringGuru\Command\Conceptual;

/**
 * The Command interface declares a method for executing a command.
 */
interface Command
{
    public function execute(): void;
}

/**
 * Some commands can implement simple operations on their own.
 */
class SimpleCommand implements Command
{
    private $payload;

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

    public function execute(): void
    {
        echo "SimpleCommand: See, I can do simple things like printing (" . $this->payload . ")\n";
    }
}

/**
 * However, some commands can delegate more complex operations to other objects,
 * called "receivers."
 */
class ComplexCommand implements Command
{
    /**
     * @var Receiver
     */
    private $receiver;

    /**
     * Context data, required for launching the receiver's methods.
     */
    private $a;

    private $b;

    /**
     * Complex commands can accept one or several receiver objects along with
     * any context data via the constructor.
     */
    public function __construct(Receiver $receiver, string $a, string $b)
    {
        $this->receiver = $receiver;
        $this->a = $a;
        $this->b = $b;
    }

    /**
     * Commands can delegate to any methods of a receiver.
     */
    public function execute(): void
    {
        echo "ComplexCommand: Complex stuff should be done by a receiver object.\n";
        $this->receiver->doSomething($this->a);
        $this->receiver->doSomethingElse($this->b);
    }
}

/**
 * The Receiver classes contain some important business logic. They know how to
 * perform all kinds of operations, associated with carrying out a request. In
 * fact, any class may serve as a Receiver.
 */
class Receiver
{
    public function doSomething(string $a): void
    {
        echo "Receiver: Working on (" . $a . ".)\n";
    }

    public function doSomethingElse(string $b): void
    {
        echo "Receiver: Also working on (" . $b . ".)\n";
    }
}

/**
 * The Invoker is associated with one or several commands. It sends a request to
 * the command.
 */
class Invoker
{
    /**
     * @var Command
     */
    private $onStart;

    /**
     * @var Command
     */
    private $onFinish;

    /**
     * Initialize commands.
     */
    public function setOnStart(Command $command): void
    {
        $this->onStart = $command;
    }

    public function setOnFinish(Command $command): void
    {
        $this->onFinish = $command;
    }

    /**
     * The Invoker does not depend on concrete command or receiver classes. The
     * Invoker passes a request to a receiver indirectly, by executing a
     * command.
     */
    public function doSomethingImportant(): void
    {
        echo "Invoker: Does anybody want something done before I begin?\n";
        if ($this->onStart instanceof Command) {
            $this->onStart->execute();
        }

        echo "Invoker: ...doing something really important...\n";

        echo "Invoker: Does anybody want something done after I finish?\n";
        if ($this->onFinish instanceof Command) {
            $this->onFinish->execute();
        }
    }
}

/**
 * The client code can parameterize an invoker with any commands.
 */
$invoker = new Invoker();
$invoker->setOnStart(new SimpleCommand("Say Hi!"));
$receiver = new Receiver();
$invoker->setOnFinish(new ComplexCommand($receiver, "Send email", "Save report"));

$invoker->doSomethingImportant();

Output.txt: Resultados da execução

Invoker: Does anybody want something done before I begin?
SimpleCommand: See, I can do simple things like printing (Say Hi!)
Invoker: ...doing something really important...
Invoker: Does anybody want something done after I finish?
ComplexCommand: Complex stuff should be done by a receiver object.
Receiver: Working on (Send email.)
Receiver: Also working on (Save report.)

Exemplo do mundo real

Neste exemplo, o padrão Command é usado para enfileirar chamadas de web scraping no site do IMDB e executá-las uma a uma. A fila em si é mantida em um banco de dados que ajuda a preservar comandos entre ativações de script.

index.php: Exemplo do mundo real

<?php

namespace RefactoringGuru\Command\RealWorld;

/**
 * The Command interface declares the main execution method as well as several
 * helper methods for retrieving a command's metadata.
 */
interface Command
{
    public function execute(): void;

    public function getId(): int;

    public function getStatus(): int;
}

/**
 * The base web scraping Command defines the basic downloading infrastructure,
 * common to all concrete web scraping commands.
 */
abstract class WebScrapingCommand implements Command
{
    public $id;

    public $status = 0;

    /**
     * @var string URL for scraping.
     */
    public $url;

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

    public function getId(): int
    {
        return $this->id;
    }

    public function getStatus(): int
    {
        return $this->status;
    }

    public function getURL(): string
    {
        return $this->url;
    }

    /**
     * Since the execution methods for all web scraping commands are very
     * similar, we can provide a default implementation and let subclasses
     * override them if needed.
     *
     * Psst! An observant reader may spot another behavioral pattern in action
     * here.
     */
    public function execute(): void
    {
        $html = $this->download();
        $this->parse($html);
        $this->complete();
    }

    public function download(): string
    {
        $html = file_get_contents($this->getURL());
        echo "WebScrapingCommand: Downloaded {$this->url}\n";

        return $html;
    }

    abstract public function parse(string $html): void;

    public function complete(): void
    {
        $this->status = 1;
        Queue::get()->completeCommand($this);
    }
}

/**
 * The Concrete Command for scraping the list of movie genres.
 */
class IMDBGenresScrapingCommand extends WebScrapingCommand
{
    public function __construct()
    {
        $this->url = "https://www.imdb.com/feature/genre/";
    }

    /**
     * Extract all genres and their search URLs from the page:
     * https://www.imdb.com/feature/genre/
     */
    public function parse($html): void
    {
        preg_match_all("|href=\"(https://www.imdb.com/search/title\?genres=.*?)\"|", $html, $matches);
        echo "IMDBGenresScrapingCommand: Discovered " . count($matches[1]) . " genres.\n";

        foreach ($matches[1] as $genre) {
            Queue::get()->add(new IMDBGenrePageScrapingCommand($genre));
        }
    }
}

/**
 * The Concrete Command for scraping the list of movies in a specific genre.
 */
class IMDBGenrePageScrapingCommand extends WebScrapingCommand
{
    private $page;

    public function __construct(string $url, int $page = 1)
    {
        parent::__construct($url);
        $this->page = $page;
    }

    public function getURL(): string
    {
        return $this->url . '?page=' . $this->page;
    }

    /**
     * Extract all movies from a page like this:
     * https://www.imdb.com/search/title?genres=sci-fi&explore=title_type,genres
     */
    public function parse(string $html): void
    {
        preg_match_all("|href=\"(/title/.*?/)\?ref_=adv_li_tt\"|", $html, $matches);
        echo "IMDBGenrePageScrapingCommand: Discovered " . count($matches[1]) . " movies.\n";

        foreach ($matches[1] as $moviePath) {
            $url = "https://www.imdb.com" . $moviePath;
            Queue::get()->add(new IMDBMovieScrapingCommand($url));
        }

        // Parse the next page URL.
        if (preg_match("|Next &#187;</a>|", $html)) {
            Queue::get()->add(new IMDBGenrePageScrapingCommand($this->url, $this->page + 1));
        }
    }
}

/**
 * The Concrete Command for scraping the movie details.
 */
class IMDBMovieScrapingCommand extends WebScrapingCommand
{
    /**
     * Get the movie info from a page like this:
     * https://www.imdb.com/title/tt4154756/
     */
    public function parse(string $html): void
    {
        if (preg_match("|<h1 itemprop=\"name\" class=\"\">(.*?)</h1>|", $html, $matches)) {
            $title = $matches[1];
        }
        echo "IMDBMovieScrapingCommand: Parsed movie $title.\n";
    }
}

/**
 * The Queue class acts as an Invoker. It stacks the command objects and
 * executes them one by one. If the script execution is suddenly terminated, the
 * queue and all its commands can easily be restored, and you won't need to
 * repeat all of the executed commands.
 *
 * Note that this is a very primitive implementation of the command queue, which
 * stores commands in a local SQLite database. There are dozens of robust queue
 * solution available for use in real apps.
 */
class Queue
{
    private $db;

    public function __construct()
    {
        $this->db = new \SQLite3(__DIR__ . '/commands.sqlite',
            SQLITE3_OPEN_CREATE | SQLITE3_OPEN_READWRITE);

        $this->db->query('CREATE TABLE IF NOT EXISTS "commands" (
            "id" INTEGER PRIMARY KEY AUTO_INCREMENT NOT NULL,
            "command" TEXT,
            "status" INTEGER
        )');
    }

    public function isEmpty(): bool
    {
        $query = 'SELECT COUNT("id") FROM "commands" WHERE status = 0';

        return $this->db->querySingle($query) === 0;
    }

    public function add(Command $command): void
    {
        $query = 'INSERT INTO commands (command, status) VALUES (:command, :status)';
        $statement = $this->db->prepare($query);
        $statement->bindValue(':command', base64_encode(serialize($command)));
        $statement->bindValue(':status', $command->getStatus());
        $statement->execute();
    }

    public function getCommand(): Command
    {
        $query = 'SELECT * FROM "commands" WHERE "status" = 0 LIMIT 1';
        $record = $this->db->querySingle($query, true);
        $command = unserialize(base64_decode($record["command"]));
        $command->id = $record['id'];

        return $command;
    }

    public function completeCommand(Command $command): void
    {
        $query = 'UPDATE commands SET status = :status WHERE id = :id';
        $statement = $this->db->prepare($query);
        $statement->bindValue(':status', $command->getStatus());
        $statement->bindValue(':id', $command->getId());
        $statement->execute();
    }

    public function work(): void
    {
        while (!$this->isEmpty()) {
            $command = $this->getCommand();
            $command->execute();
        }
    }

    /**
     * For our convenience, the Queue object is a Singleton.
     */
    public static function get(): Queue
    {
        static $instance;
        if (!$instance) {
            $instance = new Queue();
        }

        return $instance;
    }
}

/**
 * The client code.
 */

$queue = Queue::get();

if ($queue->isEmpty()) {
    $queue->add(new IMDBGenresScrapingCommand());
}

$queue->work();

Output.txt: Resultados da execução

WebScrapingCommand: Downloaded https://www.imdb.com/feature/genre/
IMDBGenresScrapingCommand: Discovered 14 genres.
WebScrapingCommand: Downloaded https://www.imdb.com/search/title?genres=comedy
IMDBGenrePageScrapingCommand: Discovered 50 movies.
WebScrapingCommand: Downloaded https://www.imdb.com/search/title?genres=sci-fi
IMDBGenrePageScrapingCommand: Discovered 50 movies.
WebScrapingCommand: Downloaded https://www.imdb.com/search/title?genres=horror
IMDBGenrePageScrapingCommand: Discovered 50 movies.
WebScrapingCommand: Downloaded https://www.imdb.com/search/title?genres=romance
IMDBGenrePageScrapingCommand: Discovered 50 movies.
...

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