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Polecenie w języku PHP
Polecenie to behawioralny wzorzec projektowy według którego żądania lub proste działania są konwertowane na obiekty.
Wyżej wymieniona konwersja pozwala odkładać zadania w czasie, uruchamiać je zdalnie, przechowywać ich historię, itp.
Złożoność:
Popularność:
Przykłady użycia: Wzorzec Polecenie jest dość powszechny w kodzie PHP. Służy kolejkowaniu zadań, śledzeniu historii wykonanych zadań oraz wycofywaniu operacji.
Identyfikacja: Jeśli widzisz zestaw powiązanych klas odpowiadających konkretnym czynnościom (jak “Kopiuj”, “Wytnij”, “Wyślij”, “Drukuj”, itd.) — może to oznaczać użycie wzorca Polecenie. Takie klasy powinny implementować ten sam interfejs/klasę abstrakcyjną. Polecenia mogą samodzielnie implementować poszczególne czynności lub delegować zadania osobnym obiektom — zwanym odbiorcami. Ostatnim elementem układanki jest znalezienie obiektu wywołującego — będzie to klasa przyjmująca obiekty-polecenia w charakterze parametrów swoich metod lub konstruktora.
Przykład koncepcyjny
Poniższy przykład ilustruje strukturę wzorca Polecenie ze szczególnym naciskiem na następujące kwestie:
- Z jakich składa się klas?
- Jakie role pełnią te klasy?
- W jaki sposób elementy wzorca są ze sobą powiązane?
Poznawszy strukturę wzorca będzie ci łatwiej zrozumieć następujący przykład, oparty na prawdziwym przypadku użycia PHP.
index.php: Przykład koncepcyjny
<?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: Wynik działania
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.)
Przykład z prawdziwego życia
W poniższym przykładzie wzorzec Polecenie wykorzystano do ułożenia kolejki zapytań do portalu IMDB i wysyłania ich po kolei. Sama kolejka przechowywana jest w bazie danych, co umożliwia zachowanie poleceń pomiędzy kolejnymi uruchomieniami skryptu.
index.php: Przykład z prawdziwego życia
<?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 »</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: Wynik działania
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.
...
