PHP 命令模式讲解和代码示例
命令是一种行为设计模式， 它可将请求或简单操作转换为一个对象。
此类转换让你能够延迟进行或远程执行请求， 还可将其放入队列中。
复杂度：
流行度：
使用示例： 命令模式在 Java 代码中很常见。 它被用于对任务进行排序、 记录任务执行历史以及执行 “撤销” 操作。
识别方法： 命令模式可以通过抽象或接口类型 （发送者） 中的行为方法来识别， 该类型调用另一个不同的抽象或接口类型 （接收者） 实现中的方法， 该实现则是在创建时由命令模式的实现封装。 命令类通常仅限于一些特殊行为。
概念示例
本例说明了命令设计模式的结构并重点回答了下面的问题：
- 它由哪些类组成？
- 这些类扮演了哪些角色？
- 模式中的各个元素会以何种方式相互关联？
了解该模式的结构后， 你可以更容易地理解下面基于真实世界的 PHP 应用案例。
index.php: 概念示例
<?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: 执行结果
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.) 真实世界示例
在本例中， 命令模式被用于对 IMDB 网站的网页抓取操作进行排序， 使它们能依次执行。 队列本身保存在数据库中， 这有助于在脚本运行时保存命令。
index.php: 真实世界示例
<?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: 执行结果
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. ... 
