设计模式之装饰器模式的原理和实现方法

装饰模式是一种结构型设计模式，允许你通过将对象放入包含行为的特殊封装对象中来为原对象绑定新的行为。

代码示例

<?php
/**
 * Decorator Design Pattern
 *
 * Intent: Lets you attach new behaviors to objects by placing these objects
 * inside special wrapper objects that contain the behaviors.
 *
 * Example: In this example, the Decorator pattern helps you to construct
 * complex text filtering rules to clean up content before rendering it on a web
 * page. Different types of content, such as comments, forum posts or private
 * messages require different sets of filters.
 *
 * For example, while you'd want to strip out all HTML from the comments, you
 * might still want to keep some basic HTML tags in forum posts. Also, you may
 * want to allow posting in Markdown format, which shall be processed before any
 * HTML filtering takes place. All these filtering rules can be represented as
 * separate decorator classes, which can be stacked differently, depending on
 * the nature of the content you have.
 */

/**
 * The Component interface declares a filtering method that must be implemented
 * by all concrete components and decorators.
 */
interface InputFormat
{
    public function formatText(string $text): string;
}

/**
 * The Concrete Component is a core element of decoration. It contains the
 * original text, as is, without any filtering or formatting.
 */
class TextInput implements InputFormat
{
    public function formatText(string $text): string
    {
        return $text;
    }
}

/**
 * The base Decorator class doesn't contain any real filtering or formatting
 * logic. Its main purpose is to implement the basic decoration infrastructure:
 * a field for storing a wrapped component or another decorator and the basic
 * formatting method that delegates the work to the wrapped object. The real
 * formatting job is done by subclasses.
 */
class TextFormat implements InputFormat
{
    /**
     * @var InputFormat
     */
    protected $inputFormat;

    public function __construct(InputFormat $inputFormat)
    {
        $this->inputFormat = $inputFormat;
    }

    /**
     * Decorator delegates all work to a wrapped component.
     */
    public function formatText(string $text): string
    {
        return $this->inputFormat->formatText($text);
    }
}

/**
 * This Concrete Decorator strips out all HTML tags from the given text.
 */
class PlainTextFilter extends TextFormat
{
    public function formatText(string $text): string
    {
        $text = parent::formatText($text);
        return strip_tags($text);
    }
}

/**
 * This Concrete Decorator strips only dangerous HTML tags and attributes that
 * may lead to an XSS vulnerability.
 */
class DangerousHTMLTagsFilter extends TextFormat
{
    private $dangerousTagPatterns = [
        "|<script.*?>([\s\S]*)?</script>|i", // ...
    ];

    private $dangerousAttributes = [
        "onclick", "onkeypress", // ...
    ];


    public function formatText(string $text): string
    {
        $text = parent::formatText($text);

        foreach ($this->dangerousTagPatterns as $pattern) {
            $text = preg_replace($pattern, '', $text);
        }

        foreach ($this->dangerousAttributes as $attribute) {
            $text = preg_replace_callback('|<(.*?)>|', function ($matches) use ($attribute) {
                $result = preg_replace("|$attribute=|i", '', $matches[1]);
                return "<" . $result . ">";
            }, $text);
        }

        return $text;
    }
}

/**
 * This Concrete Decorator provides a rudimentary Markdown → HTML conversion.
 */
class MarkdownFormat extends TextFormat
{
    public function formatText(string $text): string
    {
        $text = parent::formatText($text);

        // Format block elements.
        $chunks = preg_split('|\n\n|', $text);
        foreach ($chunks as &$chunk) {
            // Format headers.
            if (preg_match('|^#+|', $chunk)) {
                $chunk = preg_replace_callback('|^(#+)(.*?)$|', function ($matches) {
                    $h = strlen($matches[1]);
                    return "<h$h>" . trim($matches[2]) . "</h$h>";
                }, $chunk);
            } // Format paragraphs.
            else {
                $chunk = "<p>$chunk</p>";
            }
        }
        $text = implode("\n\n", $chunks);

        // Format inline elements.
        $text = preg_replace("|__(.*?)__|", '<strong>$1</strong>', $text);
        $text = preg_replace("|\*\*(.*?)\*\*|", '<strong>$1</strong>', $text);
        $text = preg_replace("|_(.*?)_|", '<em>$1</em>', $text);
        $text = preg_replace("|\*(.*?)\*|", '<em>$1</em>', $text);

        return $text;
    }
}


/**
 * The client code might be a part of a real website, which renders user-
 * generated content. Since it works with formatters through the Component
 * interface, it doesn't care whether it gets a simple component object or a
 * decorated one.
 */
function displayCommentAsAWebsite(InputFormat $format, string $text)
{
    // ..

    echo $format->formatText($text);

    // ..
}

/**
 * Input formatters are very handy when dealing with user-generated content.
 * Displaying such content "as is" could be very dangerous, especially when
 * anonymous users can generate it (e.g. comments). Your website is not only
 * risking getting tons of spammy links but may also be exposed to XSS attacks.
 */
$dangerousComment = <<<HERE
Hello! Nice blog post!
Please visit my <a href='http://www.iwillhackyou.com'>homepage</a>.
<script src="http://www.iwillhackyou.com/script.js">
  performXSSAttack();
</script>
HERE;

/**
 * Naive comment rendering (unsafe).
 */
$naiveInput = new TextInput();
echo "Website renders comments without filtering (unsafe):\n";
displayCommentAsAWebsite($naiveInput, $dangerousComment);
echo "\n\n\n";

/**
 * Filtered comment rendering (safe).
 */
$filteredInput = new PlainTextFilter($naiveInput);
echo "Website renders comments after stripping all tags (safe):\n";
displayCommentAsAWebsite($filteredInput, $dangerousComment);
echo "\n\n\n";


/**
 * Decorator allows stacking multiple input formats to get fine-grained control
 * over the rendered content.
 */
$dangerousForumPost = <<<HERE
# Welcome

This is my first post on this **gorgeous** forum.

<script src="http://www.iwillhackyou.com/script.js">
  performXSSAttack();
</script>
HERE;

/**
 * Naive post rendering (unsafe, no formatting).
 */
$naiveInput = new TextInput();
echo "Website renders a forum post without filtering and formatting (unsafe, ugly):\n";
displayCommentAsAWebsite($naiveInput, $dangerousForumPost);
echo "\n\n\n";

/**
 * Markdown formatter + filtering dangerous tags (safe, pretty).
 */
$text = new TextInput();
$markdown = new MarkdownFormat($text);
$filteredInput = new DangerousHTMLTagsFilter($markdown);
echo "Website renders a forum post after translating markdown markup" .
    " and filtering some dangerous HTML tags and attributes (safe, pretty):\n";
displayCommentAsAWebsite($filteredInput, $dangerousForumPost);
echo "\n\n\n";