Laravel 13 访问者模式深度解析

访问者模式是一种行为型设计模式，它将算法与对象结构分离，使得可以在不修改对象结构的情况下定义新的操作。本文将深入探讨 Laravel 13 中访问者模式的高级用法。

访问者模式基础

什么是访问者模式

访问者模式允许在不修改对象结构的情况下，定义作用于这些对象的新操作。

<?php

namespace App\Contracts;

interface VisitorInterface
{
    public function visit(ElementInterface $element): mixed;
}

<?php

namespace App\Contracts;

interface ElementInterface
{
    public function accept(VisitorInterface $visitor): mixed;
}

文档元素访问者

文档元素接口

<?php

namespace App\Contracts\Document;

interface DocumentElementInterface
{
    public function accept(DocumentVisitorInterface $visitor): mixed;

    public function getContent(): string;
}

<?php

namespace App\Contracts\Document;

interface DocumentVisitorInterface
{
    public function visitParagraph(ParagraphElement $paragraph): mixed;

    public function visitHeading(HeadingElement $heading): mixed;

    public function visitImage(ImageElement $image): mixed;

    public function visitTable(TableElement $table): mixed;

    public function visitCodeBlock(CodeBlockElement $codeBlock): mixed;
}

具体元素实现

<?php

namespace App\Elements\Document;

use App\Contracts\Document\DocumentElementInterface;
use App\Contracts\Document\DocumentVisitorInterface;

class ParagraphElement implements DocumentElementInterface
{
    protected string $content;

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

    public function accept(DocumentVisitorInterface $visitor): mixed
    {
        return $visitor->visitParagraph($this);
    }

    public function getContent(): string
    {
        return $this->content;
    }
}

<?php

namespace App\Elements\Document;

use App\Contracts\Document\DocumentElementInterface;
use App\Contracts\Document\DocumentVisitorInterface;

class HeadingElement implements DocumentElementInterface
{
    protected string $content;
    protected int $level;

    public function __construct(string $content, int $level = 1)
    {
        $this->content = $content;
        $this->level = $level;
    }

    public function accept(DocumentVisitorInterface $visitor): mixed
    {
        return $visitor->visitHeading($this);
    }

    public function getContent(): string
    {
        return $this->content;
    }

    public function getLevel(): int
    {
        return $this->level;
    }
}

<?php

namespace App\Elements\Document;

use App\Contracts\Document\DocumentElementInterface;
use App\Contracts\Document\DocumentVisitorInterface;

class ImageElement implements DocumentElementInterface
{
    protected string $src;
    protected string $alt;
    protected int $width;
    protected int $height;

    public function __construct(string $src, string $alt = '', int $width = 0, int $height = 0)
    {
        $this->src = $src;
        $this->alt = $alt;
        $this->width = $width;
        $this->height = $height;
    }

    public function accept(DocumentVisitorInterface $visitor): mixed
    {
        return $visitor->visitImage($this);
    }

    public function getContent(): string
    {
        return $this->alt;
    }

    public function getSrc(): string
    {
        return $this->src;
    }

    public function getAlt(): string
    {
        return $this->alt;
    }

    public function getWidth(): int
    {
        return $this->width;
    }

    public function getHeight(): int
    {
        return $this->height;
    }
}

<?php

namespace App\Elements\Document;

use App\Contracts\Document\DocumentElementInterface;
use App\Contracts\Document\DocumentVisitorInterface;

class TableElement implements DocumentElementInterface
{
    protected array $headers;
    protected array $rows;

    public function __construct(array $headers, array $rows)
    {
        $this->headers = $headers;
        $this->rows = $rows;
    }

    public function accept(DocumentVisitorInterface $visitor): mixed
    {
        return $visitor->visitTable($this);
    }

    public function getContent(): string
    {
        return '';
    }

    public function getHeaders(): array
    {
        return $this->headers;
    }

    public function getRows(): array
    {
        return $this->rows;
    }
}

<?php

namespace App\Elements\Document;

use App\Contracts\Document\DocumentElementInterface;
use App\Contracts\Document\DocumentVisitorInterface;

class CodeBlockElement implements DocumentElementInterface
{
    protected string $code;
    protected string $language;

    public function __construct(string $code, string $language = 'php')
    {
        $this->code = $code;
        $this->language = $language;
    }

    public function accept(DocumentVisitorInterface $visitor): mixed
    {
        return $visitor->visitCodeBlock($this);
    }

    public function getContent(): string
    {
        return $this->code;
    }

    public function getCode(): string
    {
        return $this->code;
    }

    public function getLanguage(): string
    {
        return $this->language;
    }
}

HTML 渲染访问者

<?php

namespace App\Visitors\Document;

use App\Contracts\Document\DocumentVisitorInterface;
use App\Elements\Document\{ParagraphElement, HeadingElement, ImageElement, TableElement, CodeBlockElement};

class HtmlRenderVisitor implements DocumentVisitorInterface
{
    public function visitParagraph(ParagraphElement $paragraph): string
    {
        return "<p>{$paragraph->getContent()}</p>";
    }

    public function visitHeading(HeadingElement $heading): string
    {
        $level = $heading->getLevel();
        $content = htmlspecialchars($heading->getContent());
        return "<h{$level}>{$content}</h{$level}>";
    }

    public function visitImage(ImageElement $image): string
    {
        $src = htmlspecialchars($image->getSrc());
        $alt = htmlspecialchars($image->getAlt());
        $width = $image->getWidth();
        $height = $image->getHeight();

        $attrs = "src=\"{$src}\" alt=\"{$alt}\"";

        if ($width) {
            $attrs .= " width=\"{$width}\"";
        }

        if ($height) {
            $attrs .= " height=\"{$height}\"";
        }

        return "<img {$attrs}>";
    }

    public function visitTable(TableElement $table): string
    {
        $html = '<table>';

        $html .= '<thead><tr>';
        foreach ($table->getHeaders() as $header) {
            $html .= '<th>' . htmlspecialchars($header) . '</th>';
        }
        $html .= '</tr></thead>';

        $html .= '<tbody>';
        foreach ($table->getRows() as $row) {
            $html .= '<tr>';
            foreach ($row as $cell) {
                $html .= '<td>' . htmlspecialchars($cell) . '</td>';
            }
            $html .= '</tr>';
        }
        $html .= '</tbody></table>';

        return $html;
    }

    public function visitCodeBlock(CodeBlockElement $codeBlock): string
    {
        $code = htmlspecialchars($codeBlock->getCode());
        $language = htmlspecialchars($codeBlock->getLanguage());

        return "<pre><code class=\"language-{$language}\">{$code}</code></pre>";
    }
}

Markdown 渲染访问者

<?php

namespace App\Visitors\Document;

use App\Contracts\Document\DocumentVisitorInterface;
use App\Elements\Document\{ParagraphElement, HeadingElement, ImageElement, TableElement, CodeBlockElement};

class MarkdownRenderVisitor implements DocumentVisitorInterface
{
    public function visitParagraph(ParagraphElement $paragraph): string
    {
        return $paragraph->getContent() . "\n\n";
    }

    public function visitHeading(HeadingElement $heading): string
    {
        $prefix = str_repeat('#', $heading->getLevel());
        return "{$prefix} {$heading->getContent()}\n\n";
    }

    public function visitImage(ImageElement $image): string
    {
        return "![{$image->getAlt()}]({$image->getSrc()})\n\n";
    }

    public function visitTable(TableElement $table): string
    {
        $headers = $table->getHeaders();
        $rows = $table->getRows();

        $md = '| ' . implode(' | ', $headers) . ' |' . "\n";
        $md .= '| ' . implode(' | ', array_fill(0, count($headers), '---')) . ' |' . "\n";

        foreach ($rows as $row) {
            $md .= '| ' . implode(' | ', $row) . ' |' . "\n";
        }

        return $md . "\n";
    }

    public function visitCodeBlock(CodeBlockElement $codeBlock): string
    {
        return "```{$codeBlock->getLanguage()}\n{$codeBlock->getCode()}\n```\n\n";
    }
}

统计访问者

<?php

namespace App\Visitors\Document;

use App\Contracts\Document\DocumentVisitorInterface;
use App\Elements\Document\{ParagraphElement, HeadingElement, ImageElement, TableElement, CodeBlockElement};

class StatisticsVisitor implements DocumentVisitorInterface
{
    protected array $stats = [
        'paragraphs' => 0,
        'headings' => 0,
        'images' => 0,
        'tables' => 0,
        'code_blocks' => 0,
        'total_words' => 0,
        'total_characters' => 0,
    ];

    public function visitParagraph(ParagraphElement $paragraph): array
    {
        $content = $paragraph->getContent();
        $this->stats['paragraphs']++;
        $this->stats['total_words'] += str_word_count($content);
        $this->stats['total_characters'] += strlen($content);

        return $this->stats;
    }

    public function visitHeading(HeadingElement $heading): array
    {
        $content = $heading->getContent();
        $this->stats['headings']++;
        $this->stats['total_words'] += str_word_count($content);
        $this->stats['total_characters'] += strlen($content);

        return $this->stats;
    }

    public function visitImage(ImageElement $image): array
    {
        $this->stats['images']++;

        return $this->stats;
    }

    public function visitTable(TableElement $table): array
    {
        $this->stats['tables']++;

        foreach ($table->getRows() as $row) {
            foreach ($row as $cell) {
                $this->stats['total_words'] += str_word_count($cell);
                $this->stats['total_characters'] += strlen($cell);
            }
        }

        return $this->stats;
    }

    public function visitCodeBlock(CodeBlockElement $codeBlock): array
    {
        $this->stats['code_blocks']++;
        $this->stats['total_characters'] += strlen($codeBlock->getCode());

        return $this->stats;
    }

    public function getStats(): array
    {
        return $this->stats;
    }
}

产品访问者

产品元素接口

<?php

namespace App\Contracts\Product;

interface ProductElementInterface
{
    public function accept(ProductVisitorInterface $visitor): mixed;

    public function getName(): string;

    public function getPrice(): float;
}

<?php

namespace App\Contracts\Product;

interface ProductVisitorInterface
{
    public function visitSimpleProduct(SimpleProduct $product): mixed;

    public function visitBundleProduct(BundleProduct $product): mixed;

    public function visitConfigurableProduct(ConfigurableProduct $product): mixed;

    public function visitDigitalProduct(DigitalProduct $product): mixed;
}

具体产品元素

<?php

namespace App\Elements\Product;

use App\Contracts\Product\ProductElementInterface;
use App\Contracts\Product\ProductVisitorInterface;

class SimpleProduct implements ProductElementInterface
{
    protected string $name;
    protected float $price;
    protected int $stock;

    public function __construct(string $name, float $price, int $stock)
    {
        $this->name = $name;
        $this->price = $price;
        $this->stock = $stock;
    }

    public function accept(ProductVisitorInterface $visitor): mixed
    {
        return $visitor->visitSimpleProduct($this);
    }

    public function getName(): string
    {
        return $this->name;
    }

    public function getPrice(): float
    {
        return $this->price;
    }

    public function getStock(): int
    {
        return $this->stock;
    }
}

<?php

namespace App\Elements\Product;

use App\Contracts\Product\ProductElementInterface;
use App\Contracts\Product\ProductVisitorInterface;
use Illuminate\Support\Collection;

class BundleProduct implements ProductElementInterface
{
    protected string $name;
    protected Collection $items;
    protected float $discount;

    public function __construct(string $name, array $items, float $discount = 0)
    {
        $this->name = $name;
        $this->items = collect($items);
        $this->discount = $discount;
    }

    public function accept(ProductVisitorInterface $visitor): mixed
    {
        return $visitor->visitBundleProduct($this);
    }

    public function getName(): string
    {
        return $this->name;
    }

    public function getPrice(): float
    {
        $total = $this->items->sum(fn($item) => $item->getPrice());
        return $total * (1 - $this->discount);
    }

    public function getItems(): Collection
    {
        return $this->items;
    }

    public function getDiscount(): float
    {
        return $this->discount;
    }
}

价格计算访问者

<?php

namespace App\Visitors\Product;

use App\Contracts\Product\ProductVisitorInterface;
use App\Elements\Product\{SimpleProduct, BundleProduct, ConfigurableProduct, DigitalProduct};

class PriceCalculationVisitor implements ProductVisitorInterface
{
    protected float $taxRate;
    protected array $prices = [];

    public function __construct(float $taxRate = 0.1)
    {
        $this->taxRate = $taxRate;
    }

    public function visitSimpleProduct(SimpleProduct $product): array
    {
        $basePrice = $product->getPrice();
        $tax = $basePrice * $this->taxRate;

        return $this->prices[$product->getName()] = [
            'base_price' => $basePrice,
            'tax' => $tax,
            'total' => $basePrice + $tax,
        ];
    }

    public function visitBundleProduct(BundleProduct $product): array
    {
        $basePrice = $product->getPrice();
        $tax = $basePrice * $this->taxRate;
        $savings = $product->getDiscount() > 0
            ? $product->getItems()->sum(fn($item) => $item->getPrice()) - $basePrice
            : 0;

        return $this->prices[$product->getName()] = [
            'base_price' => $basePrice,
            'tax' => $tax,
            'total' => $basePrice + $tax,
            'savings' => $savings,
        ];
    }

    public function visitConfigurableProduct(ConfigurableProduct $product): array
    {
        $basePrice = $product->getPrice();
        $tax = $basePrice * $this->taxRate;

        return $this->prices[$product->getName()] = [
            'base_price' => $basePrice,
            'tax' => $tax,
            'total' => $basePrice + $tax,
        ];
    }

    public function visitDigitalProduct(DigitalProduct $product): array
    {
        $basePrice = $product->getPrice();
        $tax = $basePrice * $this->taxRate;

        return $this->prices[$product->getName()] = [
            'base_price' => $basePrice,
            'tax' => $tax,
            'total' => $basePrice + $tax,
        ];
    }

    public function getPrices(): array
    {
        return $this->prices;
    }
}

AST 访问者

AST 节点接口

<?php

namespace App\Contracts\AST;

interface AstNodeInterface
{
    public function accept(AstVisitorInterface $visitor): mixed;
}

<?php

namespace App\Contracts\AST;

interface AstVisitorInterface
{
    public function visitNumberNode(NumberNode $node): mixed;

    public function visitStringNode(StringNode $node): mixed;

    public function visitBinaryOpNode(BinaryOpNode $node): mixed;

    public function visitFunctionCallNode(FunctionCallNode $node): mixed;

    public function visitVariableNode(VariableNode $node): mixed;
}

表达式求值访问者

<?php

namespace App\Visitors\AST;

use App\Contracts\AST\AstVisitorInterface;
use App\Nodes\AST\{NumberNode, StringNode, BinaryOpNode, FunctionCallNode, VariableNode};

class EvaluatorVisitor implements AstVisitorInterface
{
    protected array $variables = [];
    protected array $functions = [];

    public function __construct(array $variables = [], array $functions = [])
    {
        $this->variables = $variables;
        $this->functions = $functions;
    }

    public function visitNumberNode(NumberNode $node): float
    {
        return $node->getValue();
    }

    public function visitStringNode(StringNode $node): string
    {
        return $node->getValue();
    }

    public function visitBinaryOpNode(BinaryOpNode $node): mixed
    {
        $left = $node->getLeft()->accept($this);
        $right = $node->getRight()->accept($this);

        return match ($node->getOperator()) {
            '+' => $left + $right,
            '-' => $left - $right,
            '*' => $left * $right,
            '/' => $left / $right,
            '==' => $left == $right,
            '!=' => $left != $right,
            '<' => $left < $right,
            '>' => $left > $right,
            default => throw new \RuntimeException("Unknown operator: {$node->getOperator()}"),
        };
    }

    public function visitFunctionCallNode(FunctionCallNode $node): mixed
    {
        $name = $node->getName();
        $args = array_map(fn($arg) => $arg->accept($this), $node->getArguments());

        if (isset($this->functions[$name])) {
            return ($this->functions[$name])(...$args);
        }

        throw new \RuntimeException("Unknown function: {$name}");
    }

    public function visitVariableNode(VariableNode $node): mixed
    {
        $name = $node->getName();

        if (array_key_exists($name, $this->variables)) {
            return $this->variables[$name];
        }

        throw new \RuntimeException("Undefined variable: {$name}");
    }
}

测试访问者模式

<?php

namespace Tests\Unit\Visitors;

use Tests\TestCase;
use App\Elements\Document\{ParagraphElement, HeadingElement, ImageElement};
use App\Visitors\Document\{HtmlRenderVisitor, MarkdownRenderVisitor, StatisticsVisitor};

class VisitorTest extends TestCase
{
    public function test_html_render_visitor(): void
    {
        $visitor = new HtmlRenderVisitor();

        $paragraph = new ParagraphElement('Hello World');
        $this->assertEquals('<p>Hello World</p>', $paragraph->accept($visitor));

        $heading = new HeadingElement('Title', 1);
        $this->assertEquals('<h1>Title</h1>', $heading->accept($visitor));
    }

    public function test_markdown_render_visitor(): void
    {
        $visitor = new MarkdownRenderVisitor();

        $paragraph = new ParagraphElement('Hello World');
        $this->assertEquals("Hello World\n\n", $paragraph->accept($visitor));

        $heading = new HeadingElement('Title', 2);
        $this->assertEquals("## Title\n\n", $heading->accept($visitor));
    }

    public function test_statistics_visitor(): void
    {
        $visitor = new StatisticsVisitor();

        $paragraph = new ParagraphElement('Hello World');
        $paragraph->accept($visitor);

        $heading = new HeadingElement('Title', 1);
        $heading->accept($visitor);

        $image = new ImageElement('image.jpg', 'Alt text');
        $image->accept($visitor);

        $stats = $visitor->getStats();

        $this->assertEquals(1, $stats['paragraphs']);
        $this->assertEquals(1, $stats['headings']);
        $this->assertEquals(1, $stats['images']);
    }
}

最佳实践

1. 访问者应该无状态

<?php

class StatelessVisitor implements VisitorInterface
{
    public function visit(ElementInterface $element): mixed
    {
        return $this->process($element);
    }

    protected function process(ElementInterface $element): mixed
    {
        return null;
    }
}

2. 使用双分派

<?php

interface ElementInterface
{
    public function accept(VisitorInterface $visitor): mixed;
}

class ConcreteElement implements ElementInterface
{
    public function accept(VisitorInterface $visitor): mixed
    {
        return $visitor->visitConcreteElement($this);
    }
}

3. 提供默认实现

<?php

abstract class BaseVisitor implements VisitorInterface
{
    public function visitElementA(ElementA $element): mixed
    {
        return $this->defaultVisit($element);
    }

    public function visitElementB(ElementB $element): mixed
    {
        return $this->defaultVisit($element);
    }

    protected function defaultVisit($element): mixed
    {
        return null;
    }
}

总结

Laravel 13 的访问者模式提供了一种优雅的方式来分离算法和对象结构。通过合理使用访问者模式，可以在不修改对象结构的情况下添加新的操作，提高代码的可扩展性和可维护性。