1. 设计模式
  2. 行为型模式
  3. 访问者模式

访问者模式

目的

访问者模式允许你将对象的操作外包给其他对象，这样做的主要原因是为了保持关注点的分离。但是，类必须定义一个允许访问者访问的契约（比如，下面例子中的 Role::accept() 方法）。

这个契约是一个抽象类，但也可以是一个接口。在那种情况下，每个访问者必须选择对访问者调用哪个方法。

UML 类图

访问者模式类图

代码

RoleVisitor.php

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<?php declare(strict_types = 1);

namespace DesignPatterns\Behavioral\Visitor;

/**
 * Note: the visitor must not choose itself which method to
 * invoke, it is the visited object that makes this decision
 */
interface RoleVisitor
{
    public function visitUser(User $role);

    public function visitGroup(Group $role);
}

RecordingVisitor.php

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<?php declare(strict_types = 1);

namespace DesignPatterns\Behavioral\Visitor;

class RecordingVisitor implements RoleVisitor
{
    /**
     * @var Role[]
     */
    private array $visited = [];

    public function visitGroup(Group $role)
    {
        $this->visited[] = $role;
    }

    public function visitUser(User $role)
    {
        $this->visited[] = $role;
    }

    /**
     * @return Role[]
     */
    public function getVisited(): array
    {
        return $this->visited;
    }
}

Role.php

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<?php declare(strict_types = 1);

namespace DesignPatterns\Behavioral\Visitor;

interface Role
{
    public function accept(RoleVisitor $visitor);
}

User.php

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<?php declare(strict_types = 1);

namespace DesignPatterns\Behavioral\Visitor;

class User implements Role
{
    private string $name;

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

    public function getName(): string
    {
        return sprintf('User %s', $this->name);
    }

    public function accept(RoleVisitor $visitor)
    {
        $visitor->visitUser($this);
    }
}

Group.php

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<?php declare(strict_types = 1);

namespace DesignPatterns\Behavioral\Visitor;

class Group implements Role
{
    private string $name;

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

    public function getName(): string
    {
        return sprintf('Group: %s', $this->name);
    }

    public function accept(RoleVisitor $visitor)
    {
        $visitor->visitGroup($this);
    }
}

测试

Tests/VisitorTest.php

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<?php declare(strict_types = 1);

namespace DesignPatterns\Tests\Visitor\Tests;

use DesignPatterns\Behavioral\Visitor\RecordingVisitor;
use DesignPatterns\Behavioral\Visitor\User;
use DesignPatterns\Behavioral\Visitor\Group;
use DesignPatterns\Behavioral\Visitor\Role;
use DesignPatterns\Behavioral\Visitor;
use PHPUnit\Framework\TestCase;

class VisitorTest extends TestCase
{
    private RecordingVisitor $visitor;

    protected function setUp(): void
    {
        $this->visitor = new RecordingVisitor();
    }

    public function provideRoles()
    {
        return [
            [new User('Dominik')],
            [new Group('Administrators')],
        ];
    }

    /**
     * @dataProvider provideRoles
     */
    public function testVisitSomeRole(Role $role)
    {
        $role->accept($this->visitor);
        $this->assertSame($role, $this->visitor->getVisited()[0]);
    }
}
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