Visitor Pattern Generator

Creates Visitor pattern infrastructure for operations on object structures without modifying classes.

When to Use

Scenario	Example
Operations on object structure	Calculate price/tax on order items
Adding operations without modification	Export visitors (JSON, XML, CSV)
Different operations on same elements	Validation, transformation, rendering
Double dispatch needed	Type-specific behavior without instanceof

Component Characteristics

Visitor Interface

• Declares visit methods for each element type
• One method per visitable element
• Returns operation result
• Enables double dispatch

Concrete Visitors

• Implement specific operations
• Process each element type differently
• Encapsulate algorithm logic
• Can accumulate state during traversal

Visitable Elements

• Accept visitor via accept() method
• Call visitor's visit method with self
• Enable operation without modification
• Maintain element structure

---

Generation Process

Step 1: Generate Visitor Interface

Path: src/Domain/{BoundedContext}/Visitor/

1. {Name}VisitorInterface.php — Visitor contract with visit methods

Step 2: Generate Concrete Visitors

Path: src/Domain/{BoundedContext}/Visitor/ or src/Application/{BoundedContext}/

1. {Operation1}Visitor.php — First operation implementation
2. {Operation2}Visitor.php — Second operation implementation
3. {Operation3}Visitor.php — Third operation implementation

Step 3: Generate Visitable Interface

Path: src/Domain/{BoundedContext}/

1. VisitableInterface.php — Element contract with accept() method

Step 4: Update Existing Elements

Path: src/Domain/{BoundedContext}/

1. Add implements VisitableInterface to element classes
2. Add accept() method to each element

Step 5: Generate Tests

1. {Operation}VisitorTest.php — Individual visitor tests
2. {Element}AcceptTest.php — Element accept() tests

---

File Placement

Component	Path
Visitor Interface	src/Domain/{BoundedContext}/Visitor/
Concrete Visitors (Domain)	src/Domain/{BoundedContext}/Visitor/
Concrete Visitors (Application)	src/Application/{BoundedContext}/Visitor/
Visitable Interface	src/Domain/{BoundedContext}/
Unit Tests	tests/Unit/Domain/{BoundedContext}/Visitor/

---

Naming Conventions

Component	Pattern	Example
Visitor Interface	{Name}VisitorInterface	OrderItemVisitorInterface
Concrete Visitor	{Operation}Visitor	PriceCalculatorVisitor
Visitable Interface	VisitableInterface	VisitableInterface
Visit Method	visit{ElementType}()	visitProduct()
Accept Method	accept()	accept()
Test	{ClassName}Test	PriceCalculatorVisitorTest

---

Quick Template Reference

Visitor Interface

interface {Name}VisitorInterface
{
    public function visit{Element1}({Element1} $element): {ReturnType};
    public function visit{Element2}({Element2} $element): {ReturnType};
    public function visit{Element3}({Element3} $element): {ReturnType};
}

Concrete Visitor

final class {Operation}Visitor implements {Name}VisitorInterface
{
    public function visit{Element1}({Element1} $element): {ReturnType}
    {
        // Element1-specific operation
    }

    public function visit{Element2}({Element2} $element): {ReturnType}
    {
        // Element2-specific operation
    }
}

Visitable Interface

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

Visitable Element

final readonly class {Element} implements VisitableInterface
{
    public function accept({Name}VisitorInterface $visitor): mixed
    {
        return $visitor->visit{Element}($this);
    }
}

---

Usage Example

// Create elements
$order = new Order(items: [
    new Product(price: 100, quantity: 2),
    new Service(price: 50, duration: 1),
    new Discount(amount: 20),
]);

// Apply different visitors
$priceVisitor = new PriceCalculatorVisitor();
$taxVisitor = new TaxCalculatorVisitor(rate: 0.2);
$exportVisitor = new JsonExportVisitor();

$totalPrice = $order->accept($priceVisitor);
$totalTax = $order->accept($taxVisitor);
$json = $order->accept($exportVisitor);

---

Common Visitor Operations

Domain	Visitors
Order Items	PriceCalculator, TaxCalculator, DiscountApplier
AST/Expression Tree	Evaluator, Formatter, Validator
Document Structure	Renderer, Counter, Searcher
File System	SizeCalculator, Permissions, Backup
Shopping Cart	TotalCalculator, ShippingCost, Export

---

Anti-patterns to Avoid

Anti-pattern	Problem	Solution
instanceof in visitor	Defeats purpose	Use proper visit methods
Mutable visitor state	Race conditions	Use readonly classes
Too many element types	Visitor interface bloat	Split into multiple visitors
Breaking element encapsulation	Tight coupling	Expose getters, not internals
Returning void	Limited usefulness	Return operation results

---

References

For complete PHP templates and examples, see:

• references/templates.md — Visitor Interface, Concrete Visitor, Visitable Element templates
• references/examples.md — PriceCalculator, TaxCalculator, Export visitors with tests