Discussion on: The Visitor Pattern in PHP

The double dispatch can be confusing and make the code harder to understand.

While it is true more indirection can create confusion, the naive implementation (a service relying on instanceof to determine the correct algorithm to use) is very rigid (creates static dependencies between components) and doesn't scale at all (imagine handling hundreds of components this way, good luck 🙄)

The visitor pattern benefits clearly outweights the added complexity and create much cleaner and understandable code IMHO.

The accept() and visit...() methods usually don't return anything, so you need to keep records on the visitor itself.

I don't see why 🤷

All Visitors need every method on the VisitorInterface while it might not have an implementation for it.

You can mitigate that problem by applying the Interface Segregation Sprinciple (ISP) ("no client should be forced to depend on methods it does not use.")

Pro tip: using traits helps keep things clean & reusable.

Using the solution below, we see the Book class now only rely on the BookPageCountVisitorInterface and not the whole visitor. The visitor concrete class doesn't have to implement visitDocument if it's not needed or possible 👍

TL;DR makin small interfaces & traits gives you the flexibility to only implement what you need 😎

interface BookPageCountVisitorInterface
{
    public function visitBook(Book $book): int;
}

interface DocumentPageCountVisitorInterface
{
    public function visitDocument(Document $doc): int;
}

// you can still assemble them into a single interface if you wish
interface PageCountVisitorInterface extends BookPageCountVisitorInterface, DocumentPageCountVisitorInterface
{
    // empty
}

class Book
{
    public function accept(BookPageCountVisitorInterface $visitor)
    {
        return $visitor->visitBook($this);
    }
}

trait BookPageCountVisitor
{
    public function visitBook(Book $book): int
    {
        return 1;
    }
}

trait DocumentPageCountVisitor
{
    public function visitDocument(Document $doc): int
    {
        return 2;
    }
}

class PageCountVisitor implements PageCountVisitorInterface
{
    use BookPageCountVisitor;
    use DocumentPageCountVisitor;
}

$visitor = new PageCountVisitor();
$book = new Book();

echo $book->accept($visitor); // 1