Hands-On Design Patterns with Swift

Visitable and visitor protocols

Thanks to Swift and its expressive generics, it is possible to abstract the visitor pattern through a series of protocols:

protocol Visitor {    func visit<T>(element: T) where T:Visitable}

A Visitor is an object that can traverse Visitable elements. It implements a single method, which is generic:

protocol Visitable {    func accept(visitor: Visitor)}

Visitable objects can be traversed by a Visitor. Here we don't use the generics, as they're not needed:

// Default implementation for our visitable nodesextension Visitable {    func accept(visitor: Visitor) {        visitor.visit(element: self)    }}

For the sake of simplicity, and because this is the general use case, we'll want the Visitable objects simply to call visit on the ...