Visitors, Walkers, and Rewriters

Markup Visitor

The core MarkupVisitor protocol provides the basis for transforming, walking, or rewriting a markup tree.

public protocol MarkupVisitor {
    associatedtype Result
}

Using its Result type, you can transform a markup tree into anything: another markup tree, or perhaps a tree of XML or HTML elements. There are two included refinements of MarkupVisitor for common uses.

Markup Walker

The first refinement of MarkupVisitor, MarkupWalker, has an associated Result type of Void, so it’s meant for summarizing or detecting aspects of a markup tree. If you wanted to append to a string as elements are visited, this might be a good tool for that.

import Markdown

/// Counts `Link`s in a `Document`.
struct LinkCounter: MarkupWalker {
    var count = 0
    mutating func visitLink(_ link: Link) {
        if link.destination == "https://swift.org" {
            count += 1
        }
        descendInto(link)
    }
}

let source = "There are [two](https://swift.org) links to <https://swift.org> here."
let document = Document(parsing: source)
print(document.debugDescription())
var linkCounter = LinkCounter()
linkCounter.visit(document)
print(linkCounter.count)
// 2

Markup Rewriter

The second refinement, MarkupRewriter, has an associated Result type of an optional Markup element, so it’s meant to change or even remove elements from a markup tree. You can return nil to delete an element, or return another element to substitute in its place.

import Markdown

/// Delete all **strong** elements in a markup tree.
struct StrongDeleter: MarkupRewriter {
    mutating func visitStrong(_ strong: Strong) -> Markup? {
        return nil
    }
}

let source = "Now you see me, **now you don't**"
let document = Document(parsing: source)
var strongDeleter = StrongDeleter()
let newDocument = strongDeleter.visit(document)

print(newDocument!.debugDescription())
// Document
// └─ Paragraph
//    └─ Text "Now you see me, "