Extension that converts Strings with basic HTML tags to SwiftUI's Text (Supports SwiftUI 3.0 / iOS 15.0).

SwiftUI+HTML.swift

//
//  SwiftUI+HTML.swift
//
//  Created by Felix Mau on 28.05.21.
//  Copyright © 2021 Felix Mau. All rights reserved.
//

import SwiftUI

@available(iOS 15.0, *)
enum HTMLToMarkdownConverter {

    // MARK: - Public methods

    /// Converts the HTML-tags in the given string to their corresponding markdown tags.
    ///
    /// - SeeAlso: See type `HTMLToMarkdownConverter.Tags` for a list of supported HTML-tags.
    static func convert(_ htmlAsString: String) -> String {
        // Convert "basic" HTML-tags that don't use an attribute.
        let markdownAsString = Tags.allCases.reduce(htmlAsString) { result, textFormattingTag in
            result
                .replacingOccurrences(of: textFormattingTag.openingHtmlTag, with: textFormattingTag.markdownTag)
                .replacingOccurrences(of: textFormattingTag.closingHtmlTag, with: textFormattingTag.markdownTag)
        }

        // Hyperlinks use an attribute and therefore need to be handled differently.
        return convertHtmlLinksToMarkdown(markdownAsString)
    }

    // MARK: - Private methods

    /// Converts hyperlinks in HTML-format to their corresponding markdown representations.
    ///
    /// - Note: Currently we only support a basic HTML syntax without any attributed other than `href`.
    ///         E.g. `<a href="URL">TEXT</a>` will be converted to `[TEXT](URL)`
    ///
    /// - Parameter htmlAsString: The string containing hyperlinks in HTML-format.
    ///
    /// - Returns: A string with hyperlinks converted to their corresponding markdown representations.
    private static func convertHtmlLinksToMarkdown(_ htmlAsString: String) -> String {
        htmlAsString.replacingOccurrences(of: "<a href=\"(.+)\">(.+)</a>",
                                          with: "[$2]($1)",
                                          options: .regularExpression,
                                          range: nil)
    }
}

extension HTMLToMarkdownConverter {

    /// The supported tags inside a string we can format.
    enum Tags: String, CaseIterable {
        case strong
        case em
        case s
        case code

        // Hyperlinks need to be handled differently, as they not only have simple opening and closing tag, but also use the attribute `href`.
        // See private method `Text.convertHtmlLinksToMarkdown(:)` for further details.
        // case a

        // MARK: - Public properties

        var openingHtmlTag: String {
            "<\(rawValue)>"
        }

        var closingHtmlTag: String {
            "</\(rawValue)>"
        }

        var markdownTag: String {
            switch self {
            case .strong:
                return "**"

            case .em:
                return "*"

            case .s:
                return "~~"

            case .code:
                return "`"
            }
        }
    }
}

@available(iOS 15.0, *)
extension Text {

    // MARK: - Initializer

    /// Renders the given string containing HTML-tags with the related formatting.
    ///
    /// - SeeAlso: See type `HTMLToMarkdownConverter.Tags` for a list of supported HTML-tags.
    init(html htmlAsString: String) {
        let markdownAsString = HTMLToMarkdownConverter.convert(htmlAsString)

        do {
            let markdownAsAttributedString = try AttributedString(markdown: markdownAsString)
            self = .init(markdownAsAttributedString)
        } catch {
            print("⚠️ – Couldn't parse markdown: \(error)")

            // Show the "plain" markdown string as a fallback.
            self = .init(markdownAsString)
        }
    }
}

Tests

Unit-Tests for HTMLToMarkdownConverter.convert(:)

class HTMLToMarkdownConverterTestCase: XCTestCase {

    func testConvertShouldIgnoreStringWithoutHTMLTags() {
        // Given
        let html = "Lorem ipsum dolor sit amet"

        // When
        let markdown = HTMLToMarkdownConverter.convert(html)

        // Then
        XCTAssertEqual(html, markdown)
    }

    func testConvertShouldIgnoreUnsupportedHTMLTags() {
        // Given
        let html = "<p>Lorem ipsum dolor sit amet</p>"

        // When
        let markdown = HTMLToMarkdownConverter.convert(html)

        // Then
        XCTAssertEqual(html, markdown)
    }

    func testConvertShouldReplaceStrongTag() {
        // Given
        let html = "<strong>Lorem</strong> ipsum <strong>dolor</strong> sit <strong>amet</strong>"

        // When
        let markdown = HTMLToMarkdownConverter.convert(html)

        // Then
        XCTAssertEqual(markdown, "**Lorem** ipsum **dolor** sit **amet**")
    }

    func testConvertShouldReplaceEmTag() {
        // Given
        let html = "<em>Lorem</em> ipsum <em>dolor</em> sit <em>amet</em>"

        // When
        let markdown = HTMLToMarkdownConverter.convert(html)

        // Then
        XCTAssertEqual(markdown, "*Lorem* ipsum *dolor* sit *amet*")
    }

    func testConvertShouldReplaceStrikeTag() {
        // Given
        let html = "<s>Lorem</s> ipsum <s>dolor</s> sit <s>amet</s>"

        // When
        let markdown = HTMLToMarkdownConverter.convert(html)

        // Then
        XCTAssertEqual(markdown, "~~Lorem~~ ipsum ~~dolor~~ sit ~~amet~~")
    }

    func testConvertShouldReplaceCodeTag() {
        // Given
        let html = "<code>Lorem</code> ipsum <code>dolor</code> sit <code>amet</code>"

        // When
        let markdown = HTMLToMarkdownConverter.convert(html)

        // Then
        XCTAssertEqual(markdown, "`Lorem` ipsum `dolor` sit `amet`")
    }

    func testConvertShouldReplaceLinkTag() {
        // Given
        let html = "Visit <a href=\"https://apple.com\">Apple</a>"

        // When
        let markdown = HTMLToMarkdownConverter.convert(html)

        // Then
        XCTAssertEqual(markdown, "Visit [Apple](https://apple.com)")
    }

    func testConvertShouldHandleMultipleTags() {
        // Given
        let html = "Visit <a href=\"https://apple.com\">Apple</a>. <strong>Lorem</strong> <em>ipsum</em>"

        // When
        let markdown = HTMLToMarkdownConverter.convert(html)

        // Then
        XCTAssertEqual(markdown, "Visit [Apple](https://apple.com). **Lorem** *ipsum*")
    }

    func testConvertShouldHandleNestedTags() {
        // Given
        let html = "Visit <strong><em><a href=\"https://apple.com\">Apple</a></em></strong>"

        // When
        let markdown = HTMLToMarkdownConverter.convert(html)

        // Then
        XCTAssertEqual(markdown, "Visit ***[Apple](https://apple.com)***")
    }
}