A playground that shows how to use Swift's AttributedString with Markdown

AttributedString.swift

import UIKit
import Foundation

// NOTE: This playground shows how to use Swift's AttributedString with Markdown.
//
// This code was used to display Markdown content in the Tot iOS Widget <https://tot.rocks>

// MARK: - Helpful Links

// NOTE: The following links helped me figure this stuff out.
/*
 https://nilcoalescing.com/blog/AttributedStringAttributeScopes/
 https://developer.apple.com/documentation/foundation/attributedstring/instantiating_attributed_strings_with_markdown_syntax
 https://developer.apple.com/forums/thread/682957
 */

// MARK: - Test Data

// NOTE: This Arabic text below is used to test breaking the string into lines using AttributedSubstring.range(of:). Also of note:
// The Markdown parser is able to find the emphasis in the text, but it's unlikely that you'll have a font that's able to render
// it faithfully since italics really aren't a thing in non-Latin language (think about where the name "italic" came from).
// Similar issues will arise in Chinese/Japanese/Korean languages.
var text = """
**غامق** و*مائل* و **_كلاهما_**

مع اكثر من واحد
سطر من النص
"""

// NOTE: This string tests the styling, including a separate color for links (which overrides the text or accent color).
text = "\n\n**Bold** and _italic_ and **_both_**\n\nWith **[more](http://example.com) than** one\nline of <http:example.com> text"

// MARK: - Configuration

let fontFamilyName = "GillSans" // must be a font that has regular, bold, italic, and bold italic variants
let fontSize: CGFloat = 18

let textColor = UIColor.black
let accentColor = UIColor.red
let linkColor = UIColor.green

// MARK: - Testing

// generate an attributed string from Markdown text using the styling parameters configured above
if let attributedString = attributedMarkdownString(from: text, fontFamilyName: fontFamilyName, fontSize: fontSize, textColor: textColor, accentColor: accentColor, linkColor: linkColor) {
	
	// the following examples show how to manipulate the AttributedString
	
	// break the attributed string into lines by scanning the AttributedSubstring from the beginning to end
	// (use the Arabic text above to challenge your assumption on what constitutes the beginning of a string)
	do {
		var substring = attributedString[attributedString.startIndex..<attributedString.endIndex]
		var lineIndex = 0
		while let lineRange = substring.range(of: "\n") {
			let lineSubstring = attributedString[substring.startIndex..<lineRange.lowerBound]
			
			emitLine(lineIndex: lineIndex, lineSubstring: lineSubstring)
			
			substring = attributedString[lineRange.upperBound..<substring.endIndex]
			lineIndex += 1
		}
		emitLine(lineIndex: lineIndex, lineSubstring: substring)
		
		print("--------")
	}
	
	// find the first non-blank line in the attributed string (a "header")
	do {
		var foundHeader = false
		var substring = attributedString[attributedString.startIndex..<attributedString.endIndex]
		while let lineRange = substring.range(of: "\n") {
			let lineSubstring = attributedString[substring.startIndex..<lineRange.lowerBound]
			let characterCount = lineSubstring.characters.count
			if characterCount > 0 {
				print("header = \(lineSubstring.unadornedString)")
				foundHeader = true
				break
			}
			
			substring = attributedString[lineRange.upperBound..<substring.endIndex]
		}
		if !foundHeader {
			print("header fallback = \(attributedString.unadornedString)")
		}
		
		print("--------")
	}
	
	// find the second non-blank line and remove everything before it (a "body")
	do {
		var attributedString = attributedString // we are going to modify the attributed string in place
		
		var foundBody = false
		var haveFirstLine = false
		var substring = attributedString[attributedString.startIndex..<attributedString.endIndex]
		while let lineRange = substring.range(of: "\n") {
			let lineSubstring = attributedString[substring.startIndex..<lineRange.lowerBound]
			let characterCount = lineSubstring.characters.count
			if characterCount > 0 {
				if !haveFirstLine {
					haveFirstLine = true
				}
				else {
					attributedString.removeSubrange(attributedString.startIndex..<substring.startIndex)
					print("body = \(attributedString.unadornedString)")
					foundBody = true
					break
				}
			}

			substring = attributedString[lineRange.upperBound..<substring.endIndex]
		}
		if !foundBody {
			attributedString.removeSubrange(attributedString.startIndex..<substring.startIndex)
			print("body fallback = \(attributedString.unadornedString)")
		}
		
		print("--------")
	}
}

func emitLine(lineIndex: Int, lineSubstring: AttributedSubstring) {
	let characterCount = lineSubstring.characters.count
	if characterCount > 0 {
		// non-empty line
		print("line \(lineIndex): '\(NSAttributedString(AttributedString(lineSubstring)).string)'")
	}
	else {
		// empty line
		print("line \(lineIndex): empty")
	}
}

// MARK: - Generate styled Markdown

func attributedMarkdownString(from text: String, fontFamilyName: String, fontSize: CGFloat, textColor: UIColor, accentColor: UIColor, linkColor: UIColor) -> AttributedString? {
	// default fonts for all variants
	var regularFont = UIFont.systemFont(ofSize: fontSize)
	var italicFont = UIFont.systemFont(ofSize: fontSize)
	var boldFont = UIFont.systemFont(ofSize: fontSize)
	var boldItalicFont = UIFont.systemFont(ofSize: fontSize)

	// use the family name to create a base font that will be used to fill in the variants
	if let baseFont = UIFont(name: fontFamilyName, size: fontSize) {
		regularFont = baseFont
		if let italicFontDescriptor = baseFont.fontDescriptor.withSymbolicTraits(.traitItalic) {
			italicFont = UIFont(descriptor: italicFontDescriptor, size: fontSize)
		}
		if let boldFontDescriptor = baseFont.fontDescriptor.withSymbolicTraits(.traitBold) {
			boldFont = UIFont(descriptor: boldFontDescriptor, size: fontSize)
		}
		if let boldItalicFontDescriptor = baseFont.fontDescriptor.withSymbolicTraits([.traitBold, .traitItalic]) {
			boldItalicFont = UIFont(descriptor: boldItalicFontDescriptor, size: fontSize)
		}
	}
	else {
		assert(false, "base font does not exist")
	}

	// build an attributed string from the Markdown syntax
	if var attributedString = try? AttributedString(markdown: text, options: AttributedString.MarkdownParsingOptions(interpretedSyntax: .inlineOnlyPreservingWhitespace)) {
		attributedString.font = regularFont
		attributedString.foregroundColor = textColor
		
		// get all the runs of attributes in the attributed string
		for run in attributedString.runs {
			let piece = attributedString[run.range] // this is only used for debugging
			
			// inline presentation intent attributes let us find the Markdown runs that need styling
			let intent = run.attributes[AttributeScopes.FoundationAttributes.InlinePresentationIntentAttribute.self]
			if intent == .emphasized {
				print("'\(piece.unadornedString)' is emphasized")
				attributedString[run.range].font = italicFont
			}
			else if intent == .stronglyEmphasized {
				print("'\(piece.unadornedString)' is strongly emphasized")
				attributedString[run.range].font = boldFont
				attributedString[run.range].foregroundColor = accentColor
			}
			else if intent == [ .stronglyEmphasized, .emphasized] {
				print("'\(piece.unadornedString)' is both")
				attributedString[run.range].font = boldItalicFont
				attributedString[run.range].foregroundColor = accentColor
			}
			else {
				print("'\(piece.unadornedString)' is normal")
			}
			
			// the link attribute lets us style the links in the Markdown
			if let link = run.attributes[AttributeScopes.FoundationAttributes.LinkAttribute.self] {
				attributedString[run.range].foregroundColor = linkColor
				print("'\(piece.unadornedString)' is a link to \(link.absoluteString)")
			}
		}
		//print(attributedString)
		print("=========")
		return attributedString
	}
	
	return nil
}

extension AttributedString {
	
	var unadornedString: String {
		get {
			return NSAttributedString(self).string.replacingOccurrences(of: "\n", with: "\\n")
		}
	}
	
}

extension AttributedSubstring {
	
	var unadornedString: String {
		get {
			return AttributedString(self).unadornedString
		}
	}
	
}

A playground with this code can be downloaded here: https://files.iconfactory.net/craig/playgrounds/AttributedString.playground.zip