StyledText.swift

//
//  TextStyle.swift
//
//  Created by Rob Napier on 12/20/19.
//  Copyright © 2019 Rob Napier. All rights reserved.
//

import SwiftUI

public struct TextStyle {
    // This type is opaque because it exposes NSAttributedString details and requires unique keys.
    // It can be extended, however, by using public static methods.
    // Properties are internal to be accessed by StyledText
    internal let key: NSAttributedString.Key
    internal let apply: (Text) -> Text
    private init(key: NSAttributedString.Key, apply: @escaping (Text) -> Text) {
        self.key = key
        self.apply = apply
    }
}

// Public methods for building styles
public extension TextStyle {
    static func foregroundColor(_ color: Color) -> TextStyle {
        TextStyle(key: .init("TextStyleForegroundColor"), apply: { $0.foregroundColor(color) })
    }

    static func bold() -> TextStyle {
        TextStyle(key: .init("TextStyleBold"), apply: { $0.bold() })
    }
}

public struct StyledText {
    // This is a value type. Don't be tempted to use NSMutableAttributedString here unless
    // you also implement copy-on-write.
    private var attributedString: NSAttributedString

    private init(attributedString: NSAttributedString) {
        self.attributedString = attributedString
    }

    public func style<S>(_ style: TextStyle,
                         ranges: (String) -> S) -> StyledText
        where S: Sequence, S.Element == Range<String.Index>
    {

        // Remember this is a value type. If you want to avoid this copy,
        // then you need to implement copy-on-write.
        let newAttributedString = NSMutableAttributedString(attributedString: attributedString)

        for range in ranges(attributedString.string) {
            let nsRange = NSRange(range, in: attributedString.string)
            newAttributedString.addAttribute(style.key, value: style, range: nsRange)
        }

        return StyledText(attributedString: newAttributedString)
    }
}

public extension StyledText {
    // A convenience extension to apply to a single range.
    func style(_ style: TextStyle,
               range: (String) -> Range<String.Index> = { $0.startIndex..<$0.endIndex }) -> StyledText {
        self.style(style, ranges: { [range($0)] })
    }
}

extension StyledText {
    public init(verbatim content: String, styles: [TextStyle] = []) {
        let attributes = styles.reduce(into: [:]) { result, style in
            result[style.key] = style
        }
        attributedString = NSMutableAttributedString(string: content, attributes: attributes)
    }
}

extension StyledText: View {
    public var body: some View { text() }

    public func text() -> Text {
        var text: Text = Text(verbatim: "")
        attributedString
            .enumerateAttributes(in: NSRange(location: 0, length: attributedString.length),
                                 options: [])
            { (attributes, range, _) in
                let string = attributedString.attributedSubstring(from: range).string
                let modifiers = attributes.values.map { $0 as! TextStyle }
                text = text + modifiers.reduce(Text(verbatim: string)) { segment, style in
                    style.apply(segment)
                }
        }
        return text
    }
}


struct ContentView: View {
    var body: some View {
        StyledText(verbatim: "👩‍👩‍👦someText1")
            .style(.highlight(), ranges: { [$0.range(of: "eTex")!, $0.range(of: "1")!] })
            .style(.bold())
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

// An internal convenience extension that could be defined outside this pacakge.
// This wouldn't be a general-purpose way to highlight, but shows how a caller could create
// their own extensions
extension TextStyle {
    static func highlight() -> TextStyle { .foregroundColor(.red) }
}

SO doesn't let me to edit this answer because the queue is full. So I'm going to write it here:

It crashes when the provided range cannot be found (returns nil). To prevent this crash I propose the following modification:

public struct StyledText {
    // This is a value type. Don't be tempted to use NSMutableAttributedString here unless
    // you also implement copy-on-write.
    private var attributedString: NSAttributedString

    private init(attributedString: NSAttributedString) {
        self.attributedString = attributedString
    }

    public func style<S>(_ style: TextStyle,
                         ranges: (String) -> S) -> StyledText
        where S: Sequence, S.Element == Range<String.Index>?  //  <-- HERE
    {

        // Remember this is a value type. If you want to avoid this copy,
        // then you need to implement copy-on-write.
        let newAttributedString = NSMutableAttributedString(attributedString: attributedString)

        for range in ranges(attributedString.string).compactMap({ $0 }) {    //  <-- HERE
            let nsRange = NSRange(range, in: attributedString.string)
            newAttributedString.addAttribute(style.key, value: style, range: nsRange)
        }

        return StyledText(attributedString: newAttributedString)
    }
}

Usage:

struct ContentView: View {
  var body: some View {
    StyledText(verbatim: "👩‍👩‍👦someText1")
      .style(.highlight(), ranges: { [$0.range(of: "eTex"), $0.range(of: "1"),  $0.range(of: "some gibberish")] })
      .style(.bold())
  }
}

@rufmirza, thank you for the addition. It took me quite some time to decide whether I liked it or not. I generally resist adding Optionals unless they have a clear meaning (the fact that this adds extra code to ignore nil values demonstrates the common problem with adding Optionals). Typically I would leave it to the caller do this compactMap. But this use case is probably common enough to make this ergonomic improvement worth it. I still wouldn't do it this way in my own code, but I accepted the edit on SO. I expect it will be helpful to people.

Thanks again.