A Swift library for efficient, flexible content-based text styling

What's New


  • Expose an invalidation API for TextViewHighlighter
  • Improve compatibility with older Swift versions
  • Correctly check for stale document versions
  • Ensure tree accessed only on main actor
  • TextViewHighlighter wasn't using a UTF-16 based limit
  • currentTree accessors (thanks @kaunteya )
  • Fix a bug with edit ordering (thanks @danielpunkass )
  • Many documentation improvements

Build Status License Platforms Documentation


A Swift library for efficient, flexible content-based text styling.

  • Minimal text invalidation
  • Support for multiple sources of token data
  • A hybrid sync/async system for targeting flicker-free styling on keystrokes
  • tree-sitter integration
  • Compatibility with lazy text loading
  • Text-system agnostic

Neon has a strong focus on efficiency and flexibility. It sits in-between your text system and wherever you get your semantic token information. Neon was developed for syntax highlighting and it can serve that need very well. However, it is more general-purpose than that and could be used for any system that needs to style text based its content.

Many people are looking for a drop-in editor View subclass that does it all. This is a lower-level library. You could, however, use Neon to drive highlighting for a view like this.

Token Data Sources

Neon was designed to accept and overlay token data from multiple sources simultaneously. Here's a real-world example of how this is used:

  • First pass: pattern-matching system with ok quality and guaranteed low-latency
  • Second pass: tree-sitter, which has good quality and can often be low-latency
  • Third pass: Language Server Protocol's semantic tokens, which can augment existing highlighting, but is high-latency


A highlighting theme is really just a mapping from semantic labels to styles. Token data sources apply the semantic labels and the TextSystemInterface uses those labels to look up styling.

This separation makes it very easy for you to do this look-up in a way that makes the most sense for whatever theming formats you'd like to support. This is also a convenient spot to adapt/modify the semantic labels coming from your data sources into a normalized form.

Tree-Sitter Integration

Neon includes a tree-sitter token source, built around SwiftTreeSitter. There is an included standalone module called TreeSitterClient that abstracts almost all of the details. It includes support for running tree-sitter "highlights.scm" queries, among other features. Its hybrid synchronous/asynchronous API makes it possible to scale tree-sitter to large documents, where its parsing/queries can introduce too much latency.

Tree-sitter uses separate compiled parsers for each language. There are a variety of ways to use tree-sitter parsers with SwiftTreeSitter. Check out that project for details.

Text System Integration

Neon is text-system independent. It makes very few assumptions about how text is stored, displayed, and styled. And, it is built around parts that can be used together to build a full system. But, there are also some helper types for use-cases with simpler needs.


An implementation of the TextSystemInterface protocol for NSTextView/UITextView. This takes care of the interface to the view and NSLayoutManager, but defers Token-style translation (themes) to an external AttributeProvider. This is compatible with both TextKit 1 and 2.


A more full-featured system that integrates NSTextView/UITextView with a TreeSitterClient. This is a good way to get going quickly, or just to see how the parts fit together. Also compatible with TextKit 1 and 2.

Will take over as the delegate of the text view's NSTextStorage.

There is also an example project that demonstrates how to use TextViewHighlighter for macOS and iOS.

Manual Integration

A minimal integration can be achieved by configuring a Highlighter to interface with an NSTextView:

func applicationDidFinishLaunching(_ aNotification: Notification) {
   let textInterface = TextViewSystemInterface(textView: textView, attributeProvider: self.attributeProvider)
   self.highlighter = Highlighter(textInterface: textInterface, tokenProvider: self.tokenProvider)
   textStorage.delegate = self

Attaching the highlighter to a text view interface tells it what to update, but not when. You have to notify the highlighter whenever the text view's content changes, and invalidate existing highlighting as needed. Such notifications can be conveyed by making yourself the delegate of your text view's textStorage, and implementing this delegate method:

func textStorage(_ textStorage: NSTextStorage, didProcessEditing editedMask: NSTextStorageEditActions, range editedRange: NSRange, changeInLength delta: Int) {
   // Map NSTextStorageDelegate editedRange to Neon's style of editedRange
   let adjustedRange = NSRange(location: editedRange.location, length: editedRange.length - delta)
   self.highlighter.didChangeContent(in: adjustedRange, delta: delta)

   DispatchQueue.main.async {

Notice that the invalidate method is dispatched asynchronously to ensure the styles are not updated until after the underlying text storage is done being edited.

The initial configuration of highlighter included references to self.tokenProvider and self.attributeProvider, which are responsible for providing the logic behind what gets highlighted, and how it should be done. At a minimum, the TokenProvider generates and supplies named tokens that correspond to ranges of text:

let paintItBlackTokenName = "paintItBlack"

func tokenProvider(_ range: NSRange, completionHandler: @escaping (Result<TokenApplication, Error>) -> Void) {
   var tokens: [Token] = []
   guard let searchString = self.textView.textStorage?.string else {
      // Could also complete with .failure(...) here
      completionHandler(.success(TokenApplication(tokens: tokens, action: .replace)))

   if let regex = try? NSRegularExpression(pattern: "[^\\s]+\\s{0,1}") {
      regex.enumerateMatches(in: searchString, range: range) { regexResult, _, _ in
         guard let result = regexResult else { return }
         for rangeIndex in 0..<result.numberOfRanges {
            let tokenRange = result.range(at: rangeIndex)
            tokens.append(Token(name: paintItBlackTokenName, range: tokenRange))

   completionHandler(.success(TokenApplication(tokens: tokens, action: .replace)))

In this trivial example, the "paint it black" token is unilaterally applied to every non-whitespace range of the text. It demonstrates how you use a token provider to associate named tokens with arbitrary ranges of text. It's important to understand though that supplying the token doesn't change anything about the appearance of the corresponding text. In order to achieve that, you need to implement the attribute provider, which effectively translates named tokens in to suitable attributes:

func attributeProvider(_ token: Token) -> [NSAttributedString.Key: Any]? {
   if == paintItBlackTokenName {
      return [.foregroundColor:, .backgroundColor:]
   return nil

Now our example achieves its goal of "painting black" any runs of non-whitespace characters, along with single whitespace characters between them:

Screenshot of 'Paint it Black' text window showing text with a black background and red text.

Using this basic structure you can annotate the text with tokens while separately determining the appropriate styling for those tokens.

Advanced Integration

Achieving better performance and guaranteed flicker-free highlighting is more challenging. Monitoring the visible rect of the text view will improve performance. You need to know when a text change has been processing by enough of the system that styling is possible. This point in the text change lifecycle is not natively supported by NSTextStorage or NSLayoutManager. It requires an NSTextStorage subclass. But, even that isn't quite enough unfortunately, as you still need to precisely control the timing of invalidation and styling. This is where HighlighterInvalidationBuffer comes in.

Relationship to TextStory

TextStory is a library that contains three very useful components when working with Neon.

  • TSYTextStorage gets you all the text change life cycle hooks without falling into the NSString/String bridging performance traps
  • TextMutationEventRouter makes it easier to route events to the components
  • LazyTextStoringMonitor allows for lazy content reading, which is essential to quickly open large documents



This is the main component that coordinates the styling and invalidation of text.

  • Connects to a text view via TextSystemInterface
  • Monitors text changes and view visible state
  • Gets token-level information from a TokenProvider

Note that Highlighter is built to handle a TokenProvider calling its completion block more than one time, potentially replacing or merging with existing styling.


In a traditional NSTextStorage/NSLayoutManager system (TextKit 1), it can be challenging to achieve flicker-free on-keypress highlighting. This class offers a mechanism for buffering invalidations, so you can precisely control how and when actual text style updates occur.


This class is an asynchronous interface to tree-sitter. It provides an UTF-16 code-point (NSString-compatible) API for edits, invalidations, and queries. It can process edits of String objects, or raw bytes. Invalidations are translated to the current content state, even if a queue of edits are still being processed. It is fully-compatible with reading the document content lazily.

  • Monitors text changes
  • Can be used to build a TokenProvider

TreeSitterClient provides APIs that can be both synchronous, asynchronous, or both depending on the state of the system. This kind of interface can be important when optimizing for flicker-free, low-latency highlighting live typing interactions like indenting.

Using it is quite involved - here's a little example:

import SwiftTreeSitter
import TreeSitterSwift // this parser is available via SPM (see SwiftTreeSitter's
import Neon

// step 1: setup

// construct the tree-sitter parser for the language you are interested in
let language = Language(language: tree_sitter_swift())

// construct your highlighting query
// remember, this is a one-time cost but can be expensive
let url = Bundle.main
let query = try! language.query(contentsOf: url)

// step 2: configure the client

let client = TreeSitterClient(language: language)

// this function will be called with a minimal set of text ranges
// that have become invalidated due to edits. These ranges
// always correspond to the *current* state of the text content,
// even if TreeSitterClient is currently processing edits in the
// background.
client.invalidationHandler = { indexSet in
    // highlighter.invalidate(.set(indexSet))

// step 3: inform it about content changes
// these APIs match up fairly closely with NSTextStorageDelegate,
// and are compatible with lazy evaluation of the text content

// call this *before* the content has been changed
client.willChangeContent(in: range)

// and call this *after*
client.didChangeContent(to: string, in: range, delta: delta, limit: limit)

// step 4: run queries
// you can execute these queries directly in the invalidationHandler, if desired

// Many tree-sitter highlight queries contain predicates. These are both expensive
// and complex to resolve. This is an optional feature - you can just skip it. Doing
// so makes the process both faster and simpler, but could result in lower-quality
// and even incorrect highlighting.

let provider: TreeSitterClient.TextProvider = { (range, _) -> String? in
   return nil

let range = NSMakeRange(0, 10) // for example
client.executeHighlightsQuery(query, in: range, textProvider: provider) { result in
   // Token values will tell you the highlights.scm name and range in your text

Suggestions or Feedback

We'd love to hear from you! Get in touch via twitter, an issue, or a pull request.

Please note that this project is released with a Contributor Code of Conduct. By participating in this project you agree to abide by its terms.


  • Swift Tools 5.5.0
View More Packages from this Author


Last updated: Tue Nov 28 2023 19:33:36 GMT-1000 (Hawaii-Aleutian Standard Time)