Issues with Code Highlighting when Highlighting needs Async

[erik-aie]

Description
I am trying to use a from-source version of HighlightSwift to perform my code highlighting as I want a macOS / iOS / visionOS unified solution for highlighting, which this library seems closest to providing. It's based in HighlightJS though, and the calls to the JS Core are done via async, which means it's root level call to get attributed text for given code is async. CodeSyntaxHighlighter from MarkdownUI only provides a synchronous highlightCode method.

I was able to make this kind of work, it works on macOS, but it freezes on iOS and visionOS. I ripped this down to a sample project (See SyntaxHighlightFreezeDemo ) and I think I've narrowed it down to the code that interfaces with MarkdownUI, not the underlying highlighting code, which seems to work cross platform as expected.

The offending code seems to be my highlightCode function, which I had to finagle to work with the async call inside the synchronous function. Maybe there's a much better way to do this? I am getting a lot of warnings regarding Swift 6 Concurrency but I don't know of a better way to do this without getting an async call from MarkdownUI's protocol.

func highlightCode(_ content: String, language: String?) -> Text {
        guard language != nil else {
            return Text(content)
        }

        let semaphore = DispatchSemaphore(value: 0)
        var result: AttributedString?


        var detectedLanguage: HighlightLanguage? = nil
        if language == "swift" {
            detectedLanguage = .swift
        } else if language == "csharp" {
            detectedLanguage = .cSharp
        }

        Task {
            do {
                if detectedLanguage != nil {
                        result = try await syntaxHighlighter.attributedText(content, language: detectedLanguage!, colors: isDarkMode ? .dark(.github) : .light(.github))
                } else {
                    result = try await syntaxHighlighter.attributedText(content, colors: isDarkMode ? .dark(.github) : .light(.github))
                }
                print("finished")
            } catch {
                // Handle error
                print(error)
            }
            semaphore.signal()
        }
        semaphore.wait()

        return Text(result ?? AttributedString(""))
    }

What's weirder is that the function executes, and it gets to the Highlight.swift function call to create NSMutableAttributedString from the HTML data generated by HighlightJS, but my debug execution stops after that call:

        let mutableString = try NSMutableAttributedString(
            data: data,
            options: [
                .documentType: NSAttributedString.DocumentType.html,
                .characterEncoding: String.Encoding.utf8.rawValue
            ],
            documentAttributes: nil
        )

Checklist

• I can reproduce this issue with a vanilla SwiftUI project.
• I can reproduce this issue using the main branch of this package.
• This bug hasn't been addressed in an existing GitHub issue.

Steps to reproduce
See SyntaxHighlightFreezeDemo

Expected behavior
MarkdownUI performing syntax highlighting cross platform

Resulting Behavior
Syntax highlighting on macOS, but a freeze on iOS and visionOS

Version information

• MarkdownUI: 2.3.1
• OS: iOS 17, macOS 18
• Xcode: 16.0 Beta 2

[malicious]

Does the UI freeze, or does it simply not update?

When you call the code from a Task (in the freeze demo), you're updating the @State var asyncLoadedAttributedText, but SwiftUI is built to handle updates from the main thread only. You can't directly update a @State variable from a background thread (well you can, but SwiftUI gets out of sync, and updates aren't pushed correctly).

Try something like:

.task {
    let attributedText = try? await…
    DispatchQueue.main.async {
        asyncLoadedAttributedText = attributedText
    }
}

This transfers execution of that block back to the main thread, where SwiftUI can see that it happened.

Also, instead of the semaphore, try withCheckedThrowingContinuation or its siblings.

[erik-aie]

edit - The UI just froze, like the navigation page would freeze when trying to navigate to a sub-page containing code-blocks, and not respond to touch on other elements until the app was force closed, which was strange behavior to me, at least, and only on iOS / visionOS. macOS was fine.

Appreciate the feedback! I ended up forking the repository and changing the protocol to be async myself which fixed my issue. It also let me adjust the code block behavior on macOS to not use scroll views since the nested ScrollViews kept messing up my vertical scrolling of my implementation on macOS, but it worked fine on other targets.

Here's the commit from the fork:
erik-aie@60c5ef7

This issue can be closed if ultimately there isn't value to an async code styling adjustment to this protocol in the main repository

[erik-aie]

Does the UI freeze, or does it simply not update?

When you call the code from a Task (in the freeze demo), you're updating the @State var asyncLoadedAttributedText, but SwiftUI is built to handle updates from the main thread only. You can't directly update a @State variable from a background thread (well you can, but SwiftUI gets out of sync, and updates aren't pushed correctly).

Try something like:

.task {
    let attributedText = try? await…
    DispatchQueue.main.async {
        asyncLoadedAttributedText = attributedText
    }
}

This transfers execution of that block back to the main thread, where SwiftUI can see that it happened.

Also, instead of the semaphore, try withCheckedThrowingContinuation or its siblings.

I tried the solutions in this comment and wanted to report back on what I found:

• Adjustments to the logic of updating the @State var asyncLoadedAttributedText is a good recommendation, I'm still a bit green with SwiftUI so I honestly didn't realize it expects UI related updates to data to occur on the main thread exclusively. I tried the adjustment listed for the main sample project and it still freezes my iOS simulator on render (nothing shows entirely on the view, even entirely static Text fields, so the render of the page is definitely frozen, 0% CPU utilization too, so I think it's a main thread block):

@State var asyncLoadedAttributedText: AttributedString?

var body: some View {
    VStack {
        //fails on iOS
        Markdown(demoMarkdown)
            .markdownCodeSyntaxHighlighter(HighlightSyntaxHighligher(isDarkMode: true))
        //works cross platform
        Text(asyncLoadedAttributedText ?? AttributedString())
        //Static Test
        Text("Hello World")
    }
    .padding()
    .task {
        let loadedAttributedText = try? await highlighter.syntaxHighlighter.attributedText(demoSwift, language: .swift)
        DispatchQueue.main.async {
            asyncLoadedAttributedText = loadedAttributedText
        }
    }
}

• I tried to adjust my highlightCode implementation to use withCheckedThrowingContinuation but it's an async method too, so I don't think I can call it from my synchronous highlightCode method. At least, Xcode is giving me an error on usage of it in this adjustment of highlightCode from earlier:

func highlightCode(_ content: String, language: String?) -> Text {
    guard let language = language else {
        return Text(content)
    }
    
    let result: AttributedString
    
    do {
        //xcode error here
        result = try withCheckedThrowingContinuation { continuation in
            Task {
                var detectedLanguage: HighlightLanguage? = nil
                
                switch language {
                    case "swift": detectedLanguage = .swift
                    case "csharp": detectedLanguage = .cSharp
                    default: break
                }
                
                do {
                    let attributedText: AttributedString
                    if let detectedLanguage = detectedLanguage {
                        attributedText = try await syntaxHighlighter.attributedText(content, language: detectedLanguage, colors: isDarkMode ? .dark(.github) : .light(.github))
                    } else {
                        attributedText = try await syntaxHighlighter.attributedText(content, colors: isDarkMode ? .dark(.github) : .light(.github))
                    }
                    
                    continuation.resume(returning: attributedText)
                } catch {
                    continuation.resume(throwing: error)
                }
            }
        }
    } catch {
        print(error)
        return Text(content)
    }
    
    return Text(result)
}

[onedayitwillmake]

@erik-aie

You could try creating your own theme instead with something like the following:

public static func theme() -> Theme {
  let theme = Theme()

// Copy everything that is in Theme+Github.swift, but swapped in `MyCoolAsyncTextView`
// ...Other stuff left out for brevity

  theme.codeBlock { configuration in
      ScrollView(.horizontal) {
        // Key part 
        MyCoolAsyncTextView(content: configuration.content, language: configuration.language ?? "")
         // Below copied from  Theme+Github.swift 
          .fixedSize(horizontal: false, vertical: true)
          .relativeLineSpacing(.em(0.225))
          .markdownTextStyle {
            FontFamilyVariant(.monospaced)
            FontSize(.em(0.85))
          }
          .padding(16)
      }
      .background(Color.codeBlockBackground)
      .clipShape(RoundedRectangle(cornerRadius: 6))
      .markdownMargin(top: 0, bottom: 16)

  return theme
}

Later

    Markdown(content)
      .markdownTheme(MyApp.theme)