What's the best way to listen to NSAttributedString attribute changes in Combine?

Question

I have a model that is a group of NSMutableAttributedStrings. Well currently it's a wrapper of a MyString class. Each string manages it's own attributes/state.

typealias NSAttrStringAttr = [NSAttributedString.Key: Any]
typealias MyStringAttribute = (NSAttrStringAttr, NSRange)
class MyString: ObservableObject {
  var attributedString: NSMutableAttributedString
  @Published var attributes: [MyStringAttribute] = []

  init(string: String) {
    self.attributedString = NSMutableAttributedString(string: string)
  }
}

The main reason for this is so that I can expose changes in the NSAttributedStrings attributes, as follows:

extension MyString {
  func updateAttributes() {
    self.attributes = []
    let range = NSMakeRange(0, self.attributedString.length - 1)
    attributedString.enumerateAttributes(in: range, using: {(attrs, range, _) in
      self.attributes.append((attrs, range))
    })
  }

  func addAttribute(_ name: NSAttributedString.Key, value: Any, range: NSRange) {
    attributedString.addAttribute(name, value: value, range: range)
    updateAttributes()
  }

  func addAttributes(_ attrs: [NSAttributedString.Key : Any] = [:], range: NSRange) {
    attributedString.addAttributes(attrs, range: range)
    updateAttributes()
  }
}

class MyStringGroup {
  private var: strings: [MyString] = []
  var _attributedString: NSMutableAttributedString
  var attributedString {
    get {
      if let str = _attributedString {
        return str
      } else {
        var newAString = NSMutableAttributedString(string: "")
        strings.forEach { self.newAString.append($0.attributedString) }
        _attributedString = newAString
      }
    }
  }
  var cancellable = Set<AnyCancellable>()

  func append(_ string: MyString) {
    string.$attributes.sink(receiveValue: { _ in
      // Handle rewriting portion of NSAttributedString based on substring update
    }).store(in: &cancellable)
    strings.append(string
  }

  func someStringUpdate(..., range: NSRange) {
    let str = strings.findRightString(range: range)
    str.someStringUpdate(..., range: offsetRangeForString(str, range: range))
  }
}

The main reason for this is that strings are linked with strings outside of the text, and they update certain attributes based on user action, responses from the server, etc. The reason for wrapping NSAttributedString is so that the logic for managing string attributes, and their connection to the other text can be managed separately of the entire text, and MyStringGroup needs to know if a MyString instance has updated its attributes so that it can re-render. I have been trying to use the reactive pattern of Combine, but can't find anyway to natively listen to attribute changes in NSAttributedString, as the only way to access attributes is via function attributes(at:effectiveRange:) and enumerateAttributes(in:using:), so currently I'm stuck with wrapping. Is there a better way to propagates changes in the substring/sentence NSAttributedString without having to wrap it and rewrite all it's methods?

You can listen/subscribe only to a **publisher**. If the framework doesn’t provide any, you have to do it. A possible way is to *publish* the changes in the `update` method for example with a `subject`. — vadian, Jun 16 '22 at 10:52

score 0 · Answer 1 · answered Jun 17 '22 at 16:53

You can use the (somewhat) new AttributedString struct.

The struct provides a similar API as the NSMutableAttributedString one, and what's best is that you can use it as a @Published property, which will automatically emit change events when any mutation happens on the attributed string (yes, this includes any attribute changes).

class MyString: ObservableObject {
    @Published var attributedString: AttributedString
}

With this change, you can remove all the boilerplate code that interfaced with the old Foundation class.

What's the best way to listen to NSAttributedString attribute changes in Combine?

1 Answers1