How do I make a UITextView inside of a UIViewRepresentable update when I add an attribute to an NSMutableAttributedString?

Question

I am trying to make a WYSIWYG editor by interfacing between SwiftUI and UIKit via a UIViewRepresentable. I am primarily using SwiftUI but am using UIKit here as it seems SwiftUI does not currently support the functionality needed.

My problem is, when I set the NSMutableAttributedString to be already containing a string with attributes, if I then select that text in the UIViewRepresentable before typing any new text and press the underline button in the UIToolBar to add the attribute, the attribute is added to the NSMutableAttributedString but the UIView does not update to show the updated NSMutableAttributedString. However, if I type a single character and then select the text and add the underline attribute, the UIView updates.

Could someone explain why this is and maybe point me towards a solution? Any help would be greatly appreciated.

Below is the code:

import SwiftUI
import UIKit

struct ContentView: View {
    @State private var mutableAttributedString: NSMutableAttributedString = NSMutableAttributedString(
        string: "this is the string before typing anything new",
        attributes: [.foregroundColor: UIColor.blue])
    
    var body: some View {
        EditorExample(outerMutableString: $mutableAttributedString)
    }
}

struct EditorExample: UIViewRepresentable {
    @Binding var outerMutableString: NSMutableAttributedString
    @State private var outerSelectedRange: NSRange = NSRange()

    func makeUIView(context: Context) -> some UITextView {
// make UITextView
        let textView = UITextView()
        textView.font = UIFont(name: "Helvetica", size: 30.0)
        textView.delegate = context.coordinator
// make toolbar
        let toolBar = UIToolbar(frame: CGRect(x: 0, y: 0, width: textView.frame.size.width, height: 44))
// make toolbar underline button
        let underlineButton = UIBarButtonItem(
            image: UIImage(systemName: "underline"),
            style: .plain,
            target: context.coordinator,
            action: #selector(context.coordinator.underline))
        toolBar.items = [underlineButton]
        textView.inputAccessoryView = toolBar
        return textView
    }
    
    func updateUIView(_ uiView: UIViewType, context: Context) {
        uiView.attributedText = outerMutableString
    }
    
    func makeCoordinator() -> Coordinator {
        Coordinator(innerMutableString: $outerMutableString, selectedRange: $outerSelectedRange)
    }
    
    class Coordinator: NSObject, UITextViewDelegate {
        @Binding var innerMutableString: NSMutableAttributedString
        @Binding var selectedRange: NSRange
    
        init(innerMutableString: Binding<NSMutableAttributedString>, selectedRange: Binding<NSRange>) {
            self._innerMutableString = innerMutableString
            self._selectedRange = selectedRange
        }
        
        func textViewDidChange(_ textView: UITextView) {
            innerMutableString = textView.textStorage
        }
        
        func textViewDidChangeSelection(_ textView: UITextView) {
            selectedRange = textView.selectedRange
        }
        
        @objc func underline() {
            if (selectedRange.length > 0) {
                innerMutableString.addAttribute(.underlineStyle, value: NSUnderlineStyle.single.rawValue, range: selectedRange)
            }
        }
    }
}

Answer 1

It's not working because NSAttributedString is a class and @State is for value types like structs. This means the dependency tracking is broken and things won't update correctly.

Also your UIViewRepresentable and Coordinator design is non-standard so I thought I would share an example of the correct way to do it. The binding is change to a string, which is a value type so it's working (minus the underline feature obviously).

struct ContentView: View {
    //@State private var mutableAttributedString: NSMutableAttributedString = NSMutableAttributedString(
    //    string: "this is the string before typing anything new",
    //    attributes: [.foregroundColor: UIColor.blue])
    
    @State var string = "this is the string before typing anything new"
    
    var body: some View {
        VStack {
          //  EditorExample(outerMutableString: $mutableAttributedString)
          //  EditorExample(outerMutableString: $mutableAttributedString) // a second to test bindings are working\
            //Text(mutableAttributedString.string)
            
            EditorExample(outerMutableString2: $string)
            EditorExample(outerMutableString2: $string)
            
        }
    }
}

struct EditorExample: UIViewRepresentable {
    //@Binding var outerMutableString: NSMutableAttributedString
    @Binding var outerMutableString2: String
    
    // this is called first
    func makeCoordinator() -> Coordinator {
        // we can't pass in any values to the Coordinator because they will be out of date when update is called the second time.
        Coordinator()
    }
    
    // this is called second
    func makeUIView(context: Context) -> UITextView {
        context.coordinator.textView
    }
    
    // this is called third and then repeatedly every time a let or `@Binding var` that is passed to this struct's init has changed from last time.
    func updateUIView(_ uiView: UITextView, context: Context) {
        //uiView.attributedText = outerMutableString
        uiView.text = outerMutableString2

        // we don't usually pass bindings in to the coordinator and instead use closures.
        // we have to set a new closure because the binding might be different.
        
         context.coordinator.stringDidChange2 = { string in
            outerMutableString2 = string
        }
    }
    
    class Coordinator: NSObject, UITextViewDelegate {
        
        lazy var textView: UITextView = {
            let textView = UITextView()
            textView.font = UIFont(name: "Helvetica", size: 30.0)
            textView.delegate = self
            // make toolbar
            let toolBar = UIToolbar(frame: CGRect(x: 0, y: 0, width: textView.frame.size.width, height: 44))
            // make toolbar underline button
            let underlineButton = UIBarButtonItem(
                image: UIImage(systemName: "underline"),
                style: .plain,
                target: self,
                action: #selector(underline))
            toolBar.items = [underlineButton]
            textView.inputAccessoryView = toolBar
            return textView
        }()
        
        //var stringDidChange: ((NSMutableAttributedString) -> ())?
        var stringDidChange2: ((String) -> ())?
        
        func textViewDidChange(_ textView: UITextView) {
            //innerMutableString = textView.textStorage
            //stringDidChange?(textView.textStorage)
            stringDidChange2?(textView.text)
        }
        
        func textViewDidChangeSelection(_ textView: UITextView) {
           // selectedRange = textView.selectedRange
        }
        
        @objc func underline() {
            let range = textView.selectedRange
            if (range.length > 0) {
                textView.textStorage.addAttribute(.underlineStyle, value: NSUnderlineStyle.single.rawValue, range: range)
              //  stringDidChange?(textView.textStorage)
            }
        }
    }
}