SwiftUI get NSViewRepresentable to automatically redraw

Question

I have introduced a styled NSViewRepresentable text field on to my layout in order to use an attributed Text Editor field. I want to influence the content of this text field through some buttons.

I can change the styled attributes of the field, but it never refreshes unless I hit it manually. There must be a way of getting to do this automatically. What have I missed?

import SwiftUI
import Combine

let publisher = PassthroughSubject<Bool,Never>()

struct ContentView: View
{
    @State var mytext = NSMutableAttributedString(string: "")
    var attributedString: NSMutableAttributedString
    @State var textViewID: Int = 0
    
    init()
    {
        attributedString = NSMutableAttributedString(
            string: "Hello World!",
            attributes: [.font: NSFont.systemFont(ofSize: 18)])
        _mytext = State(initialValue: attributedString)
    }
    
    var body: some View
    {
        VStack
        {
            Button("Bold")
            {
                attributedString.addAttribute(.font, value: NSFont.boldSystemFont(ofSize: 18), range: NSRange(location: 0, length: 5))
                mytext = attributedString
                publisher.send(true)
            }
        }
        TextView(text: $mytext)
            .frame(maxWidth: .infinity, maxHeight: .infinity)
            .id(textViewID)
            .onReceive(publisher) { ( stateTo ) in
                textViewID += 1
            }

    }
}

struct TextView: NSViewRepresentable
{
    @Binding var text: NSMutableAttributedString
    
    func makeNSView(context: Context) -> NSTextView 
    {
        NSTextView()
    }
    
    func updateNSView(_ nsView: NSTextView, context: Context)
    {
        nsView.textStorage?.setAttributedString(text)
    }

}

Or am I missing something and there is a much easier way?

This should be helpful https://stackoverflow.com/a/63761738/12299030. — Asperi, Apr 18 '22 at 09:47

score 0 · Answer 1 · answered Aug 31 '22 at 19:57

Suspect worked around this and moved onto bigger fish. But for the benefit of those bumping into something similar :-)

The problem with the code in the example is that there is nothing in it that SwiftUI can use to determine when it needs to redraw.

Specifically @State var mytext: NSMutableAttributedString is holding a reference to the same attributed string as attributedString does. Consequently the button action mytext = attributedString is from pov of SwiftUI a noop.

Two approaches to making it work.

First as now, where the same string is kept and mutated in place and a redraw is in effect explicity requested. In this case the Button action can be cleaned up to remove the need for Combine and the onReceive thus:

Button("Bold") {
   mytext.addAttribute(.font, value: NSFont.boldSystemFont(ofSize: 18), range: NSRange(location: 0, length: 5))
   textViewID = mytext.hashValue
}

Or second, get rid of the need for Combine, onReceive and id attribute by removing the mutation in place. Instead replace with a new attribute string that SwiftUI will notice, e.g.:

Button("Bold") {
   let newAttributedString:NSMutableAttributedString = mytext.mutableCopy() as! NSMutableAttributedString
   newAttributedString.addAttribute(.font, value: NSFont.boldSystemFont(ofSize: 18), range: NSRange(location: 0, length: 5))
   mytext = newAttributedString
}

Later is probably slightly more idiomatic SwiftUI.

SwiftUI get NSViewRepresentable to automatically redraw

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