FileDocument

ContentView.swift

import SwiftUI

struct ContentView: View {
    @Binding var document: TextFile
    
    var body: some View {
        HStack {
            TextEditor(text: $document.text)
        }
    }
}

MyApp.swift

import SwiftUI
import UniformTypeIdentifiers

struct TextFile: FileDocument {
    static var readableContentTypes: [UTType] { [.plainText] }
    
    var text = ""
    
    init(initialText: String = "") {
        text = initialText
    }
    
    init(configuration: ReadConfiguration) throws {
        if let data = configuration.file.regularFileContents {
            text = String(decoding: data, as: UTF8.self)
        }
    }
    
    func fileWrapper(configuration: WriteConfiguration) throws -> FileWrapper {
        let data = Data(text.utf8)
        return FileWrapper(regularFileWithContents: data)
    }
}

@main
struct MyApp: App {
    var body: some Scene {
        DocumentGroup(newDocument: TextFile()) { file in
            ContentView(document: file.$document)
        }
    }
}
struct MyApp_Previews: PreviewProvider {
    static var previews: some View {
        ContentView(document: .constant(TextFile()))
    }
}