A basic FileDocument template created by ChatGPT

ContentView.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)
	}
}

struct ContentView: View {
	@Binding var document: TextFile

	var body: some View {
		TextEditor(text: $document.text)
			.padding()
			.onAppear {
				print("ContentView appeared with text: \(document.text)")
			}
	}
}

struct ContentView_Previews: PreviewProvider {
	static var previews: some View {
		ContentView(document: .constant(TextFile()))
	}
}

FileDocumentChatGPTTemplateApp.swift

//
//  FileDocumentChatGPTTemplateApp.swift
//  FileDocumentChatGPTTemplate
//
//  Created by Xcode Developer on 7/5/24.
//

import SwiftUI

@main
struct FileDocumentChatGPTTemplateApp: App {
	var body: some Scene {
		DocumentGroup(newDocument: TextFile()) { file in
			ContentView(document: file.$document)
				.onAppear {
					print("DocumentGroup content view appeared")
				}
		}
	}
}

Info.plist

<?xml version = "1.0" encoding = "UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version = "1.0">
<dict>
	<key > LSSupportsOpeningDocumentsInPlace </ key>
	<true/>
	<key > UISupportsDocumentBrowser </ key>
	<true/>
	<key > CFBundleDocumentTypes </ key>
	<array>
	<dict>
		<key > CFBundleTypeName </ key>
		<string > Plain Text Document </ string>
		<key > LSHandlerRank </ key>
		<string > Owner </ string>
		<key > LSItemContentTypes </ key>
		<array>
		<string>public .plain - text </ string>
		</array>
		<key > NSDocumentClass </ key>
		<string > $(PRODUCT_MODULE_NAME).TextFile </ string>
		</dict>
		</array>
		<key > UTExportedTypeDeclarations </ key>
		<array>
		<dict>
			<key > UTTypeConformsTo </ key>
			<array>
			<string>public .plain - text </ string>
			</array>
			<key > UTTypeDescription </ key>
			<string > Plain Text Document </ string>
			<key > UTTypeIdentifier </ key>
			<string > $(PRODUCT_BUNDLE_IDENTIFIER).plaintext </ string>
			<key > UTTypeTagSpecification </ key>
			<dict>
			<key>public .filename- extension </key>
			<array>
				<string > txt </ string>
				</array>
				<key>public .mime - type </ key>
				<array>
					<string > text / plain </ string>
					</array>
					</dict>
					</dict>
					</array>
					</dict>
					</plist>