Document-Based App with MVVM

import SwiftUI
import UniformTypeIdentifiers
import Foundation

@main
struct DocumentBasedMVVMApp: App {
    @StateObject var viewModel = TheViewModel()
    
    var body: some Scene {
        #if os(iOS)
        DocumentGroupLaunchScene("Title") {
            NewDocumentButton("New motor")
        }
        #endif
        DocumentGroup(newDocument: TheDocument()) { file in
            DocumentView(doc: file.$document, viewModel: viewModel)
                .onChange(of: file.document) { oldDoc, newDoc in
                    viewModel.updateFromDocument(newDoc)
                }
                .onAppear() {
                    viewModel.updateFromDocument(file.document)
                    viewModel.updateArea()
                }
        }
    }
}

/// The main page for any documents
struct DocumentView: View {
    @Binding var doc: TheDocument
    @ObservedObject var viewModel: TheViewModel
    @Environment(\.dismiss) var dismiss
    @State private var showingEditingView = false
    
    var body: some View {
        Text("Computed area: \(viewModel.computedArea)")
        HStack {
            Button("Cancel") {
                dismiss()
            }
            Button("Geometry") {
                showingEditingView.toggle()
            }
            .fullScreenCover(isPresented: $showingEditingView) {
                EditingView(doc: $doc, viewModel: viewModel)
            }
        }
        .padding()
    }
}

/// Example of a view dedicated to modifying the document and doing some computation in the model, via the model viewer.
struct EditingView: View {
    @Binding var doc: TheDocument
    @ObservedObject var viewModel: TheViewModel
    @Environment(\.dismiss) var dismiss
    
    var body: some View {
        ScrollView {
            HStack {
                Text("Stator OD")
                TextField("Stator OD", value: $doc.staOD, format: .number)
            }
            HStack {
                Text("Stator ID")
                TextField("Stator ID", value: $doc.staID, format: .number)
            }
        }
        HStack {
            Button("Cancel", systemImage: "xmark") {
                dismiss()
            }
            Button("Save", systemImage: "hand.thumbsup") {
                viewModel.updateArea()
                dismiss()
            }
        }
        .padding()
    }
}

/// Default file extension from the template
extension UTType {
    static var exampleText: UTType {
        UTType(importedAs: "com.example.plain-text")
    }
}

/// This is the structure I have so far
/// Equatable because of .onChange(of: file.document) in the main app view
struct TheDocument: FileDocument, Equatable {
    var staOD = 80.0 // Stator Outter Diameter
    var staID = 50.0 // Stator Inner Diameter
    
    init() {
    }

    static var readableContentTypes: [UTType] { [.exampleText] }

    init(configuration: ReadConfiguration) throws {
        guard let data = configuration.file.regularFileContents,
              let content = String(data: data, encoding: .utf8) else {
            throw CocoaError(.fileReadCorruptFile)
        }
        
        let lines = content.split(separator: "\n").map { String($0) }
        
        for line in lines {
            let components = line.split(separator: "=", maxSplits: 1).map { String($0) }
            if components.count == 2 {
                let key = components[0]
                let value = components[1]
                
                switch key {
                case "staOD": self.staOD = Double(value) ?? 80.0
                case "staID": self.staID = Double(value) ?? 50.0
                default:
                    print("Unknown key in TheDocument initializer: \(key)")
                    continue
                }
            }
        }
    }
    
    func fileWrapper(configuration: WriteConfiguration) throws -> FileWrapper {
        let content = """
        staOD=\(staOD)
        staID=\(staID)
        """

        guard let data = content.data(using: .utf8) else {
            throw CocoaError(.fileWriteInapplicableStringEncoding)
        }

        return .init(regularFileWithContents: data)
    }
    
    /// For Equatable
    static func == (lhs: TheDocument, rhs: TheDocument) -> Bool {
        return lhs.staOD == rhs.staOD &&
               lhs.staID == rhs.staID
    }
}

/// Previously, this ViewModel had `@Published var` instead of `private(set) var`
/// I used to edit directly the values of the view model in my TextFields.
/// However, I thought the document would now play that role, so the ViewModel is left with  button callbacks and interfacing with the model
class TheViewModel: ObservableObject {
    private static func createTheModel() -> (TheModel) {
        return TheModel()
    }
    
    private(set) var theModel = createTheModel()
    private(set) var staOD = 80.0
    private(set) var staID = 50.0
    private(set) var computedArea = 0.0
    
    public func updateFromDocument(_ doc: TheDocument) {
        self.staOD = doc.staOD
        self.staID = doc.staID
    }
    
    private func transferToModel() {
        theModel.staOD = staOD
        theModel.staID = staID
    }
    
    public func updateArea() {
        transferToModel()
        theModel.calcArea()
        computedArea = theModel.area
    }
}

/// Huge structure with a lot of parameters and complex computations
struct TheModel {
    var area = 0.0
    var staOD = 0.0
    var staID = 0.0
    
    mutating func calcArea() {
        area = .pi/4 * (pow(staOD,2) - pow(staID,2))
    }
}