a Reminder List with control of focus, adding new values with double click, auto deleting nil values, using Combine for delay

ReminderList

//
//  ReminderList.swift
//
//  Demonstrates:-
//    list settings (style, colouring etc )
//    autoscrolling
//    control of focus in List, 
//    adding new values with enter, or double click, 
//    auto deleting nil values, 
//    using Combine for managed delay/UI updates
//
//  Created by Andrew Cowley on 29/10/2021.
//

import SwiftUI
import Combine

struct ReminderList: View {
    @EnvironmentObject
    var vm: ReminderListVM
    
    @FocusState
    private var focusedField: RowID?
    
    var body: some View {
        ScrollViewReader { proxy in
            List {
                ForEach($vm.reminders) { $reminder in
                    ReminderRow(reminder: $reminder)
                        .listRowBackground(Color.blue)
                        .listRowInsets(EdgeInsets(top: 0, leading: 10, bottom: 0, trailing: 10))
                        .swipeActions() {
                            Button(role: .destructive) { vm.deleteReminder(reminder) }
                                label: { Label("Delete", systemImage: "trash" ) }
                        }
                        .focused($focusedField, equals: .row(id: reminder.id))
                        .onSubmit { vm.submit(reminder) }
                        .onTapGesture { vm.tap(reminder: reminder) }
                }
                .background(Color.teal)
            }
            .onChange(of: vm.reminders) { _ in proxy.scrollTo(vm.focusedID, anchor: .bottom) } // keep new entries visible
            .listStyle(.plain)
            .background(Color.indigo)
            .animation(.easeInOut(duration: 0.6), value: vm.reminders)
            .sync($vm.focusedField, $focusedField)
            // a tap on a valid row will first fire the background tap below, then the tap on the reminder
            .gesture(TapGesture(count: 1).onEnded(vm.tap))
            .highPriorityGesture(TapGesture(count: 2) .onEnded( vm.doubleTap))
        }
        .background(Color.secondary)
        .environment(\.defaultMinListRowHeight, 10)
    }
}

enum RowID : Hashable { 
    case row(id: String) 
}

class ReminderListVM : ObservableObject{
    @Published var reminders: [Reminder] = Reminder.examples
    @Published var focusedField: RowID? = nil
    @Published var delayedFocusedField: RowID? = nil
    private var cancellables = Set<AnyCancellable>()
    var previousFocusedField: RowID? // intermediate value that adds delay
    
    var focusedID: String { rowid(focusedField) }
    // Helper function for debugging/printing FocusState fields
    func rowid(_ row: RowID?) -> String {
        guard case.row(let id) = row else { return "" }
        return id
    }
    
    init() {
        // push the values to focusedField after a small delay
        $delayedFocusedField
            .removeDuplicates()
            .delay(for: 0.01, scheduler: RunLoop.current )
            .assign(to: &$focusedField)
        
        // Remove last focused entry if it is empty
        $focusedField
            .removeDuplicates()
            .receive(on: RunLoop.main)
            .compactMap { focusedField -> Int? in
                defer { self.previousFocusedField = focusedField }
                guard
                    self.previousFocusedField != nil,
                    case .row(let previousId) = self.previousFocusedField,
                    let previousIndex = self.reminders.firstIndex(where: { $0.id == previousId } ),
                    self.reminders[previousIndex].text.isEmpty
                else { return nil }
                return previousIndex
            }
            .sink { self.reminders.remove(at: $0) }
            .store(in: &cancellables)
    }
    
    func submit(_ reminder: Reminder) {
        if reminder.text.count > 0 {
            newReminder(after: reminder)
        } else { // put cursor back on the reminder ( onSubmit cancels focus )
            setFocus(reminder)
        }
    }
    /// for a tap anywhere on the List, including blank areas
    func tap() {
        // Important! Do not delay this action, since we may need to override it
        // on the reminder row. Setting the same delay causes indeterminate results
        focusedField = nil
    }
    /// for a tap on a valid reminder list row
    func tap(reminder: Reminder) {
         focusedField = .row(id: reminder.id) // override background tap that sets nil focus
    }
    
    func doubleTap() {
        if let lastReminder = reminders.last {
            if lastReminder.text.count > 0 {
                newReminder(after: lastReminder)
            }
            else {
                focusedField = .row(id: lastReminder.id)
            }
        }
    }
    
    func newReminder(after reminder: Reminder) {
        let newReminder = Reminder()
        if let index = reminders.firstIndex(where: {$0.id == reminder.id }) {
            reminders.insert(newReminder,at: index+1)
        } else {
            reminders.append(newReminder)
        }
        setFocus(newReminder)
    }

    func deleteReminder(_ reminder: Reminder) {
        Just(reminder)
          .delay(for: .seconds(0.25), scheduler: RunLoop.main)
          .sink { reminder in
            self.reminders.removeAll { $0.id == reminder.id }
          }
          .store(in: &cancellables)
      }
    
    func setFocus(_ reminder: Reminder) {
        delayedFocusedField = .row(id: reminder.id)
    }
}

struct Reminder: Identifiable, Equatable {
    var id = UUID().uuidString
    var text: String = ""
    static var examples = [ 
        Reminder(text: "one"), 
        Reminder(text: "two"), 
        Reminder(text: "three")
    ]
}

struct ReminderRow: View {
    @Binding var reminder: Reminder
    var body: some View {
        HStack{
            TextField("Reminder name", text: $reminder.text)
                .accentColor(.primary) // cursor color
                .disableAutocorrection(true)
        }
    }
}

extension View {
    /// Mirror changes between an @Published variable (typically in your View Model) and an @FocusedState variable in a view
    func sync<T: Equatable>(_ field1: Binding<T>, _ field2: FocusState<T>.Binding ) -> some View {
        return self
           .onChange(of: field1.wrappedValue) { field2.wrappedValue = $0 }
           .onChange(of: field2.wrappedValue) { field1.wrappedValue = $0 }
    }
}


struct ReminderList_Previews: PreviewProvider {
    static var previews: some View {
        ReminderList().environmentObject(ReminderListVM())
    }
}