Why is an ObservedObject array not updated in my SwiftUI application? - swiftui

Question

I'm playing with SwiftUI, trying to understand how ObservableObject works. I have an array of Person objects. When I add a new Person into the array, it is reloaded in my View, however if I change the value of an existing Person, it is not reloaded in the View.
// NamesClass.swift
import Foundation
import SwiftUI
import Combine
class Person: ObservableObject,Identifiable{
var id: Int
#Published var name: String
init(id: Int, name: String){
self.id = id
self.name = name
}
}
class People: ObservableObject{
#Published var people: [Person]
init(){
self.people = [
Person(id: 1, name:"Javier"),
Person(id: 2, name:"Juan"),
Person(id: 3, name:"Pedro"),
Person(id: 4, name:"Luis")]
}
}
struct ContentView: View {
#ObservedObject var mypeople: People
var body: some View {
VStack{
ForEach(mypeople.people){ person in
Text("\(person.name)")
}
Button(action: {
self.mypeople.people[0].name="Jaime"
//self.mypeople.people.append(Person(id: 5, name: "John"))
}) {
Text("Add/Change name")
}
}
}
}
If I uncomment the line to add a new Person (John), the name of Jaime is shown properly, however if I just change the name this is not shown in the View.
I'm afraid I'm doing something wrong or maybe I don't get how the ObservedObjects work with arrays.

Answer 1

You can use a struct instead of a class. Because of a struct's value semantics, a change to a person's name is seen as a change to Person struct itself, and this change is also a change to the people array so #Published will send the notification and the View body will be recomputed.
import Foundation
import SwiftUI
import Combine
struct Person: Identifiable{
var id: Int
var name: String
init(id: Int, name: String){
self.id = id
self.name = name
}
}
class Model: ObservableObject{
#Published var people: [Person]
init(){
self.people = [
Person(id: 1, name:"Javier"),
Person(id: 2, name:"Juan"),
Person(id: 3, name:"Pedro"),
Person(id: 4, name:"Luis")]
}
}
struct ContentView: View {
#StateObject var model = Model()
var body: some View {
VStack{
ForEach(model.people){ person in
Text("\(person.name)")
}
Button(action: {
self.mypeople.people[0].name="Jaime"
}) {
Text("Add/Change name")
}
}
}
}
Alternatively (and not recommended), Person is a class, so it is a reference type. When it changes, the People array remains unchanged and so nothing is emitted by the subject. However, you can manually call it, to let it know:
Button(action: {
self.mypeople.objectWillChange.send()
self.mypeople.people[0].name="Jaime"
}) {
Text("Add/Change name")
}

Answer 2

I think there is a more elegant solution to this problem. Instead of trying to propagate the objectWillChange message up the model hierarchy, you can create a custom view for the list rows so each item is an #ObservedObject:
struct PersonRow: View {
#ObservedObject var person: Person
var body: some View {
Text(person.name)
}
}
struct ContentView: View {
#ObservedObject var mypeople: People
var body: some View {
VStack{
ForEach(mypeople.people){ person in
PersonRow(person: person)
}
Button(action: {
self.mypeople.people[0].name="Jaime"
//self.mypeople.people.append(Person(id: 5, name: "John"))
}) {
Text("Add/Change name")
}
}
}
}
In general, creating a custom view for the items in a List/ForEach allows each item in the collection to be monitored for changes.

Answer 3

For those who might find it helpful. This is a more generic approach to #kontiki 's answer.
This way you will not have to be repeating yourself for different model class types
import Foundation
import Combine
import SwiftUI
class ObservableArray<T>: ObservableObject {
#Published var array:[T] = []
var cancellables = [AnyCancellable]()
init(array: [T]) {
self.array = array
}
func observeChildrenChanges<T: ObservableObject>() -> ObservableArray<T> {
let array2 = array as! [T]
array2.forEach({
let c = $0.objectWillChange.sink(receiveValue: { _ in self.objectWillChange.send() })
// Important: You have to keep the returned value allocated,
// otherwise the sink subscription gets cancelled
self.cancellables.append(c)
})
return self as! ObservableArray<T>
}
}
class Person: ObservableObject,Identifiable{
var id: Int
#Published var name: String
init(id: Int, name: String){
self.id = id
self.name = name
}
}
struct ContentView : View {
//For observing changes to the array only.
//No need for model class(in this case Person) to conform to ObservabeObject protocol
#ObservedObject var mypeople: ObservableArray<Person> = ObservableArray(array: [
Person(id: 1, name:"Javier"),
Person(id: 2, name:"Juan"),
Person(id: 3, name:"Pedro"),
Person(id: 4, name:"Luis")])
//For observing changes to the array and changes inside its children
//Note: The model class(in this case Person) must conform to ObservableObject protocol
#ObservedObject var mypeople: ObservableArray<Person> = try! ObservableArray(array: [
Person(id: 1, name:"Javier"),
Person(id: 2, name:"Juan"),
Person(id: 3, name:"Pedro"),
Person(id: 4, name:"Luis")]).observeChildrenChanges()
var body: some View {
VStack{
ForEach(mypeople.array){ person in
Text("\(person.name)")
}
Button(action: {
self.mypeople.array[0].name="Jaime"
//self.mypeople.people.append(Person(id: 5, name: "John"))
}) {
Text("Add/Change name")
}
}
}
}

Answer 4

ObservableArray is very useful, thank you! Here's a more generalised version that supports all Collections, which is handy when you need to react to CoreData values indirected through a to-many relationship (which are modelled as Sets).
import Combine
import SwiftUI
private class ObservedObjectCollectionBox<Element>: ObservableObject where Element: ObservableObject {
private var subscription: AnyCancellable?
init(_ wrappedValue: AnyCollection<Element>) {
self.reset(wrappedValue)
}
func reset(_ newValue: AnyCollection<Element>) {
self.subscription = Publishers.MergeMany(newValue.map{ $0.objectWillChange })
.eraseToAnyPublisher()
.sink { _ in
self.objectWillChange.send()
}
}
}
#propertyWrapper
public struct ObservedObjectCollection<Element>: DynamicProperty where Element: ObservableObject {
public var wrappedValue: AnyCollection<Element> {
didSet {
if isKnownUniquelyReferenced(&observed) {
self.observed.reset(wrappedValue)
} else {
self.observed = ObservedObjectCollectionBox(wrappedValue)
}
}
}
#ObservedObject private var observed: ObservedObjectCollectionBox<Element>
public init(wrappedValue: AnyCollection<Element>) {
self.wrappedValue = wrappedValue
self.observed = ObservedObjectCollectionBox(wrappedValue)
}
public init(wrappedValue: AnyCollection<Element>?) {
self.init(wrappedValue: wrappedValue ?? AnyCollection([]))
}
public init<C: Collection>(wrappedValue: C) where C.Element == Element {
self.init(wrappedValue: AnyCollection(wrappedValue))
}
public init<C: Collection>(wrappedValue: C?) where C.Element == Element {
if let wrappedValue = wrappedValue {
self.init(wrappedValue: wrappedValue)
} else {
self.init(wrappedValue: AnyCollection([]))
}
}
}
It can be used as follows, let's say for example we have a class Fridge that contains a Set and our view needs to react to changes in the latter despite not having any subviews that observe each item.
class Food: ObservableObject, Hashable {
#Published var name: String
#Published var calories: Float
init(name: String, calories: Float) {
self.name = name
self.calories = calories
}
static func ==(lhs: Food, rhs: Food) -> Bool {
return lhs.name == rhs.name && lhs.calories == rhs.calories
}
func hash(into hasher: inout Hasher) {
hasher.combine(self.name)
hasher.combine(self.calories)
}
}
class Fridge: ObservableObject {
#Published var food: Set<Food>
init(food: Set<Food>) {
self.food = food
}
}
struct FridgeCaloriesView: View {
#ObservedObjectCollection var food: AnyCollection<Food>
init(fridge: Fridge) {
self._food = ObservedObjectCollection(wrappedValue: fridge.food)
}
var totalCalories: Float {
self.food.map { $0.calories }.reduce(0, +)
}
var body: some View {
Text("Total calories in fridge: \(totalCalories)")
}
}

Answer 5

The ideal thing to do would be to chain #ObservedObject or #StateObject and some other property wrapper that is suitable for sequences, e.g. #StateObject #ObservableObjects. But you can't use more than one property wrapper, so you need to make different types to handle the two different cases. Then you can use either one of the following, as appropriate.
(Your People type is unnecessary—its purpose can be abstracted to all sequences.)
#StateObjects var people = [
Person(id: 1, name:"Javier"),
Person(id: 2, name:"Juan"),
Person(id: 3, name:"Pedro"),
Person(id: 4, name:"Luis")
]
#ObservedObjects var people: [Person]
import Combine
import SwiftUI
#propertyWrapper
public final class ObservableObjects<Objects: Sequence>: ObservableObject
where Objects.Element: ObservableObject {
public init(wrappedValue: Objects) {
self.wrappedValue = wrappedValue
assignCancellable()
}
#Published public var wrappedValue: Objects {
didSet { assignCancellable() }
}
private var cancellable: AnyCancellable!
}
// MARK: - private
private extension ObservableObjects {
func assignCancellable() {
cancellable = Publishers.MergeMany(wrappedValue.map(\.objectWillChange))
.sink { [unowned self] _ in objectWillChange.send() }
}
}
// MARK: -
#propertyWrapper
public struct ObservedObjects<Objects: Sequence>: DynamicProperty
where Objects.Element: ObservableObject {
public init(wrappedValue: Objects) {
_objects = .init(
wrappedValue: .init(wrappedValue: wrappedValue)
)
}
public var wrappedValue: Objects {
get { objects.wrappedValue }
nonmutating set { objects.wrappedValue = newValue }
}
public var projectedValue: Binding<Objects> { $objects.wrappedValue }
#ObservedObject private var objects: ObservableObjects<Objects>
}
#propertyWrapper
public struct StateObjects<Objects: Sequence>: DynamicProperty
where Objects.Element: ObservableObject {
public init(wrappedValue: Objects) {
_objects = .init(
wrappedValue: .init(wrappedValue: wrappedValue)
)
}
public var wrappedValue: Objects {
get { objects.wrappedValue }
nonmutating set { objects.wrappedValue = newValue }
}
public var projectedValue: Binding<Objects> { $objects.wrappedValue }
#StateObject private var objects: ObservableObjects<Objects>
}