Passing data between two views - swiftui

I wanted to create quiet a simple app on watchOS 6, but after Apple has changed the ObjectBindig in Xcode 11 beta 5 my App does not run anymore. I simply want to synchronize data between two Views.
So I have rewritten my App with the new #Published, but I can't really set it up:
class UserInput: ObservableObject {
#Published var score: Int = 0
}
struct ContentView: View {
#ObservedObject var input = UserInput()
var body: some View {
VStack {
Text("Hello World\(self.input.score)")
Button(action: {self.input.score += 1})
{
Text("Adder")
}
NavigationLink(destination: secondScreen()) {
Text("Next View")
}
}
}
}
struct secondScreen: View {
#ObservedObject var input = UserInput()
var body: some View {
VStack {
Text("Button has been pushed \(input.score)")
Button(action: {self.input.score += 1
}) {
Text("Adder")
}
}
}
}

Your code has a couple of errors:
1) You didn't put your ContentView in a NavigationView, so the navigation between the two views never happened.
2) You used data binding in a wrong way. If you need the second view to rely on some state belonging to the first view you need to pass a binding to that state to the second view. Both in your first view and in your second view you had an #ObservedObject created inline:
#ObservedObject var input = UserInput()
so, the first view and the second one worked with two totally different objects. Instead, you are interested in sharing the score between the views. Let the first view own the UserInput object and just pass a binding to the score integer to the second view. This way both the views will work on the same value (you can copy paste the code below and try yourself).
import SwiftUI
class UserInput: ObservableObject {
#Published var score: Int = 0
}
struct ContentView: View {
#ObservedObject var input = UserInput()
var body: some View {
NavigationView {
VStack {
Text("Hello World\(self.input.score)")
Button(action: {self.input.score += 1})
{
Text("Adder")
}
NavigationLink(destination: secondScreen(score: self.$input.score)) {
Text("Next View")
}
}
}
}
}
struct secondScreen: View {
#Binding var score: Int
var body: some View {
VStack {
Text("Button has been pushed \(score)")
Button(action: {self.score += 1
}) {
Text("Adder")
}
}
}
}
#if DEBUG
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
#endif
If you really need it you can even pass the entire UserInput object to the second view:
import SwiftUI
class UserInput: ObservableObject {
#Published var score: Int = 0
}
struct ContentView: View {
#ObservedObject var input = UserInput() //please, note the difference between this...
var body: some View {
NavigationView {
VStack {
Text("Hello World\(self.input.score)")
Button(action: {self.input.score += 1})
{
Text("Adder")
}
NavigationLink(destination: secondScreen(input: self.input)) {
Text("Next View")
}
}
}
}
}
struct secondScreen: View {
#ObservedObject var input: UserInput //... and this!
var body: some View {
VStack {
Text("Button has been pushed \(input.score)")
Button(action: {self.input.score += 1
}) {
Text("Adder")
}
}
}
}
#if DEBUG
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
#endif

I tried a lot of different approaches on how to pass data from one view to another and came up with a solution that fits for simple and complex views / view models.
Version
Apple Swift version 5.3.1 (swiftlang-1200.0.41 clang-1200.0.32.8)
This solution works with iOS 14.0 upwards, because you need the .onChange() view modifier. The example is written in Swift Playgrounds. If you need an onChange like modifier for lower versions, you should write your own modifier.
Main View
The main view has a #StateObject viewModel handling all of the views logic, like the button tap and the "data" (testingID: String) -> Check the ViewModel
struct TestMainView: View {
#StateObject var viewModel: ViewModel = .init()
var body: some View {
VStack {
Button(action: { self.viewModel.didTapButton() }) {
Text("TAP")
}
Spacer()
SubView(text: $viewModel.testingID)
}.frame(width: 300, height: 400)
}
}
Main View Model (ViewModel)
The viewModel publishes a testID: String?. This testID can be any kind of object (e.g. configuration object a.s.o, you name it), for this example it is just a string also needed in the sub view.
final class ViewModel: ObservableObject {
#Published var testingID: String?
func didTapButton() {
self.testingID = UUID().uuidString
}
}
So by tapping the button, our ViewModel will update the testID. We also want this testID in our SubView and if it changes, we also want our SubView to recognize and handle these changes. Through the ViewModel #Published var testingID we are able to publish changes to our view. Now let's take a look at our SubView and SubViewModel.
SubView
So the SubView has its own #StateObject to handle its own logic. It is completely separated from other views and ViewModels. In this example the SubView only presents the testID from its MainView. But remember, it can be any kind of object like presets and configurations for a database request.
struct SubView: View {
#StateObject var viewModel: SubviewModel = .init()
#Binding var test: String?
init(text: Binding<String?>) {
self._test = text
}
var body: some View {
Text(self.viewModel.subViewText ?? "no text")
.onChange(of: self.test) { (text) in
self.viewModel.updateText(text: text)
}
.onAppear(perform: { self.viewModel.updateText(text: test) })
}
}
To "connect" our testingID published by our MainViewModel we initialize our SubView with a #Binding. So now we have the same testingID in our SubView. But we don't want to use it in the view directly, instead we need to pass the data into our SubViewModel, remember our SubViewModel is a #StateObject to handle all the logic. And we can't pass the value into our #StateObject during view initialization. Also if the data (testingID: String) changes in our MainViewModel, our SubViewModel should recognize and handle these changes.
Therefore we are using two ViewModifiers.
onChange
.onChange(of: self.test) { (text) in
self.viewModel.updateText(text: text)
}
The onChange modifier subscribes to changes in our #Binding property. So if it changes, these changes get passed to our SubViewModel. Note that your property needs to be Equatable. If you pass a more complex object, like a Struct, make sure to implement this protocol in your Struct.
onAppear
We need onAppear to handle the "first initial data" because onChange doesn't fire the first time your view gets initialized. It is only for changes.
.onAppear(perform: { self.viewModel.updateText(text: test) })
Ok and here is the SubViewModel, nothing more to explain to this one I guess.
class SubviewModel: ObservableObject {
#Published var subViewText: String?
func updateText(text: String?) {
self.subViewText = text
}
}
Now your data is in sync between your MainViewModel and SubViewModel and this approach works for large views with many subviews and subviews of these subviews and so on. It also keeps your views and corresponding viewModels enclosed with high reusability.
Working Example
Playground on GitHub:
https://github.com/luca251117/PassingDataBetweenViewModels
Additional Notes
Why I use onAppear and onChange instead of only onReceive: It appears that replacing these two modifiers with onReceive leads to a continuous data stream firing the SubViewModel updateText multiple times. If you need to stream data for presentation, it could be fine but if you want to handle network calls for example, this can lead to problems. That's why I prefer the "two modifier approach".
Personal Note: Please don't modify the StateObject outside the corresponding view's scope. Even if it is somehow possible, it is not what its meant for.

My question is still related to how to pass data between two views but I have a more complicated JSON data set and I am running into problems both with the passing the data and with it's initialization. I have something that works but I am sure it is not correct. Here is the code. Help!!!!
/ File: simpleContentView.swift
import SwiftUI
// Following is the more complicated #ObservedObject (Buddy and class Buddies)
struct Buddy : Codable, Identifiable, Hashable {
var id = UUID()
var TheirNames: TheirNames
var dob: String = ""
var school: String = ""
enum CodingKeys1: String, CodingKey {
case id = "id"
case Names = "Names"
case dob = "dob"
case school = "school"
}
}
struct TheirNames : Codable, Identifiable, Hashable {
var id = UUID()
var first: String = ""
var middle: String = ""
var last: String = ""
enum CodingKeys2: String, CodingKey {
case id = "id"
case first = "first"
case last = "last"
}
}
class Buddies: ObservableObject {
#Published var items: [Buddy] {
didSet {
let encoder = JSONEncoder()
if let encoded = try? encoder.encode(items) {UserDefaults.standard.set(encoded, forKey: "Items")}
}
}
#Published var buddy: Buddy
init() {
if let items = UserDefaults.standard.data(forKey: "Items") {
let decoder = JSONDecoder()
if let decoded = try? decoder.decode([Buddy].self, from: items) {
self.items = decoded
// ??? How to initialize here
self.buddy = Buddy(TheirNames: TheirNames(first: "c", middle: "r", last: "c"), dob: "1/1/1900", school: "hard nocks")
return
}
}
// ??? How to initialize here
self.buddy = Buddy(TheirNames: TheirNames(first: "c", middle: "r", last: "c"), dob: "1/1/1900", school: "hard nocks")
self.items = []
}
}
struct simpleContentView: View {
#Environment(\.presentationMode) var presentationMode
#State private var showingSheet = true
#ObservedObject var buddies = Buddies()
var body: some View {
VStack {
Text("Simple View")
Button(action: {self.showingSheet.toggle()}) {Image(systemName: "triangle")
}.sheet(isPresented: $showingSheet) {
simpleDetailView(buddies: self.buddies, item: self.buddies.buddy)}
}
}
}
struct simpleContentView_Previews: PreviewProvider {
static var previews: some View {
simpleContentView()
}
}
// End of File: simpleContentView.swift
// This is in a separate file: simpleDetailView.swift
import SwiftUI
struct simpleDetailView: View {
#Environment(\.presentationMode) var presentationMode
#ObservedObject var buddies = Buddies()
var item: Buddy
var body: some View {
VStack {
Text(/*#START_MENU_TOKEN#*/"Hello, World!"/*#END_MENU_TOKEN#*/)
Text("First Name = \(item.TheirNames.first)")
Button(action: {self.presentationMode.wrappedValue.dismiss()}){ Text("return"); Image(systemName: "gobackward")}
}
}
}
// ??? Correct way to make preview call
struct simpleDetailView_Previews: PreviewProvider {
static var previews: some View {
// ??? Correct way to call here
simpleDetailView(item: Buddy(TheirNames: TheirNames(first: "", middle: "", last: ""), dob: "", school: "") )
}
}
// end of: simpleDetailView.swift

Using directly #State variable will help you to achieve this, but if you want to sync that variable for both the screens using view model or #Published, this is what you can do. As the #State won't be binded to the #Published property. To achieve this follow these steps.
Step1: - Create a delegate to bind the value on pop or disappearing.
protocol BindingDelegate {
func updateOnPop(value : Int)
}
Step 2:- Follow the code base for Content View
class UserInput: ObservableObject {
#Published var score: Int = 0
}
struct ContentView: View , BindingDelegate {
#ObservedObject var input = UserInput()
#State var navIndex : Int? = nil
var body: some View {
NavigationView {
VStack {
Text("Hello World\(self.input.score)")
Button(action: {self.input.score += 1}) {
Text("Adder")
}
ZStack {
NavigationLink(destination: secondScreen(score: self.$input.score,
del: self, navIndex: $navIndex),
tag: 1, selection: $navIndex) {
EmptyView()
}
Button(action: {
self.navIndex = 1
}) {
Text("Next View")
}
}
}
}
}
func updateOnPop(value: Int) {
self.input.score = value
}
}
Step 3: Follow these steps for secondScreen
final class ViewModel : ObservableObject {
#Published var score : Int
init(_ value : Int) {
self.score = value
}
}
struct secondScreen: View {
#Binding var score: Int
#Binding var navIndex : Int?
#ObservedObject private var vm : ViewModel
var delegate : BindingDelegate?
init(score : Binding<Int>, del : BindingDelegate, navIndex : Binding<Int?>) {
self._score = score
self._navIndex = navIndex
self.delegate = del
self.vm = ViewModel(score.wrappedValue)
}
private var btnBack : some View { Button(action: {
self.delegate?.updateOnPop(value: self.vm.score)
self.navIndex = nil
}) {
HStack {
Text("Back")
}
}
}
var body: some View {
VStack {
Text("Button has been pushed \(vm.score)")
Button(action: {
self.vm.score += 1
}) {
Text("Adder")
}
}
.navigationBarBackButtonHidden(true)
.navigationBarItems(leading: btnBack)
}
}

Related

SwiftUI publishing an environment change from within view update

The app has a model that stores the user's current preference for light/dark mode, which the user can change by clicking on a button:
class DataModel: ObservableObject {
#Published var mode: ColorScheme = .light
The ContentView's body tracks the model, and adjusts the colorScheme when the model changes:
struct ContentView: View {
#StateObject private var dataModel = DataModel()
var body: some View {
NavigationStack(path: $path) { ...
}
.environmentObject(dataModel)
.environment(\.colorScheme, dataModel.mode)
As of Xcode Version 14.0 beta 5, this is producing a purple warning: Publishing changes from within view updates is not allowed, this will cause undefined behavior. Is there another way to do this? Or is it a hiccup in the beta release? Thanks!
Update: 2022-09-28
Xcode 14.1 Beta 3 (finally) fixed the "Publishing changes from within view updates is not allowed, this will cause undefined behavior"
See: https://www.donnywals.com/xcode-14-publishing-changes-from-within-view-updates-is-not-allowed-this-will-cause-undefined-behavior/
Full disclosure - I'm not entirely sure why this is happening but these have been the two solutions I have found that seem to work.
Example Code
// -- main view
#main
struct MyApp: App {
#StateObject private var vm = ViewModel()
var body: some Scene {
WindowGroup {
ViewOne()
.environmentObject(vm)
}
}
}
// -- initial view
struct ViewOne: View {
#EnvironmentObject private var vm: ViewModel
var body: some View {
Button {
vm.isPresented.toggle()
} label: {
Text("Open sheet")
}
.sheet(isPresented: $vm.isPresented) {
SheetView()
}
}
}
// -- sheet view
struct SheetView: View {
#EnvironmentObject private var vm: ViewModel
var body: some View {
Button {
vm.isPresented.toggle()
} label: {
Text("Close sheet")
}
}
}
// -- view model
class ViewModel: ObservableObject {
#Published var isPresented: Bool = false
}
Solution 1
Note: from my testing and the example below I still get the error to appear. But if I have a more complex/nested app then the error disappears..
Adding a .buttonStyle() to the button that does the initial toggling.
So within the ContentView on the Button() {} add in a .buttonStyle(.plain) and it will remove the purple error:
struct ViewOne: View {
#EnvironmentObject private var vm: ViewModel
var body: some View {
Button {
vm.isPresented.toggle()
} label: {
Text("Open sheet")
}
.buttonStyle(.plain) // <-- here
.sheet(isPresented: $vm.isPresented) {
SheetView()
}
}
}
^ This is probably more of a hack than solution since it'll output a new view from the modifier and that is probably what is causing it to not output the error on larger views.
Solution 2
This one is credit to Alex Nagy (aka. Rebeloper)
As Alex explains:
.. with SwiftUI 3 and SwiftUI 4 the data handling kind of changed. How SwiftUI handles, more specifically the #Published variable ..
So the solution is to have the boolean trigger to be a #State variable within the view and not as a #Published one inside the ViewModel. But as Alex points out it can make your views messy and if you have a lot of states in it, or not be able to deep link, etc.
However, since this is the way that SwiftUI 4 wants these to operate, we run the code as such:
// -- main view
#main
struct MyApp: App {
var body: some Scene {
WindowGroup {
ViewOne()
}
}
}
// -- initial view
struct ViewOne: View {
#State private var isPresented = false
var body: some View {
Button {
isPresented.toggle()
} label: {
Text("Open sheet")
}
.sheet(isPresented: $isPresented) {
SheetView(isPresented: $isPresented)
// SheetView() <-- if using dismiss() in >= iOS 15
}
}
}
// -- sheet view
struct SheetView: View {
// I'm showing a #Binding here for < iOS 15
// but you can use the dismiss() option if you
// target higher
// #Environment(\.dismiss) private var dismiss
#Binding var isPresented: Bool
var body: some View {
Button {
isPresented.toggle()
// dismiss()
} label: {
Text("Close sheet")
}
}
}
Using the #Published and the #State
Continuing from the video, if you need to still use the #Published variable as it might tie into other areas of your app you can do so with a .onChange and a .onReceive to link the two variables:
struct ViewOne: View {
#EnvironmentObject private var vm: ViewModel
#State private var isPresented = false
var body: some View {
Button {
vm.isPresented.toggle()
} label: {
Text("Open sheet")
}
.sheet(isPresented: $isPresented) {
SheetView(isPresented: $isPresented)
}
.onReceive(vm.$isPresented) { newValue in
isPresented = newValue
}
.onChange(of: isPresented) { newValue in
vm.isPresented = newValue
}
}
}
However, this can become really messy in your code if you have to trigger it for every sheet or fullScreenCover.
Creating a ViewModifier
So to make it easier for you to implement it you can create a ViewModifier which Alex has shown works too:
extension View {
func sync(_ published: Binding<Bool>, with binding: Binding<Bool>) -> some View {
self
.onChange(of: published.wrappedValue) { newValue in
binding.wrappedValue = newValue
}
.onChange(of: binding.wrappedValue) { newValue in
published.wrappedValue = newValue
}
}
}
And in use on the View:
struct ViewOne: View {
#EnvironmentObject private var vm: ViewModel
#State private var isPresented = false
var body: some View {
Button {
vm.isPresented.toggle()
} label: {
Text("Open sheet")
}
.sheet(isPresented: $isPresented) {
SheetView(isPresented: $isPresented)
}
.sync($vm.isPresented, with: $isPresented)
// .onReceive(vm.$isPresented) { newValue in
// isPresented = newValue
// }
// .onChange(of: isPresented) { newValue in
// vm.isPresented = newValue
// }
}
}
^ Anything denoted with this is my assumptions and not real technical understanding - I am not a technical knowledgeable :/
Try running the code that's throwing the purple error asynchronously, for example, by using DispatchQueue.main.async or Task.
DispatchQueue.main.async {
// environment changing code comes here
}
Task {
// environment changing code comes here
}
Improved Solution of Rebel Developer
as a generic function.
Rebeloper solution
It helped me a lot.
1- Create extension for it:
extension View{
func sync<T:Equatable>(_ published:Binding<T>, with binding:Binding<T>)-> some View{
self
.onChange(of: published.wrappedValue) { published in
binding.wrappedValue = published
}
.onChange(of: binding.wrappedValue) { binding in
published.wrappedValue = binding
}
}
}
2- sync() ViewModel #Published var to local #State var
struct ContentView: View {
#EnvironmentObject var viewModel:ViewModel
#State var fullScreenType:FullScreenType?
var body: some View {
//..
}
.sync($viewModel.fullScreenType, with: $fullScreenType)

Swiftui how to update variable once view is closed

I am using SwiftUi 3.0 and I am new to it . I am learning about ObservedObjects . What I am trying to do is update the count of a variable every time that I close a view . This is the entire small app . The screen starts at DataUpdateView view when I click Next View I go to DataUpdateView2 view . Once I close DataUpdateView2 and go back to the original view I want to have the
Text("Score Count \(progress.score)")
score number increase by 1 since in the second view I do a +1 every time that I close that view . Any suggestions would be great
import SwiftUI
class UserProgress: ObservableObject {
#Published var score = 0
}
struct DataUpdateView: View {
#State var nextView = false
#StateObject var progress = UserProgress()
var body: some View {
VStack {
Text("Score Count \(progress.score)")
Text("Next View")
.onTapGesture {
nextView = true
}.fullScreenCover(isPresented: $nextView, content: {
DataUpdateView2()
})
}
}
}
struct DataUpdateView_Previews: PreviewProvider {
static var previews: some View {
DataUpdateView()
}
}
struct DataUpdateView2: View {
#ObservedObject var progress = UserProgress()
#Environment(\.presentationMode) var presentationMode
var body: some View {
Text("Back")
.onTapGesture {
progress.score += 1
self.presentationMode.wrappedValue.dismiss()
}
}
}
struct DataUpdateView2_Previews: PreviewProvider {
static var previews: some View {
DataUpdateView2()
}
}
You're probably not seeing the first view update since both views are instantiating their own UserProgress(). You need to pass the object you already created in the first view along to the second in the initializer
So In DataUpdateView:
.fullScreenCover(isPresented: $nextView, content: {
DataUpdateView2(progress: progress)
})
}
And then in DataUpdateView2:
struct DataUpdateView2: View {
#ObservedObject var progress: UserProgress
#Environment(\.presentationMode) var presentationMode
// ...
}
So now the second view is receiving the object from the first rather than creating its own.
Note: If you are not using an ObservableObject, then take a look at the second part.
In this specific situation, you don't even need a Binding variable, you can just use the .onDisappear method. .onDisappear Documentation.
import SwiftUI
class UserProgress: ObservableObject {
#Published var score = 0
}
struct DataUpdateView: View {
#State var nextView = false
#StateObject var progress = UserProgress()
var body: some View {
VStack {
Text("Score Count \(progress.score)")
Text("Next View")
.onTapGesture {
nextView = true
}.fullScreenCover(isPresented: $nextView, content: {
DataUpdateView2()
})
}
}
}
struct DataUpdateView_Previews: PreviewProvider {
static var previews: some View {
DataUpdateView()
}
}
struct DataUpdateView2: View {
#ObservedObject var progress = UserProgress()
#Environment(\.presentationMode) var presentationMode
var body: some View {
Text("Back")
.onTapGesture{
presentationMode.wrappedValue.dismiss()
print("Dismissed!")
}
.onDisappear{
//This is called when the view disappears.
progress.score += 1
}
}
}
struct DataUpdateView2_Previews: PreviewProvider {
static var previews: some View {
DataUpdateView2()
}
}
Second Part
If you want the variable to update when the view closes, you could use the .onDisappear method and a Binding value. An example implementation of this is below:
struct ViewOne: View{
#State var number = 0
var body: some View{
VStack{
Text("Number: \(number)")
NavigationLink(destination: ViewTwo(variable: $number)){
Text("Go To View Two")
}
}
}
}
struct ViewTwo: View{
#Binding var variable: Int
var body: some View{
//Content of view 2 here
Text("View Two")
.onDisappear{
//This is called when the view disappears
variable += 1
}
}
}
In short you need to use same view model in both views. A possible and seems simplest approach in your code is to inject view model from first view to second via environment object, like
#StateObject var progress = UserProgress()
var body: some View {
VStack {
Text("Score Count \(progress.score)")
Text("Next View")
.onTapGesture {
nextView = true
}.fullScreenCover(isPresented: $nextView, content: {
DataUpdateView2()
.environmentObject(progress) // << here !!
})
and use it internally, like
struct DataUpdateView2: View {
#EnvironmentObject var progress: UserProgress // << injected automatically !!

swiftui cannot change #State value in sink

i am learning swiftui now and I am newbie for stackoverflow, I find a question,this is my code. I want to change the #State nopubName in sink ,but it's not work,the print is always "Nimar", I don't know why
struct ContentView: View {
#State var nopubName: String = "Nimar"
private var cancellable: AnyCancellable?
var stringSubject = PassthroughSubject<String, Never>()
init() {
cancellable = stringSubject.sink(receiveValue: handleValue(_:))
}
func handleValue(_ value: String) {
print("handleValue: '\(value)'")
self.nopubName = value
print("in sink "+nopubName)
}
var body: some View {
VStack {
Text(self.nopubName)
.font(.title).bold()
.foregroundColor(.red)
Spacer()
Button("sink"){
stringSubject.send("World")
print(nopubName)
}
}
}
}
You should only access a state property from inside the view’s body, or from methods called by it.
https://developer.apple.com/documentation/swiftui/state
You can get that functionality working in an ObservableObject and update an #Published To keep the UI updated
https://developer.apple.com/documentation/swiftui/managing-model-data-in-your-app
You don't need to use Combine, If you are within the View, you can change the value of #State variables directly
struct ContentView: View {
#State var nopubName: String = "Nimar"
var body: some View {
VStack {
Text(self.nopubName)
.font(.title).bold()
.foregroundColor(.red)
Spacer()
Button("sink"){
nopubName = "World"
}
}
}
}

SwiftUI NavigationLink behaviour is different when selection is set directly, rather than from viewModel

I've been working with SwiftUI and ran into unexpected behavior.
I have View A and View B and View C. View C has EnviromentObject that changes AppState from View A
View B has ViewModel with selection
If I call function from ViewModel to change the selection then
View C is shown for a few seconds and then it automatically pops back to View B
If I change selection directly from View B (not from ViewModel), everything works as expected.
Also, if I comment out onDissapear, it also works. But, I need to change environmentObject when screen dissapeared
Here is View B and ViewModel
import SwiftUI
class AppState: ObservableObject {
#Published
var shouldHideUserInfo = false
}
struct ContentView: View {
#EnvironmentObject
var appState: AppState
#State
var selection: Int? = nil
var body: some View {
NavigationView {
VStack {
if !appState.shouldHideUserInfo {
Text("USER INFO")
}
NavigationLink(
destination: ViewA(),
tag: 1,
selection: $selection,
label: { EmptyView()})
Button("MOVE TO VIEW A") {
selection = 1
}
}
}
}
}
class ViewAModel: ObservableObject {
#Published
var selection: Int? = nil
func navigate() {
selection = 2 //<- this doesnt
}
}
struct ViewA: View {
#ObservedObject
var viewModel: ViewAModel
init() {
viewModel = ViewAModel()
}
#State
var selection: Int? = nil //<- this works
var body: some View {
VStack
{
Text("VIEW A")
NavigationLink(
destination: ViewB(),
tag: 2,
selection: $viewModel.selection,
label: { EmptyView()})
Button("MOVE TO VIEW B") {
//selection = 2 <-- this works
viewModel.navigate() //<- this doesnt
}
}
}
}
struct ViewB: View {
#EnvironmentObject
var appState: AppState
#State
var selection: Int? = nil
var body: some View {
VStack
{
Text("VIEW B")
}
.onAppear {
appState.shouldHideUserInfo = true
}
}
}
Factory pattern didn't solve the issue:
static func makeViewA(param: Int?) -> some View {
let viewModel = ViewAModel(param: param)
return ViewA(viewModel: viewModel)
}
}
I see... it is a bit different than in post. The issue is because view model is recreated (this is long observed behavior of NavigationView) and thus binding lost.
The quick fix is
struct ViewA: View {
#StateObject
var viewModel: ViewAModel = ViewAModel()
init() {
// viewModel = ViewAModel()
}
// ... other code
}
alternate is to keep ownership of view model outside of ViewA.
Update: SwiftUI 1.0 compatible - here is variant that works everywhere. The reason of the issue is in AppState. The code in ViewB updates appState
.onAppear {
appState.shouldHideUserInfo = true
}
that causes rebuild of ContentView body, which recreates ViewA, which recreates NavigationLink, which drops previous link and ViewB got closed.
To prevent this we need to avoid rebuild ViewA. This can be done by making ViewA is-a Equatable, so SwiftUI check if ViewA needs to be recreated and we will answer NO.
Here is how it goes:
NavigationLink(
destination: ViewA().equatable(), // << here !!
tag: 1,
selection: $selection,
label: { EmptyView()})
and
struct ViewA: View, Equatable {
static func == (lhs: ViewA, rhs: ViewA) -> Bool {
true
}
// .. other code

How to add an observable property when other properties change

I have the following model object that I use to populate a List with a Toggle for each row, which is bound to measurement.isSelected
final class Model: ObservableObject {
struct Measurement: Identifiable {
var id = UUID()
let name: String
var isSelected: Binding<Bool>
var selected: Bool = false
init(name: String) {
self.name = name
let selected = CurrentValueSubject<Bool, Never>(false)
self.isSelected = Binding<Bool>(get: { selected.value }, set: { selected.value = $0 })
}
}
#Published var measurements: [Measurement]
#Published var hasSelection: Bool = false // How to set this?
init(measurements: [Measurement]) {
self.measurements = measurements
}
}
I'd like the hasSelection property to be true whenever any measurement.isSelected is true. I'm guessing somehow Model needs to observe changes in measurements and then update its hasSelection property… but I've no idea where to start!
The idea is that hasSelection will be bound to a Button to enable or disable it.
Model is used as follows…
struct MeasurementsView: View {
#ObservedObject var model: Model
var body: some View {
NavigationView {
List(model.measurements) { measurement in
MeasurementView(measurement: measurement)
}
.navigationBarTitle("Select Measurements")
.navigationBarItems(trailing: NavigationLink(destination: NextView(), isActive: $model.hasSelection, label: {
Text("Next")
}))
}
}
}
struct MeasurementView: View {
let measurement: Model.Measurement
var body: some View {
HStack {
Text(measurement.name)
.font(.subheadline)
Spacer()
Toggle(measurement.name, isOn: measurement.isSelected)
.labelsHidden()
}
}
}
For info, here's a screenshot of what I'm trying to achieve. A list of selectable items, with a navigation link that is enabled when one or more is selected, and disabled when no items are selected.
#user3441734 hasSelection should ideally be a get only property, that
is true if any of measurement.isSelected is true
struct Data {
var bool: Bool
}
class Model: ObservableObject {
#Published var arr: [Data] = []
var anyTrue: Bool {
arr.map{$0.bool}.contains(true)
}
}
example (as before) copy - paste - run
import SwiftUI
struct Data: Identifiable {
let id = UUID()
var name: String
var on_off: Bool
}
class Model: ObservableObject {
#Published var data = [Data(name: "alfa", on_off: false), Data(name: "beta", on_off: false), Data(name: "gama", on_off: false)]
var bool: Bool {
data.map {$0.on_off} .contains(true)
}
}
struct ContentView: View {
#ObservedObject var model = Model()
var body: some View {
VStack {
List(0 ..< model.data.count) { idx in
HStack {
Text(verbatim: self.model.data[idx].name)
Toggle(isOn: self.$model.data[idx].on_off) {
EmptyView()
}
}
}
Text("\(model.bool.description)").font(.largeTitle).padding()
}
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
When the model.data is updated
#Published var data ....
its publisher calls objectWillChange on ObservableObject.
Next SwiftUI recognize that ObservedObject needs the View to be "updated". The View is recreated, and that will force the model.bool.description will have fresh value.
LAST UPDATE
change this part of code
struct ContentView: View {
#ObservedObject var model = Model()
var body: some View {
NavigationView {
List(0 ..< model.data.count) { idx in
HStack {
Text(verbatim: self.model.data[idx].name)
Toggle(isOn: self.$model.data[idx].on_off) {
EmptyView()
}
}
}.navigationBarTitle("List")
.navigationBarItems(trailing:
NavigationLink(destination: Text("next"), label: {
Text("Next")
}).disabled(!model.bool)
)
}
}
}
and it is EXACTLY, WHAT YOU HAVE in your updated question
Try it on real device, otherwise the NavigationLink is usable only once (this is well known simulator bug in current Xcode 11.3.1 (11C504)).
The problem with your code at the moment is that even if you observe the changes to measurements, they will not get updated when the selection updates, because you declared the var isSelected: Binding<Bool> as a Binding. This means that SwiftUI is storing it outside of your struct, and the struct itself doesn't update (stays immutable).
What you could try instead is declaring #Published var selectedMeasurementId: UUID? = nil on your model So your code would be something like this:
import SwiftUI
import Combine
struct NextView: View {
var body: some View {
Text("Next View")
}
}
struct MeasurementsView: View {
#ObservedObject var model: Model
var body: some View {
let hasSelection = Binding<Bool> (
get: {
self.model.selectedMeasurementId != nil
},
set: { value in
self.model.selectedMeasurementId = nil
}
)
return NavigationView {
List(model.measurements) { measurement in
MeasurementView(measurement: measurement, selectedMeasurementId: self.$model.selectedMeasurementId)
}
.navigationBarTitle("Select Measurements")
.navigationBarItems(trailing: NavigationLink(destination: NextView(), isActive: hasSelection, label: {
Text("Next")
}))
}
}
}
struct MeasurementView: View {
let measurement: Model.Measurement
#Binding var selectedMeasurementId: UUID?
var body: some View {
let isSelected = Binding<Bool>(
get: {
self.selectedMeasurementId == self.measurement.id
},
set: { value in
if value {
self.selectedMeasurementId = self.measurement.id
} else {
self.selectedMeasurementId = nil
}
}
)
return HStack {
Text(measurement.name)
.font(.subheadline)
Spacer()
Toggle(measurement.name, isOn: isSelected)
.labelsHidden()
}
}
}
final class Model: ObservableObject {
#Published var selectedMeasurementId: UUID? = nil
struct Measurement: Identifiable {
var id = UUID()
let name: String
init(name: String) {
self.name = name
}
}
#Published var measurements: [Measurement]
init(measurements: [Measurement]) {
self.measurements = measurements
}
}
I'm not sure exactly how you want the navigation button in the navbar to behave. For now I just set the selection to nil when it's tapped. You can modify it depending on what you want to do.
If you want to support multi-selection, you can use a Set of selected ids instead.
Also, seems like the iOS simulator has some problems with navigation, but I tested on a physical device and it worked.