I want to change Picker's content dynamically.
But I think you cannot pass Binding property into ForEach.
#Binding var options: [String]
#Binding var selectedIndex: Int
var body: some View {
Picker(selection: self.$selectedIndex, label: Text("")) {
ForEach(0..<self.$options.count) { // error: Cannot assign to property: 'count' is a get-only property
Text(self.options[$0])
}
}
}
Here is a demo of possible solution. Tested with Xcode 11.4 / iOS 13.4
struct TestPickerView: View {
#Binding var options: [String]
#Binding var selectedIndex: Int
var body: some View {
Picker(selection: self.$selectedIndex, label: Text("")) {
ForEach(Array(self.options.enumerated()), id: \.element) { index, item in
Text(item).tag(index)
}
}.id(options) // << important !!
}
}
Note: a Picker have to be explicitly depend on options to be updated/rebuilt when number of options changed, this is what id is needed for.
Related
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)
I am making an app where I am showing different views based of user's selection by a picker. The binding value of the picker is initially set by UserDefaults in a viewModel. The problem is when I choose a picker value in my app, The picker automatically go back to initial state, as if someone forcing the picker not the change the values.
Settings ViewModel :
import Foundation
class SettingsViewModel:ObservableObject{
#Published var showSettings = false
//Here is the problem
#Published var choosenUserType = UserDefaults.standard.string(forKey: "userType"){
didSet{
UserDefaults.standard.set(self.choosenUserType, forKey: "userType")
}
}
static var userTypes = ["Client", "Worker"]
}
Home View:
import SwiftUI
struct HomeView: View {
#StateObject var settingsVM = SettingsViewModel()
var body: some View {
VStack{
switch settingsVM.choosenUserType{
case "Client":
Text("This is client")
case "Worker":
Text("This is worker")
default:
Text("This is default")
}
}.navigationTitle("Tanvirgeek Co")
.navigationBarItems(trailing: Button(action: {
settingsVM.showSettings.toggle()
}, label: {
Text("Settings")
}))
.sheet(isPresented: $settingsVM.showSettings, content: {
SettingsView(dissmiss: $settingsVM.showSettings)
.environmentObject(settingsVM)
})
}
}
Settings View:
import SwiftUI
struct SettingsView: View {
#EnvironmentObject var settingVM:SettingsViewModel
#Binding var dissmiss:Bool
var body: some View {
VStack{
Picker(selection: $settingVM.choosenUserType, label: Text("Choose User Type"), content: {
ForEach(SettingsViewModel.userTypes, id: \.self) { userType in
Text("\(userType)")
}
})
Button(action: {
dissmiss.toggle()
}, label: {
Text("Dismiss")
})
}
}
}
What I am doing wrong? How to change the picker's binding variable value through the picked value here?
Your choosenUserType ends up with an inferred type of String? because that's what UserDefaults.string(forKey:) returns.
The Picker's selection type needs to match exactly with the tag type. The tags (which are inferred in this case as well) are of type String.
I've solved this by giving a default value to choosenUserType so that it can be a String (not String?):
class SettingsViewModel:ObservableObject{
#Published var showSettings = false
#Published var choosenUserType : String = UserDefaults.standard.string(forKey: "userType") ?? SettingsViewModel.userTypes[0] {
didSet{
UserDefaults.standard.set(self.choosenUserType, forKey: "userType")
}
}
static var userTypes = ["Client", "Worker"]
}
Also, in your SettingsView, you don't have to interpolate the userType in the Text -- you can just provide it directly:
struct SettingsView: View {
#EnvironmentObject var settingVM:SettingsViewModel
#Binding var dissmiss:Bool
var body: some View {
VStack{
Picker(selection: $settingVM.choosenUserType, label: Text("Choose User Type")) {
ForEach(SettingsViewModel.userTypes, id: \.self) { userType in
Text(userType)
}
}
Button(action: {
dissmiss.toggle()
}, label: {
Text("Dismiss")
})
}
}
}
I'm trying to implement the logic of programmatic view pop in SwiftUI
I set a tag on the items and create a corresponding variable for the selected item, but when I click the button on the DetaiView screen, nothing happens.
The problem disappears if I use VStack instead of List or use the presentationMode Environment variable to release the screen, but I'd like to use List.
Code:
struct ContentView: View {
#State var data: [Int] = [1, 2, 3, 4, 5]
#State var selectedItem: Int? = nil
var body: some View {
NavigationView {
// NavigationLink( // This make dismiss work
// destination: DetailView(selectedItem: $selectedItem),
// tag: 123,
// selection: $selectedItem,
// label: { Text("\(123)") }
// )
List { // VStack here make dismiss work
ForEach(data, id: \.self) { element in
NavigationLink(
destination: DetailView(selectedItem: $selectedItem),
tag: element,
selection: $selectedItem,
label: { Text("\(element)") }
)
.isDetailLink(false)
}
}
}
}
}
struct DetailView: View {
#Binding var selectedItem: Int?
// #Environment(\.presentationMode) var presentationMode: Binding<PresentationMode>
var body: some View {
VStack {
Text("DETAIL")
Button("BUTTON") {
selectedItem = nil // Variable set to nil but nothing happens
// presentationMode.wrappedValue.dismiss()
}
}
}
}
Am I doing something wrong or is it a SwiftUI bug?
Thank you!
Update: Fixed
Tested with Xcode 13.3 / iOS 15.4
Demo with just copy-pasted code:
NavigationLinks selected parameeter takes in Binding but my ObservedObject has Bool variable not an Optional value.
How to convert it so that selected would accept it?
Code:
class RegistrationFlowEnvironment: ObservableObject {
#Published var done = false
}
the view:
struct ContentView: View {
#ObservedObject var regFlowEnv = RegistrationFlowEnvironment()
var body: some View {
NavigationView {
VStack{
NavigationLink("Go to other place", destination: Dest(),
tag: true,
selection: $regFlowEnv.done)
}
}
}
}
Here is fixed variant (tested with Xcode 11.7 / iOS 13.7)
NavigationView {
VStack{
NavigationLink("Go to other place", destination: Text("Dest"),
tag: true,
selection: Binding($regFlowEnv.done))
}
}
I'm trying to get a list of Datacenters from a Rest API and show them in a Picker, so the user can choose one. When I do it with a static list it works fine. However, retrieving the Datacenters dinamically seems not to work fine.
I'm using Xcode 11 (GM)
This is the Datacenter Object
struct Datacenter:Codable, Hashable, Identifiable{
let id: String
var location: String
}
This is the ObservedObject (it has the property datacenters that is an array of Datacenter objects)
#ObservedObject var datacenters_controller : DatacentersController
#State private var selectedDatacenter = 0
This was my first attempt:
Picker(selection: $selectedDatacenter, label: Text("Datacenter")) {
ForEach(0 ..< datacenters_controller.datacenters.count) {
Text(self.datacenters_controller.datacenters[$0].location)
}
}
Swift complained with the following error:
ForEach<Range<Int>, Int, Text> count (4) != its initial count (0). `ForEach(_:content:)` should only be used for *constant* data. Instead conform data to `Identifiable` or use `ForEach(_:id:content:)` and provide an explicit `id`!
Then I switched to:
Picker(selection: $selectedDatacenter, label: Text("Datacenter")) {
ForEach(datacenters_controller.datacenters) { datacenter in
Text(datacenter.location)
}
}
It "works" (no error), but the result is not the expected because although I can select a datacenter, it is not "stored", not shown in the Picker as selected.
Actual result
Expected result
Any idea? What I'm doing wrong?
Here's a working example. The key is that selectedDatacenter needs to be the same type as Datacenter.id (in this case, String).
struct ContentView: View {
#ObservedObject var datacenters_controller = DatacentersController()
#State private var selectedDatacenter = ""
var body: some View {
NavigationView {
Form {
Picker(selection: $selectedDatacenter, label: Text("Datacenter")) {
ForEach(datacenters_controller.datacenters) { datacenter in
Text(datacenter.location)
}
}
// Just here for demonstration
Text("selectedDatacenter (id): \(selectedDatacenter.isEmpty ? "Nothing yet" : selectedDatacenter)")
}
}
}
}
Here's the supporting code
struct Datacenter:Codable, Hashable, Identifiable{
let id: String
var location: String
}
class DatacentersController: ObservableObject {
#Published var datacenters: [Datacenter] = []
init() {
datacenters = [
Datacenter(id: "ABQ", location: "Albuquerque"),
Datacenter(id: "BOS", location: "Boston"),
Datacenter(id: "COS", location: "Colorado Springs")
]
}
}
I think you are missing tag on your picker:
Picker(selection: $selectedDatacenter, label: Text("Datacenter")) {
ForEach(datacenters_controller.datacenters) {
Text($0.location).tag($0)
}
}
Apple docs on tag:
Sets the tag of the view, used for selecting from a list of View
options.
In your second attempt, you need to use the tag modifier (as described by LuLuGaGa). You also need to change the type of selectedDatacenter to match. For example:
struct ContentView: View {
init(_ controller: DatacentersController) {
self.datacenters_controller = controller
self._selectedDatacenter = State(initialValue: controller.datacenters[0].id)
}
var body: some View {
NavigationView {
Form {
Picker(selection: $selectedDatacenter, label: Text("Datacenter")) {
ForEach(datacenters_controller.datacenters) {
Text($0.location).tag($0)
}
}
}
}
}
#ObservedObject private var datacenters_controller: DatacentersController
#State private var selectedDatacenter: String
}