I'm trying to bind properties together in the view and couldn't find anything better specifically for it. This is what I'm doing:
struct ContentView: View {
#ObservedObject var model: MyModel
#State var selectedID: Int
var body: some View {
Picker("Choose", selection: $selectedID) {
Text("Abc").tag(0)
Text("Def").tag(1)
Text("Ghi").tag(2)
}
.onChange(of: model.item?.selectedID) {
selectedID = $0
}
}
}
Is there a better way to bind properties together?
If it is about unidirectional flow then the only change I see needed in provided snapshot is make similar types. Everything else is ok.
struct ContentView: View {
#ObservedObject var model: MyModel
#State var selectedID: Int? // << make optional as well !!
...
Related
I am reading a FileDocument using SwiftUI. This document is available though a binding to the top-most view:
struct ContentView: View {
#Binding var document: SomeDocument
var body: some View {
Text(self.document.someString)
NestedChildViews().environmentObject(self.document)
}
}
Is there are a way to turn this #Binding of the document into an #EnvironmentObject to make it available to all nested views? Simply replacing #Binding with #EnvironmentObject will not work, since FileDocument is a struct.
Property type 'SomeDocument' does not match that of the 'wrappedValue' property of its wrapper type 'EnvironmentObject'
Creating an in-between ObservableObject that is created as a #StateObject inside ContentView won't work, since it has to be initiated before the Binding to document is available:
struct ContentView: View {
#Binding var document: SomeDocument
#StateObject var sharedState = SomeSharedState(document: self.document) // won't work
}
What would be the best way to handle this in-between a document and a series of nested views without passing bindings between all levels of them?
You should initialize your #StateObject from within the view's init, since you are passing in the document into the init:
struct ContentView: View {
#Binding var document: SomeDocument
#StateObject var sharedState: SomeSharedState
init(document: Binding<SomeDocument>) {
_document = document
_sharedState = StateObject(wrappedValue: SomeSharedState(document: document.projectedValue))
}
var body: some View {
Text(document.someString)
.environmentObject(sharedState)
/* TEST TO SHOW IT WORKS
.onAppear {
DispatchQueue.main.asyncAfter(deadline: .now() + 3) {
document = SomeDocument(someString: "New!")
}
}
*/
}
}
class SomeSharedState: ObservableObject {
#Published var document: Binding<SomeDocument>
init(document: Binding<SomeDocument>) {
_document = Published(initialValue: document)
}
}
In the code below (a stripped-down version of some code in a project) I'm using a MVVM pattern with two views:
ViewA - displays a value stored in an ObservableObject ViewModel;
ViewB - displays the same value and has a Slider that changes that value, which is passed to the view using Binding.
Inside of ViewModelA I have a computed property which serves both to avoid the View from accessing the Model directly and to perform some other operations when the value inside the model (the one being displayed) is changed.
I'm also passing that computed value to a ViewModelB, using Binding, which acts as a StateObject for ViewB. However, when dragging the Slider to change that value, the value changes on ViewA but doesn't change on ViewB and the slider itself doesn't slide. As expected, when debugging, the wrappedValue inside the Binding is not changing. But how is the change propagated upwards (through the Binding's setters, I imagine) but not downwards back to ViewB?? I imagine this can only happen if the variable is being duplicated somewhere and changed only in one place, but I can't seem to understand where or if that's what's actually happening.
Thanks in advance!
Views:
import SwiftUI
struct ContentView: View {
#StateObject var viewModelA = ViewModelA()
var body: some View {
VStack{
ViewA(value: viewModelA.value)
ViewB(value: $viewModelA.value)
}
}
}
struct ViewA: View {
let value: Double
var body: some View {
Text("\(value)").padding()
}
}
struct ViewB: View {
#StateObject var viewModelB: ViewModelB
init(value: Binding<Double>){
_viewModelB = StateObject(wrappedValue: ViewModelB(value: value))
}
var body: some View {
VStack{
Text("\(viewModelB.value)")
Slider(value: $viewModelB.value, in: 0...1)
}
}
}
ViewModels:
class ViewModelA: ObservableObject {
#Published var model = Model()
var value: Double {
get {
model.value
}
set {
model.value = newValue
// perform other checks and operations
}
}
}
class ViewModelB: ObservableObject {
#Binding var value: Double
init(value: Binding<Double>){
self._value = value
}
}
Model:
struct Model {
var value: Double = 0
}
If you only look where you can't go, you might just miss the riches below
Breaking single source of truth, and breaching local (private) property of #StateObjectby sharing it via Binding are two places where you can't go.
#EnvironmentObject or more generally the concept of "shared object" between views are the riches below.
This is an example of doing it without MVVM nonsense:
import SwiftUI
final class EnvState: ObservableObject {#Published var value: Double = 0 }
struct ContentView: View {
#EnvironmentObject var eos: EnvState
var body: some View {
VStack{
ViewA()
ViewB()
}
}
}
struct ViewA: View {
#EnvironmentObject var eos: EnvState
var body: some View {
Text("\(eos.value)").padding()
}
}
struct ViewB: View {
#EnvironmentObject var eos: EnvState
var body: some View {
VStack{
Text("\(eos.value)")
Slider(value: $eos.value, in: 0...1)
}
}
}
Isn't this easier to read, cleaner, less error-prone, with fewer overheads, and without serious violation of fundamental coding principles?
MVVM does not take value type into consideration. And the reason Swift introduces value type is so that you don't pass shared mutable references and create all kinds of bugs.
Yet the first thing MVVM devs do is to introduce shared mutable references for every view and pass references around via binding...
Now to your question:
the only options I see are either using only one ViewModel per Model, or having to pass the Model (or it's properties) between ViewModels through Binding
Another option is to drop MVVM, get rid of all view models, and use #EnvironmentObject instead.
Or if you don't want to drop MVVM, pass #ObservedObject (your view model being a reference type) instead of #Binding.
E.g.;
struct ContentView: View {
#ObservedObject var viewModelA = ViewModelA()
var body: some View {
VStack{
ViewA(value: viewModelA)
ViewB(value: viewModelA)
}
}
}
On a side note, what's the point of "don't access model directly from view"?
It makes zero sense when your model is value type.
Especially when you pass view model reference around like cookies in a party so everyone can have it.
Really it looks like broken single-source or truth concept. Instead the following just works (ViewModelB might probably be needed for something, but not for this case)
Tested with Xcode 12 / iOS 14
Only modified parts:
struct ContentView: View {
#StateObject var viewModelA = ViewModelA()
var body: some View {
VStack{
ViewA(value: viewModelA.value)
ViewB(value: $viewModelA.model.value)
}
}
}
struct ViewB: View {
#Binding var value: Double
var body: some View {
VStack{
Text("\(value)")
Slider(value: $value, in: 0...1)
}
}
}
I'm trying to understand Combine a little bit and have an issue and can't wrap my head around it:
I got this data-source
class NoteManager: ObservableObject, Identifiable {
#Published var notes: [Note] = []
var cancellable: AnyCancellable?
init() {
cancellable = $notes.sink(receiveCompletion: { completion in
print("receiveCompletion \(completion)")
}, receiveValue: { notes in
print("receiveValue \(notes)")
})
}
}
which is used here:
struct ContentView: View {
#State var noteManager: NoteManager
var body: some View {
VStack {
NavigationView {
VStack {
List {
ForEach(noteManager.notes) { note in
NoteCell(note: note)
}
}
...
And I can change the values here:
struct NoteCell: View {
#State var note: Note
var body: some View {
NavigationLink(destination: TextField("title", text: $note.title)
...
Anyways - I'm not receiving the receiveValue event after changing the value (which is also correctly reflected in the ui). receiveValue is only called initially when setting it - is there some other way to receive an event for updated fields?
Add on:
struct Note: Identifiable, Codable {
var id = UUID()
var title: String
var information: String
}
make manager observed object (as it is design pair for observable)
struct ContentView: View {
#ObservedObject var noteManager: NoteManager
make cell editing original note via binding (as note is struct)
struct NoteCell: View {
#Binding var note: Note
transfer note by projected value directly from manager (single source of truth) to cell (editor)
ForEach(Array(noteManager.notes.enumerated()), id: \.element) { i, note in
NoteCell(note: $noteManager.notes[i])
}
Problem
In Order to achieve a clean look and feel of the App's code, I create ViewModels for every View that contains logic.
A normal ViewModel looks a bit like this:
class SomeViewModel: ObservableObject {
#Published var state = 1
// Logic and calls of Business Logic goes here
}
and is used like so:
struct SomeView: View {
#ObservedObject var viewModel = SomeViewModel()
var body: some View {
// Code to read and write the State goes here
}
}
This workes fine when the Views Parent is not being updated. If the parent's state changes, this View gets redrawn (pretty normal in a declarative Framework). But also the ViewModel gets recreated and does not hold the State afterward. This is unusual when you compare to other Frameworks (eg: Flutter).
In my opinion, the ViewModel should stay, or the State should persist.
If I replace the ViewModel with a #State Property and use the int (in this example) directly it stays persisted and does not get recreated:
struct SomeView: View {
#State var state = 1
var body: some View {
// Code to read and write the State goes here
}
}
This does obviously not work for more complex States. And if I set a class for #State (like the ViewModel) more and more Things are not working as expected.
Question
Is there a way of not recreating the ViewModel every time?
Is there a way of replicating the #State Propertywrapper for #ObservedObject?
Why is #State keeping the State over the redraw?
I know that usually, it is bad practice to create a ViewModel in an inner View but this behavior can be replicated by using a NavigationLink or Sheet.
Sometimes it is then just not useful to keep the State in the ParentsViewModel and work with bindings when you think of a very complex TableView, where the Cells themself contain a lot of logic.
There is always a workaround for individual cases, but I think it would be way easier if the ViewModel would not be recreated.
Duplicate Question
I know there are a lot of questions out there talking about this issue, all talking about very specific use-cases. Here I want to talk about the general problem, without going too deep into custom solutions.
Edit (adding more detailed Example)
When having a State-changing ParentView, like a list coming from a Database, API, or cache (think about something simple). Via a NavigationLink you might reach a Detail-Page where you can modify the Data. By changing the data the reactive/declarative Pattern would tell us to also update the ListView, which would then "redraw" the NavigationLink, which would then lead to a recreation of the ViewModel.
I know I could store the ViewModel in the ParentView / ParentView's ViewModel, but this is the wrong way of doing it IMO. And since subscriptions are destroyed and/or recreated - there might be some side effects.
Finally, there is a Solution provided by Apple: #StateObject.
By replacing #ObservedObject with #StateObject everything mentioned in my initial post is working.
Unfortunately, this is only available in ios 14+.
This is my Code from Xcode 12 Beta (Published June 23, 2020)
struct ContentView: View {
#State var title = 0
var body: some View {
NavigationView {
VStack {
Button("Test") {
self.title = Int.random(in: 0...1000)
}
TestView1()
TestView2()
}
.navigationTitle("\(self.title)")
}
}
}
struct TestView1: View {
#ObservedObject var model = ViewModel()
var body: some View {
VStack {
Button("Test1: \(self.model.title)") {
self.model.title += 1
}
}
}
}
class ViewModel: ObservableObject {
#Published var title = 0
}
struct TestView2: View {
#StateObject var model = ViewModel()
var body: some View {
VStack {
Button("StateObject: \(self.model.title)") {
self.model.title += 1
}
}
}
}
As you can see, the StateObject Keeps it value upon the redraw of the Parent View, while the ObservedObject is being reset.
I agree with you, I think this is one of many major problems with SwiftUI. Here's what I find myself doing, as gross as it is.
struct MyView: View {
#State var viewModel = MyViewModel()
var body : some View {
MyViewImpl(viewModel: viewModel)
}
}
fileprivate MyViewImpl : View {
#ObservedObject var viewModel : MyViewModel
var body : some View {
...
}
}
You can either construct the view model in place or pass it in, and it gets you a view that will maintain your ObservableObject across reconstruction.
Is there a way of not recreating the ViewModel every time?
Yes, keep ViewModel instance outside of SomeView and inject via constructor
struct SomeView: View {
#ObservedObject var viewModel: SomeViewModel // << only declaration
Is there a way of replicating the #State Propertywrapper for #ObservedObject?
No needs. #ObservedObject is-a already DynamicProperty similarly to #State
Why is #State keeping the State over the redraw?
Because it keeps its storage, ie. wrapped value, outside of view. (so, see first above again)
You need to provide custom PassThroughSubject in your ObservableObject class. Look at this code:
//
// Created by Франчук Андрей on 08.05.2020.
// Copyright © 2020 Франчук Андрей. All rights reserved.
//
import SwiftUI
import Combine
struct TextChanger{
var textChanged = PassthroughSubject<String,Never>()
public func changeText(newValue: String){
textChanged.send(newValue)
}
}
class ComplexState: ObservableObject{
var objectWillChange = ObservableObjectPublisher()
let textChangeListener = TextChanger()
var text: String = ""
{
willSet{
objectWillChange.send()
self.textChangeListener.changeText(newValue: newValue)
}
}
}
struct CustomState: View {
#State private var text: String = ""
let textChangeListener: TextChanger
init(textChangeListener: TextChanger){
self.textChangeListener = textChangeListener
print("did init")
}
var body: some View {
Text(text)
.onReceive(textChangeListener.textChanged){newValue in
self.text = newValue
}
}
}
struct CustomStateContainer: View {
//#ObservedObject var state = ComplexState()
var state = ComplexState()
var body: some View {
VStack{
HStack{
Text("custom state View: ")
CustomState(textChangeListener: state.textChangeListener)
}
HStack{
Text("ordinary Text View: ")
Text(state.text)
}
HStack{
Text("text input: ")
TextInput().environmentObject(state)
}
}
}
}
struct TextInput: View {
#EnvironmentObject var state: ComplexState
var body: some View {
TextField("input", text: $state.text)
}
}
struct CustomState_Previews: PreviewProvider {
static var previews: some View {
return CustomStateContainer()
}
}
First, I using TextChanger to pass new value of .text to .onReceive(...) in CustomState View. Note, that onReceive in this case gets PassthroughSubject, not the ObservableObjectPublisher. In last case you will have only Publisher.Output in perform: closure, not the NewValue. state.text in that case would have old value.
Second, look at the ComplexState class. I made an objectWillChange property to make text changes send notification to subscribers manually. Its almost the same like #Published wrapper do. But, when the text changing it will send both, and objectWillChange.send() and textChanged.send(newValue). This makes you be able to choose in exact View, how to react on state changing. If you want ordinary behavior, just put the state into #ObservedObject wrapper in CustomStateContainer View. Then, you will have all the views recreated and this section will get updated values too:
HStack{
Text("ordinary Text View: ")
Text(state.text)
}
If you don't want all of them to be recreated, just remove #ObservedObject. Ordinary text View will stop updating, but CustomState will. With no recreating.
update:
If you want more control, you can decide while changing the value, who do you want to inform about that change.
Check more complex code:
//
//
// Created by Франчук Андрей on 08.05.2020.
// Copyright © 2020 Франчук Андрей. All rights reserved.
//
import SwiftUI
import Combine
struct TextChanger{
// var objectWillChange: ObservableObjectPublisher
// #Published
var textChanged = PassthroughSubject<String,Never>()
public func changeText(newValue: String){
textChanged.send(newValue)
}
}
class ComplexState: ObservableObject{
var onlyPassthroughSend = false
var objectWillChange = ObservableObjectPublisher()
let textChangeListener = TextChanger()
var text: String = ""
{
willSet{
if !onlyPassthroughSend{
objectWillChange.send()
}
self.textChangeListener.changeText(newValue: newValue)
}
}
}
struct CustomState: View {
#State private var text: String = ""
let textChangeListener: TextChanger
init(textChangeListener: TextChanger){
self.textChangeListener = textChangeListener
print("did init")
}
var body: some View {
Text(text)
.onReceive(textChangeListener.textChanged){newValue in
self.text = newValue
}
}
}
struct CustomStateContainer: View {
//var state = ComplexState()
#ObservedObject var state = ComplexState()
var body: some View {
VStack{
HStack{
Text("custom state View: ")
CustomState(textChangeListener: state.textChangeListener)
}
HStack{
Text("ordinary Text View: ")
Text(state.text)
}
HStack{
Text("text input with full state update: ")
TextInput().environmentObject(state)
}
HStack{
Text("text input with no full state update: ")
TextInputNoUpdate().environmentObject(state)
}
}
}
}
struct TextInputNoUpdate: View {
#EnvironmentObject var state: ComplexState
var body: some View {
TextField("input", text: Binding( get: {self.state.text},
set: {newValue in
self.state.onlyPassthroughSend.toggle()
self.state.text = newValue
self.state.onlyPassthroughSend.toggle()
}
))
}
}
struct TextInput: View {
#State private var text: String = ""
#EnvironmentObject var state: ComplexState
var body: some View {
TextField("input", text: Binding(
get: {self.text},
set: {newValue in
self.state.text = newValue
// self.text = newValue
}
))
.onAppear(){
self.text = self.state.text
}.onReceive(state.textChangeListener.textChanged){newValue in
self.text = newValue
}
}
}
struct CustomState_Previews: PreviewProvider {
static var previews: some View {
return CustomStateContainer()
}
}
I made a manual Binding to stop broadcasting objectWillChange. But you still need to gets new value in all the places you changing this value to stay synchronized. Thats why I modified TextInput too.
Is that what you needed?
My solution is use EnvironmentObject and don't use ObservedObject at view it's viewModel will be reset, you pass through hierarchy by
.environmentObject(viewModel)
Just init viewModel somewhere it will not be reset(example root view).
Aim:
I have a model which is an ObservableObject. It has a Bool property, I would like to use this Bool property to initialise a #Binding variable.
Questions:
How to convert an #ObservableObject to a #Binding ?
Is creating a #State the only way to initialise a #Binding ?
Note:
I do understand I can make use of #ObservedObject / #EnvironmentObject, and I see it's usefulness, but I am not sure a simple button needs to have access to the entire model.
Or is my understanding incorrect ?
Code:
import SwiftUI
import Combine
import SwiftUI
import PlaygroundSupport
class Car : ObservableObject {
#Published var isReadyForSale = true
}
struct SaleButton : View {
#Binding var isOn : Bool
var body: some View {
Button(action: {
self.isOn.toggle()
}) {
Text(isOn ? "On" : "Off")
}
}
}
let car = Car()
//How to convert an ObservableObject to a Binding
//Is creating an ObservedObject or EnvironmentObject the only way to handle a Observable Object ?
let button = SaleButton(isOn: car.isReadyForSale) //Throws a compilation error and rightly so, but how to pass it as a Binding variable ?
PlaygroundPage.current.setLiveView(button)
Binding variables can be created in the following ways:
#State variable's projected value provides a Binding<Value>
#ObservedObject variable's projected value provides a wrapper from which you can get the Binding<Subject> for all of it's properties
Point 2 applies to #EnvironmentObject as well.
You can create a Binding variable by passing closures for getter and setter as shown below:
let button = SaleButton(isOn: .init(get: { car.isReadyForSale },
set: { car.isReadyForSale = $0} ))
Note:
As #nayem has pointed out you need #State / #ObservedObject / #EnvironmentObject / #StateObject (added in SwiftUI 2.0) in the view for SwiftUI to detect changes automatically.
Projected values can be accessed conveniently by using $ prefix.
You have several options to observe the ObservableObject. If you want to be in sync with the state of the object, it's inevitable to observe the state of the stateful object. From the options, the most commons are:
#State
#ObservedObject
#EnvironmentObject
It is upto you, which one suits your use case.
No. But you need to have an object which can be observed of any change made to that object in any point in time.
In reality, you will have something like this:
class Car: ObservableObject {
#Published var isReadyForSale = true
}
struct ContentView: View {
// It's upto you whether you want to have other type
// such as #State or #ObservedObject
#EnvironmentObject var car: Car
var body: some View {
SaleButton(isOn: $car.isReadyForSale)
}
}
struct SaleButton: View {
#Binding var isOn: Bool
var body: some View {
Button(action: {
self.isOn.toggle()
}) {
Text(isOn ? "Off" : "On")
}
}
}
If you are ready for the #EnvironmentObject you will initialize your view with:
let contentView = ContentView().environmentObject(Car())
struct ContentView: View {
#EnvironmentObject var car: Car
var body: some View {
SaleButton(isOn: self.$car.isReadyForSale)
}
}
class Car: ObservableObject {
#Published var isReadyForSale = true
}
struct SaleButton: View {
#Binding var isOn: Bool
var body: some View {
Button(action: {
self.isOn.toggle()
}) {
Text(isOn ? "On" : "Off")
}
}
}
Ensure you have the following in your SceneDelegate:
// Create the SwiftUI view that provides the window contents.
let contentView = ContentView()
.environmentObject(Car())
In my case i used .constant(viewModel) to pass viewModel to ListView #Binding var viewModel
Example
struct CoursesView: View {
#StateObject var viewModel = CoursesViewModel()
var body: some View {
ZStack {
ListView(viewModel: .constant(viewModel))
ProgressView().opacity(viewModel.isShowing)
}
}
}
struct ListView: View {
#Binding var viewModel: CoursesViewModel
var body: some View {
List {
ForEach(viewModel.courses, id: \.id) { course in
Text(couse.imageUrl)
}
}
}
}