As seen in the handling user input tutorial.
struct LandmarkList: View {
#State var showFavoritesOnly = true
var body: some View {
NavigationView {
List {
Toggle(isOn: $showFavoritesOnly) {
Text("Favorites only")
}
...
What is the showFavoritesOnly / $showFavoritesOnly syntax ?
Is it something unique to Binding<T> or can we use it in our own code ?
#State is designed to be used as a binding for SwiftUI properties. Any access to it outside the body accessor of your View will crash with:
Thread 1: Fatal error: Accessing State<Bool> outside View.body
SwiftUI automatically tracks all the #State declarations and re-calculates the appropriate body whenever any of them change.
#State is implemented using the Swift 5.1 #propertyDelegate feature, which enables the storage behavior of properties to be customized.
Related
Update:
This question is already solved (see responses below). The correct way to do this is to get your Binding by projecting the
ObservableObject For example, $options.refreshRate.
TLDR version:
How do I get a SwiftUI Picker (or other API that relies on a local Binding) to immediately update my ObservedObject/EnvironmentObject. Here is more context...
The scenario:
Here is something I consistently need to do in every SwiftUI app I create...
I always make some class that stores any user preference (let's call this class Options and I make it an ObservableObject.
Any setting that needs to be consumed is marked with #Published
Any view that consumes this brings it in as a #ObservedObject or #EnvironmentObject and subscribes to changes.
This all works quite nicely. The trouble I always face is how to set this from the UI. From the UI, here is usually what I'm doing (and this should all sound quite normal):
I have some SwiftUI view like OptionsPanel that drives the Options class above and allows the user to choose their options.
Let's say we have some option defined by an enum:
enum RefreshRate {
case low, medium, high
}
Naturally, I'd choose a Picker in SwiftUI to set this... and the Picker API requires that my selection param be a Binding. This is where I find the issue...
The issue:
To make the Picker work, I usually have some local Binding that is used for this purpose. But, ultimately, I don't care about that local value. What I care about is immediately and instantaneously broadcasting that new value to the rest of the app. The moment I select a new refresh rate, I'd like immediately know that instant about the change. The ObservableObject (the Options class) object does this quite nicely. But, I'm just updating a local Binding. What I need to figure out is how to immediately translate the Picker's state to the ObservableObject every time it's changed.
I have a solution that works... but I don't like it. Here is my non-ideal solution:
The non-ideal solution:
The first part of the solution is quite actually fine, but runs into a snag...
Within my SwiftUI view, rather than do the simplest way to set a Binding with #State I can use an alternate initializer...
// Rather than this...
#ObservedObject var options: Options
#State var refreshRate: RefreshRate = .medium
// Do this...
#ObservedObject var options: Options
var refreshRate: Binding<RefreshRate>(
get: { self.options.refreshRate },
set: { self.options.refreshRate = $0 }
)
So far, this is great (in theory)! Now, my local Binding is directly linked to the ObservableObject. All changes to the Picker are immediately broadcast to the entire app.
But this doesn't actually work. And this is where I have to do something very messy and non-ideal to get it to work.
The code above produces the following error:
Cannot use instance member 'options' within property initializer; property initializers run before 'self' is available
Here my my (bad) workaround. It works, but it's awful...
The Options class provides a shared instance as a static property. So, in my options panel view, I do this:
#ObservedObject var options: Options = .shared // <-- This is still needed to tell SwiftUI to listen for updates
var refreshRate: Binding<RefreshRate>(
get: { Options.shared.refreshRate },
set: { Options.shared.refreshRate = $0 }
)
In practice, this actually kinda works in this case. I don't really need to have multiple instances... just that one. So, as long as I always reference that shared instance, everything works. But it doesn't feel well architected.
So... does anyone have a better solution? This seems like a scenario EVERY app on the face of the planet has to tackle, so it seems like someone must have a better way.
(I am aware some use an .onDisapear to sync local state to the ObservedObject but this isn't ideal either. This is non-ideal because I value having immediate updates for the rest of the app.)
The good news is you're trying way, way, way too hard.
The ObservedObject property wrapper can create this Binding for you. All you need to say is $options.refreshRate.
Here's a test playground for you to try out:
import SwiftUI
enum RefreshRate {
case low, medium, high
}
class Options: ObservableObject {
#Published var refreshRate = RefreshRate.medium
}
struct RefreshRateEditor: View {
#ObservedObject var options: Options
var body: some View {
// vvvvvvvvvvvvvvvvvvvv
Picker("Refresh Rate", selection: $options.refreshRate) {
// ^^^^^^^^^^^^^^^^^^^^
Text("Low").tag(RefreshRate.low)
Text("Medium").tag(RefreshRate.medium)
Text("High").tag(RefreshRate.high)
}
.pickerStyle(.segmented)
}
}
struct ContentView: View {
#StateObject var options = Options()
var body: some View {
VStack {
RefreshRateEditor(options: options)
Text("Refresh rate: \(options.refreshRate)" as String)
}
.padding()
}
}
import PlaygroundSupport
PlaygroundPage.current.setLiveView(ContentView())
It's also worth noting that if you want to create a custom Binding, the code you wrote almost works. Just change it to be a computed property instead of a stored property:
var refreshRate: Binding<RefreshRate> {
.init(
get: { self.options.refreshRate },
set: { self.options.refreshRate = $0 }
)
}
If I understand your question correctly, you want
to Set a Published value in an ObservableObject from the UI (Picker, etc.) in SwiftUI.
There are many ways to do that, I suggest you use a ObservableObject class, and use it directly wherever you need a binding in a view, such as in a Picker.
The following example code shows one way of setting up your code to do that:
import Foundation
import SwiftUI
// declare your ObservableObject class
class Options: ObservableObject {
#Published var name = "Mickey"
}
struct ContentView: View {
#StateObject var optionModel = Options() // <-- initialise the model
let selectionSet = ["Mickey", "Mouse", "Goofy", "Donald"]
#State var showSheet = false
var body: some View {
VStack {
Text(optionModel.name).foregroundColor(.red)
Picker("names", selection: $optionModel.name) { // <-- use the model directly as a $binding
ForEach (selectionSet, id: \.self) { value in
Text(value).tag(value)
}
}
Button("Show other view") { showSheet = true }
}
.sheet(isPresented: $showSheet) {
SheetView(optionModel: optionModel) // <-- pass the model to other view, see also #EnvironmentObject
}
}
}
struct SheetView: View {
#ObservedObject var optionModel: Options // <-- receive the model
var body: some View {
VStack {
Text(optionModel.name).foregroundColor(.green) // <-- show updated value
}
}
}
If you really want to have a "useless" intermediate local variable, then use this approach:
struct ContentView: View {
#StateObject var optionModel = Options() // <-- initialise the model
let selectionSet = ["Mickey", "Mouse", "Goofy", "Donald"]
#State var showSheet = false
#State var localVar = "" // <-- the local var
var body: some View {
VStack {
Text(optionModel.name).foregroundColor(.red)
Picker("names", selection: $localVar) { // <-- using the localVar
ForEach (selectionSet, id: \.self) { value in
Text(value).tag(value)
}
}
.onChange(of: localVar) { newValue in
optionModel.name = newValue // <-- update the model
}
Button("Show other view") { showSheet = true }
}
.sheet(isPresented: $showSheet) {
SheetView(optionModel: optionModel) // <-- pass the model to other view, see also #EnvironmentObject
}
}
}
Have the following situation. I have a view model that is an observable object with a computed property of type Bool. I want to be able to enable/disable a navigation link based on the computed property, but I need a binding to do so. Here a simplified example:
struct Project {
var name: String
var duration: Int
}
class MyViewModel: Observable Object {
#Published var project: Project
var isProjectValid: Bool {
return project.name != "" && project.duration > 0
}
}
struct MyView: View {
#EnvironmentObject var myVM: MyViewModel
var body: some View {
...
NavigationLink("Click Link", isActive: ?????, destination: NextView())
...
}
}
Since isActive expects a binding I am not able to access computed property such as myVM.isProjectValid. Tried also with the computed property in project class, still same problem.
Also considered creating custom binding related to the computed property, but not quite sure if/how to go about it.
First question I have posted, so if I am missing some details please be kind :)
Make it a #Published property and update it when project is changed
class MyViewModel: ObservableObject {
#Published var project: Project {
didSet {
isProjectValid = project.name != "" && project.duration > 0
}
}
#Published var isProjectValid: Bool
//...
}
The use of the computed property suggests some type of design where the user is not supposed to trigger the NavigationLink directly. But instead the NavigationLink is expected to be triggered programatically as a side-effect of some other mechanism elsewhere in the code. Such as might be done at the completion of a form or similar process by the user.
Not 100% if this is what's being aimed for, but if it is, then one option would be to pass a constant Binding to the NavigationLink, e.g.
NavigationLink("Click Link", isActive: .constant(myVM.isProjectValid), destination: NextView())`
Hi everyone I have a question about #State vs #ObservableObject with SwiftUI
I have a view that contains a LazyHGrid
To have a custom cell of the LazyHGrid I preferred to create a new struct with the custom cell.
The view hierarchy is composed as follows:
struct View1 -> struct LazyHGrid -> struct LazyHGridCustomCell
In View1 I have a text that must be replaced with content of the LazyHGridCustomCell every time it is selected.
At this point in view of my hierarchy should I use #State & #Binding to update the text or would it be better #ObservableObject?
In case I wanted to use the #State wrapper I would find myself like this:
struct View1 (#State)
struct LazyHGrid (#Binding)
struct LazyHGridCustomCell (#Binding)
I was wondering if this is the right way or consider #ObservableObject
I created a code example based on my question .. It was created just to let you understand what I mean to avoid being misunderstood
I was wondering if it is right to create such a situation or use an #ObservableObject
In case this path is wrong can you show me an example of the right way to go to get the correct result?
Thanks for suggestion
struct View1: View {
#State private var name: String
var body: some View {
Text(name)
LazyHGridView(name: $name)
}
}
struct LazyHGridView: View {
#Binding var name: String
var body: some View {
LazyHGrid(rows: Array(repeating: GridItem(), count: 2)) {
ForEach(reservationTimeItems) { item in
LazyHGridCustomCell(name: $name)
}
}
}
}
struct LazyHGridCustomCell: View {
#Binding var name: String
var body: some View {
Text(name)
.foregroundColor(.white)
}
}
According to Data Essentials in SwiftUI (WWDC 2020) at 9:46, you should be using State because ObservableObject is for model data.
State is designed for transient UI state that is local to a view. In
this section, I want to move your attention to designing your model
and explain all the tools that SwiftUI provides to you. Typically, in
your app, you store and process data by using a data model that is
separate from its UI. This is when you reach a critical point where
you need to manage the life cycle of your data, including persisting
and syncing it, handle side-effects, and, more generally, integrate it
with existing components. This is when you should use
ObservableObject. First, let's take a look at how ObservableObject is
defined.
I have an ObservedObject AppStatus class that has multiple Published classes inside itself. If I have only level in terms of children, everything is working great.
The problem comes when I have a class RecordingTimeManager that has another variable inside (2-levels of children). The variable maxRecordingTime is changing properly when I press the button, it prints '15 fifteenSeconds' but the foregroundColor is not triggering the change. I am not sure if this is a SwiftUI bug or I should structure the relationships in another way:
// AppStatus
// Recording
#Published var recordingTimeManager: RecordingTimeManager = RecordingTimeManager()
// RecordingTimeManager
class RecordingTimeManager {
#Published var maxRecordingTime: TimeSeletedTime = .sixteenSeconds
...
// SwiftUI component that would need to change the opacity based on a maxRecordingTime change (.foregroundColor is not changing)
Button {
appStatus.recordingTimeManager.maxRecordingTime = .fifteenSeconds
print("15 \(appStatus.recordingTimeManager.maxRecordingTime)")
} label: {
Text("15")
.font(Font.custom("BwGradual-Bold", size: 15))
.foregroundColor(appStatus.recordingTimeManager.maxRecordingTime == .fifteenSeconds ? CLAPSOFFWHITE : TRIBESGREY)
}
Many Thanks,
Observable objects don't just work when you have multiple levels of classes. The #Published property wrapper notifies the view when the property has changed, but because this property is a class - a reference type - it doesn't actually change when you change one of its properties. In other words, the reference remains the same.
And the inner #Published doesn't do anything because there's nothing directly observing it (even if RecordingTimeManager conformed to an ObservableObject)
So, you either need to make RecordingTimeManager a value-type - a struct:
struct RecordingTimeManager {
var maxRecordingTime: TimeSeletedTime = .sixteenSeconds
}
Or, if it must be a class (perhaps because it has some internal state and life cycle), then you could create an inner view that directly observes it.
First, it needs to be an ObservableObject:
class RecordingTimeManager: ObservableObject {
#Published var maxRecordingTime: TimeSeletedTime = .sixteenSeconds
and then create a view (could be a private internal view) that observes it:
struct MainView: View {
#StateObject var appStatus: AppStatus = .init()
private struct InnerView: View {
#ObservedObject var recordingManager: RecordingTimeManager
var body: some View {
Button {
recordingManager.maxRecordingTime = .fifteenSeconds
} label: {
Text("15")
.font(Font.custom("BwGradual-Bold", size: 15))
.foregroundColor(
recordingManager.maxRecordingTime == .fifteenSeconds
? CLAPSOFFWHITE : TRIBESGREY)
}
}
}
var body: some View {
InnerView(recordingManager: appStatus.recordingTimeManager)
}
}
If I pass a binding to another view can that 2nd view then pass the binding on and have the third view change the values in the first view or can this cause unexpected behavior?
For instance, if I have
struct FirstView: View {
#State var input: String = ""
var body: some View {
Form {
CustomTextField("Placeholder", $input)
}
}
}
struct CustomTextField: View {
#Binding var text: String
var body: some View {
ThirdView(text: $text)
}
}
struct ThirdView: View {
#Binding var text: String
var body: some View {
TextField("Result", $text)
}
}
I know the above is nonsensical - I'm only using it for demonstration purposes - but would the ThirdView properly update the state of the first?
I have had instances where it works fine and others where it doesn't but can't really find much of an explanation.
If I pass a binding to another view can that 2nd view then pass the binding on and have the third view change the values in the first view or can this cause unexpected behavior?
By my observation there are following variants:
1) If those views live in one view hierarchy (or one in another) then such combination work
2) If those views are in different view hierarchies (or views are replaced, eg. by navigation) then binding works only on one level, but deeper transfer has unexpected behavior (most usual defect is that intermediate views are not updated).
Code in question works (tested with Xcode 11.4 / iOS 13.4), because fits variant 1.