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())`
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
}
}
}
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)
}
}
Currently I have the code below which I'm trying to use as a navigation switch so I can navigate through different views without using the crappy NavigationLinks and otherwise. I'm by default a WebDev, so I've been having a mountain of issues transferring my knowledge over to Swift, the syntax feels completely dissimilar to any code I've written before. Anyways, here's the code;
import Foundation
import Combine
import SwiftUI
class ViewRouter: ObservableObject {
let objectWillChange: PassthroughSubject<ViewRouter,Never>
#Published var currentPage: String = "page1" {
didSet {
objectWillChange.send(self)
}
}
init(currentPage: String) {
self.currentPage = currentPage
}
}
As you can see, it's really simple, and I just use the object to switch values and display different views on another file, the only errors which prevent me from building it is the fact that the initializer is saying "Return from initializer without initializing all stored properties", even though the only variable is the currentPage variable which is defined. I know it's saying that objectWillChange is not defined by the message, but objectWillChange doesn't have any value to be assigned. Any help would be appreciated.
You just declare objectWillChange, but don't initialise it.
Simply change the declaration from
let objectWillChange: PassthroughSubject<ViewRouter,Never>
to
let objectWillChange = PassthroughSubject<ViewRouter,Never>()
However, using a PassthroughSubject shouldn't be necessary. currentPage is already #Published, so you can simply subscribe to its publisher. What you are trying to achieve using a PassthroughSubject and didSet is already defined by the swiftUI property wrappers, ObservableObject and Published.
class ViewRouter: ObservableObject {
#Published var currentPage: String
init(currentPage: String) {
self.currentPage = currentPage
}
}
Then you can simply do
let router = ViewRouter(currentPage: "a")
router.$currentPage.sink { page in print(page) }
router.currentPage = "b" // the above subscription prints `"b"`
I have a class, a “clock face” with regular updates; it should display an array of metrics that change over time.
Because I’d like the clock to also be displayed in a widget, I’ve found that I had to put the class into a framework (perhaps there’s another way, but I’m too far down the road now). This appears to have caused a problem with SwiftUI and observable objects.
In my View I have:
#ObservedObject var clockFace: myClock
In the clock face I have:
class myClock: ObservableObject, Identifiable {
var id: Int
#Publish public var metric:[metricObject] = []
....
// at some point the array is mutated and the display updates
}
I don’t know if Identifiable is needed but it’s doesn’t make any difference to the outcome. The public is demanded by the compiler, but it’s always been like that anyway.
With these lines I get a runtime error as the app starts:
objc[31175] no class for metaclass
So I took off the #Published and changed to a manual update:
public var metric:[metricObject] = [] {
didSet {
self.objectWillChange.send()`
}
}
And now I get a display and by setting a breakpoint I can see the send() is being called at regular intervals. But the display won’t update unless I add/remove from the array. I’m guessing the computed variables (which make up the bulk of the metricObject change isn’t being seen by SwiftUI. I’ve subsequently tried adding a “dummy” Int to the myClock class and setting that to a random value to trying to trigger a manual refresh via a send() on it’s didSet with no luck.
So how can I force a periodic redraw of the display?
What is MetricObject and can you make it a struct so you get Equatable for free?
When I do this with an Int it works:
class PeriodicUpdater: ObservableObject {
#Published var time = 0
var subscriptions = Set<AnyCancellable>()
init() {
Timer
.publish(every: 1, on: .main, in: .default)
.autoconnect()
.sink(receiveValue: { _ in
self.time = self.time + 1
})
.store(in: &subscriptions)
}
}
struct ContentView: View {
#ObservedObject var updater = PeriodicUpdater()
var body: some View {
Text("\(self.updater.time)")
}
}
So it's taken a while but I've finally got it working. The problem seemed to be two-fold.
I had a class defined in my framework which controls the SwiftUI file. This class is sub-classed in both the main app and the widget.
Firstly I couldn't use #Published in the main class within the framework. That seemed to cause the error:
objc[31175] no class for metaclass
So I used #JoshHomman's idea of an iVar that's periodically updated but that didn't quite work for me. With my SwiftUI file, I had:
struct FRMWRKShape: Shape {
func drawShape(in rect: CGRect) -> Path {
// draw and return a shape
}
}
struct ContentView: View {
#ObservedObject var updater = PeriodicUpdater()
var body: some View {
FRMWRKShape()
//....
FRMWRKShape() //slightly different parameters are passed in
}
}
The ContentView was executed every second as I wanted, however the FRMWRKShape code was called but not executed(?!) - except on first starting up - so the view doesn't update. When I changed to something far less D.R.Y. such as:
struct ContentView: View {
#ObservedObject var updater = PeriodicUpdater()
var body: some View {
Path { path in
// same code as was in FRMWRKShape()
}
//....
Path { path in
// same code as was in FRMWRKShape()
// but slightly different parameters
}
}
}
Magically, the View was updated as I wanted it to be. I don't know if this is expected behaviour, perhaps someone can say whether I should file a Radar....
In order to make my code testable, I'm trying to adhere to Liskov's substitution principle by having my SwiftUI views depend on protocols rather than concrete types. This allows me to easily swap implementations and allows me to easily build mocks for testing. Here's an example of what I'm trying to do:
protocol DashboardViewModel: ObservableObject {
var orders: [Order] { get }
}
My DashboardViewModel needs to communicate changes back to its dependents, so I've also attached ObservableObject as a transitive requirement.
This seems to be a problem. You cannot achieve LSP if you have associated type requirements. Here's the error I got from my SwiftUI view class that depended on my view model:
struct DashboardView: View {
#ObservedObject var viewModel: DashboardViewModel
}
Protocol 'DatastoreProtocol' can only be used as a generic constraint because it has Self or associated type requirements
I ended up doing this instead:
protocol DashboardViewModel {
var orders: [Order] { get }
var objectWillChange: AnyPublisher<Void, Never> { get }
}
This also requires dependents to do additional work to observe for state changes. This takes away the conveniences of using property wrappers - mainly the ability for dependents to observe for state changes using #ObservedObject. Using this alternative leads us to code like this:
struct DashboardView: View {
let viewModel: DashboardViewModel
var viewModelSubscriber: AnyCancellable!
// MARK: - Used only to force a re-render of this view
#State private var reload = false
init(viewModel: DashboardViewModel) {
self.viewModel = viewModel
viewModelSubscriber = viewModel.objectWillChange.sink { _ in
self.reload.toggle()
}
}
}
This is a tad distasteful to look at:
Created a #State variable that is only used to force an update of the view, because we cannot utilize SwiftUI property wrappers to observe for state changes
Created a AnyCancellable! variable to hold a subscription to objectWillChange from the view model. This is necessary to detect state changes from the DashboardViewModel
Added the subscription call in the initializer that only toggles the #State variable to force a retrieval of new data from the view model
I feel like there should be a much better way of handling this. Looking for help!
One way to solve this is to make your view generic:
protocol DashboardViewModel: ObservableObject {
var orders: [Order] { get }
}
struct DashboardView<Model: DashboardViewModel>: View {
#ObservedObject var viewModel: Model
}