How to add an observable property when other properties change - swiftui

Question

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.

Answer 1

#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)).

Answer 2

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.