Why doesn't code in a body property of a View run each time an #State variable of its parent View changes? - swift

Question

I wish to run the function calculateBedtime() when the app first loads, and each time any of the #State variables of ContentView change, so that an updated bedtime is displayed constantly at the bottom of the app in the lowermost Section. However, the app acts as if variable bedtime just keeps its initial value all the time and never changes.
What I am expecting to happen is that when I change any #State variable, say using the DatePicker to change wakeUp, the body property is reinvoked, the first line of which is a call to calculateBedtime(), and so this function runs and updates bedtime as frequently as I want it to.
import SwiftUI
struct ContentView: View {
#State private var wakeUp = defaultWakeTime
#State private var bedtime = ""
#State private var sleepAmount = 8.0
#State private var coffeeAmount = 1
#State private var alertTitle = ""
#State private var alertMessage = ""
#State private var showingAlert = false
var body: some View {
bedtime = calculateBedtime()
return NavigationView
{
Form
{
Section(header: Text("When do you want to wake up?").font(.headline))
{
Text("When do you want to wake up?")
.font(.headline)
DatePicker("Please enter a time", selection: $wakeUp, displayedComponents: .hourAndMinute)
.labelsHidden()
.datePickerStyle(WheelDatePickerStyle())
}
Section(header: Text("Desired amount of sleep")
.font(.headline))
{
Stepper(value: $sleepAmount, in: 4...12, step: 0.25)
{
Text("\(sleepAmount, specifier: "%g") hours")
}
}
Section(header: Text("Daily coffee intake")
.font(.headline))
{
Picker("\(coffeeAmount+1) cup(s)", selection: $coffeeAmount)
{
ForEach(1..<21)
{ num in
if num==1
{
Text("\(num) cup")
}
else
{
Text("\(num) cups")
}
}
}
.pickerStyle(MenuPickerStyle())
}
Section(header: Text("Your Ideal Bedtime")
.font(.headline))
{
Text("\(bedtime)")
.font(.largeTitle)
}
}
.navigationBarTitle("BetterRest")
}
/*.onAppear(perform: {
calculateBedtime()
})
.onChange(of: wakeUp, perform: { value in
calculateBedtime()
})
.onChange(of: sleepAmount, perform: { value in
calculateBedtime()
})
.onChange(of: coffeeAmount, perform: { value in
calculateBedtime()
})*/
}
static var defaultWakeTime: Date
{
var components = DateComponents()
components.hour = 7
components.minute = 0
return Calendar.current.date(from: components) ?? Date()
}
func calculateBedtime() -> String
{
let model = SleepCalculator()
let components = Calendar.current.dateComponents([.hour, .minute], from: wakeUp)
let hour = (components.hour ?? 0) * 60 * 60
let minute = (components.minute ?? 0) * 60
var sleepTime = ContentView.defaultWakeTime
do
{
let prediction = try
model.prediction(wake: Double(hour + minute), estimatedSleep: sleepAmount, coffee: Double(coffeeAmount))
sleepTime = wakeUp - prediction.actualSleep
let formatter = DateFormatter()
formatter.timeStyle = .short
alertMessage = formatter.string(from: sleepTime)
alertTitle = "Your ideal bedtime is..."
} catch {
alertTitle = "Error"
alertMessage = "Sorry, there was a problem calculating your bedtime."
}
showingAlert = true
return alertMessage
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
What is the problem here? I am new to SwiftUI and feel that I must have a crucial misunderstanding of how the #State wrapper works. And what would be a good way to get the behavior I desire?

Answer 1

#State variables can only be mutated from within the body of your view and methods invoked by it; for anything else, you need to use ObservableObject which I think will solve your problem here.
You should only access a state property from inside the view’s body, or from methods called by it. For this reason, declare your state properties as private, to prevent clients of your view from accessing them. It is safe to mutate state properties from any thread.
More or less the scaffolding of the code below should achieve the results you want:
class SleepTimerViewModel: ObservableObject {
#Published public var bedTimeMessage: String?
}
struct ContentView: View {
#ObservedObject public var sleepTimerViewModel: SleepTimerViewModel
var body: some View {
Text(sleepTimerViewModel.bedTimeMessage)
}
public func updateBedTimeMessage() {
sleepTimerViewModel.bedTimeMessage = "Hello World"
}
}
I do think it's kind of annoying that Swift just don't care to let you know that you're updating a #State variable incorrectly. It just silently ignores the value you're trying to set, which is super annoying!