I'm coming from a C#/.NET background and my instinct was to look for an onChange event handler for the DatePicker, Stepper and Picker controls but I couldn't find one., I found one, but I am completely stymied in my attempted use of it. It takes two parameters, "of" and "perform". The "of" is no problem, it's just the relevant variable I'm checking, but it will not accept calculateBedtime for the "perform" parameter. It complains that it cannot convert value of type '()' to equatable. It is not clear to me why it would insist that my function take an equatable as a parameter.

It then occurred to me that I could use didSet with the wakeUp, sleepAmount and coffeeAmount @State variables, however, while it compiles fine it doesn't work. Nothing is recalculated/updated unless I tap the Calculate button.

How can I get the calculateBedtime() function to fire whenever wakeUp, sleepAmount or coffeeAmount are changed?

I got it to work, but I don't like the solution. And the question still remains as to why the didSets didn't work. Anyway, I had to supply an unused dummy parameter to the closure.

Section {
      DatePicker("Please enter a time", selection: $wakeUp, displayedComponents: .hourAndMinute)
          .labelsHidden()
  } header: {
      Text("When do you want to wake up?")
          .font(.subheadline)
  }.onChange(of: wakeUp) { bahTimApple in
      calculateBedtime() }

  Section {
      Stepper("\(sleepAmount.formatted()) hours", value: $sleepAmount, in: 4...12, step: 0.25)
  } header: {
      Text("Desired amount of sleep")
          .font(.subheadline)
  }.onChange(of: sleepAmount) { whyDoYouForceMe in
      calculateBedtime()
  }

  Section {
      Picker("Number of cups", selection: $coffeeAmount) {
          ForEach(1...20, id: \.self) {
              Text("\($0)")
          }
      }
  } header: {
      Text("Daily coffee intake")
          .font(.subheadline)
  }.onChange(of: coffeeAmount) { toUseThrowAwayParams in
      calculateBedtime()
  }

Thanks for the explanation. Your idea about using a computed property worked great, much cleaner that way. Understood about @State and didSet, will remember that going forward.

One last thing that you might be able to clear up for me. When I'm populating the Picker I have to put quotes around $0 or it complains "No exact matches in call to initializer".

                Picker("Number of cups", selection: $coffeeAmount) {
                    ForEach(1...20, id: \.self) {
                        Text("\($0)")
                        //Text($0)  This doesn't work
                    }
                }

Any idea what the problem is?

If you examine the defintion of Text you will see that it requires that the parameter initializer needs to follow the StringProtocol.

public init<S>(_ content: S) where S : StringProtocol

The anonymous parameter $0 does not follow the StringProtocol, hence you have to use double-quotes around it. Of course you cannot leave it like that as it would just literally show $0, and you need the value behind it, so you use string interpolation \($0) inbetween the quotes.

OK, makes sense. Thanks