Change the user interface so that it always shows their recommended bedtime using a nice and large font. You should be able to remove the “Calculate” button entirely.

I've created a computed string property, using the code that was called when we hit the calculate button. It works just like I was hoping, but I'm not entirely sure why it works.

How does SwiftUI know to "recompute" the property each time I change one of the inputs? Is the entire view being rebuilt each time one of the pickers/steppers is changed, causing that property to be accessed again?

Nick wonders about computed vars:

How does SwiftUI know to "recompute" the property each time I change one of the inputs?

Gakkie has the correct answer!
Here's a bit of code to illustrate some of the concepts. Paste into Playgrounds for jollies!

struct LightBulb: View {
    var lightIsOn: Bool

    var body: some View {
        Circle()
            .frame(width: 100, height: 100)
            // randomColor is recalculated
            // whenever this view is drawn on screen.
            .foregroundColor(lightIsOn ? randomColor : .white)
    }
    let bulbColors = [Color.teal, .indigo, .yellow, .red]

    // computed var recalculates whenever it is
    // needed in the code above.
    var randomColor: Color {
        (bulbColors.randomElement()!.opacity(0.4))
    }
}

struct LightSwitch: View {
    @State private var switchOne = true
    @State private var switchTwo = false

    // computed var recalculates whenever
    // the view below is redrawn
    var bothOn: Bool {
        switchOne && switchTwo
    }

    var body: some View {
        VStack {
            VStack {
                Toggle("Switch One", isOn: $switchOne) // trigger redraw
                Toggle("Switch Two", isOn: $switchTwo) // trigger redraw
            }.padding().frame(width: 200, height: 100)
            Divider()
            HStack (spacing: 10){
                // When do these redraw themselves?
                LightBulb( lightIsOn: switchOne )
                LightBulb( lightIsOn: switchTwo )
                ZStack {
                    LightBulb( lightIsOn: bothOn    )
                    Text(bothOn ? "Recalculated" : "" ).font(.caption)
                }
            }
            .padding()
            .animation(.easeIn,                  value: switchTwo)
            .animation(.easeIn(duration: 1.25),  value: switchOne)
        }
    }
}

// Run this line in Playgrounds.
PlaygroundPage.current.setLiveView( LightSwitch() )

@Obelix, as elaborate and fantastic as always ✌️