tricksy-timer3.swift

import SwiftUI

struct ContentView: View {
    var body: some View {
        IceCreamView(viewModel: ViewModel(iceCream: "banana"))
    }
}

struct IceCreamView: View {
    @State var viewModel: ViewModel
    
    var body: some View {
        VStack {
            Text("Flavor: \(viewModel.iceCream)")
            
            // This random number changes every 1 second
            // and ideally it wouldn't
            Text("Enjoy your ice cream! \(arc4random_uniform(9999999))")
        }
        .overlay {
            // Option 1: ❌
            if let secondsSinceCreation = viewModel.secondsSinceCreation {
                // Still will cause updates as view model is still rebuilding the body
                IceCreamOverlayView(secondsSinceCreation: secondsSinceCreation)
            }
            
            // Option 2: ✅
            MaybeExistsIceCreamOverlayView(viewModel: viewModel)
        }
    }
}

// ❌ This would not be enough, as it would be recreated
// each time secondsSinceCreation changes
struct IceCreamOverlayView: View {
    let secondsSinceCreation: Int
    
    var body: some View {
        Text("Ordered \(secondsSinceCreation) second(s) ago")
            .padding(.top, 100)
    }
}

// ✅ This WOULD be enough, but feels weird. What
// do you even call it?
struct MaybeExistsIceCreamOverlayView: View {
    let viewModel: ViewModel
    
    var body: some View {
        if let secondsSinceCreation = viewModel.secondsSinceCreation {
            // Still will cause updates as view model is still rebuilding the body
            IceCreamOverlayView(secondsSinceCreation: secondsSinceCreation)
        }
    }
}

@Observable class ViewModel {
    let iceCream: String
    var secondsSinceCreation: Int?
    
    init(iceCream: String) {
        self.iceCream = iceCream
        
        Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { timer in
            if let secondsSinceCreation = self.secondsSinceCreation {
                self.secondsSinceCreation = secondsSinceCreation + 1
            } else {
                self.secondsSinceCreation = 1
            }
        }
    }
}

Here's my attempt to refactor in a way that I would

1. View isolation for the flavour view so the view doesn't compute if none of its values change
2. Option 1 is ok in my mind because there is very little overhead for a view to be reinstantiated

import SwiftUI

struct ContentView: View {
    var body: some View {
        IceCreamView(viewModel: ViewModel(iceCream: "banana"))
    }
}

struct IceCreamView: View {
    @State var viewModel: ViewModel

    var body: some View {
        IceCreamFlavorView(flavor: viewModel.iceCream)
            .overlay {
                IceCreamOverlayView(secondsSinceCreation: viewModel.secondsSinceCreation)
            }
    }
}

struct IceCreamFlavorView: View {
    let flavor: String

    var body: some View {
        VStack {
            Text("Flavor: \(flavor)")
            Text("Enjoy your ice cream! \(arc4random_uniform(9999999))")
        }
    }
}

struct IceCreamOverlayView: View {
    let secondsSinceCreation: Int?

    init(secondsSinceCreation: Int?) {
        self.secondsSinceCreation = secondsSinceCreation
    }

    var body: some View {
        if let secondsSinceCreation {
            Text("Ordered \(secondsSinceCreation) second(s) ago")
                .padding(.top, 100)
        }
    }
}

@Observable class ViewModel {
    let iceCream: String
    var secondsSinceCreation: Int?

    init(iceCream: String) {
        self.iceCream = iceCream

        Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { timer in
            if let secondsSinceCreation = self.secondsSinceCreation {
                self.secondsSinceCreation = secondsSinceCreation + 1
            } else {
                self.secondsSinceCreation = 1
            }
        }
    }
}

@aerickson14 This would still result in the number updating, I believe

The seconds ago number will updated but the random number in IceCreamFlavorView will remain the same. I just double checked and still seeing that behaviour.

@aerickson14 Interesting, okay thanks! Will have to dig a bit deeper and try to understand