1

I believe that the use of @AppStorage or @SceneStorage is causing view equality to fail in a way that @State does not. This is unexpected and can lead to much unneeded computation and wasted cycles.

Comments?

import SwiftUI

struct InnerView: View {
    // Using @State, the toggle starts at false whenever you start the app.
    // Using @AppStorage can let the app remember what the value of the toggle was.
    // But if we comment out the @State version and uncomment the @AppStorage
    // version, now we are hit by excessive recomputation.
    @State var initialValue = false

    // If you comment out the above @State variable and uncomment the @AppStorage
    // version below, although the starting toggle state is preserved, we have a serious issue:
    // the function someText() is called while dragging the slider in ContentView.
    // I believe that view equality on InnerView is failing due to the AppStorage
    // property wrapper in a way it does not for the State property wrapper.

    //@AppStorage("initialValue") var initialValue = false

    func someText() -> String {
        print("someText called: \(Date())")
        return "Blah"
    }

    var body: some View {
        VStack {
            Toggle("Toggle: ", isOn: $initialValue)
            Text(someText())
        }
    }
}

struct ContentView: View {
    @State var sliderValue = 0.0
    
    var body: some View {
        VStack {
            Slider(value: $sliderValue)
            InnerView()
        }.padding()
        
    }
}
2

Something strange, if you add _printChanges():

    var body: some View {
        let _ = Self._printChanges()
        VStack {
            Toggle("Toggle: ", isOn: $initialValue)
            Text(someText())
        }
    }

It show @self is the change, not initialValue.

3

I have not heard of _printChanges() but it sounds extremely useful! What exactly does it tell me?

2 Likes
4
5

This looks extremely useful. Thanks for sharing for those of us who are not on Twitter.

Interestingly by searching for "_printChanges" on Apple site I found there is a SwiftUI Q&A session tomorrow Dec 14. You'd need to register to join it.

PS. I believe we need a place dedicated to swiftUI questions. More alive than Apple Developer Forums, something like this discourse forum but dedicated to swiftUI. I can see a few categories here in "Related Projects", quite a few with just a few messages there and but SwiftUI is not one of the categories. Could we establish SwiftUI as one of the subcategories of this forum?