SwiftUI onScroll

OnScroll.swift

import SwiftUI
import Combine

struct OnScroll: ViewModifier {
    @Binding var offset: CGFloat
    
    //we can have a version with a closure instead of the binding, but that triggers an infinite loop if content depends on the same Store
    //    var onOffset: (CGFloat) -> ()
    func body(content: Content) -> some View {
        return VStack {
                GeometryReader { geometry -> ForEach<Range<Int>, EmptyView> in
                    let newOffset = geometry.frame(in: .global).minY
//                    self.onOffset(newOffset)
//                    //this can trigger a rerender. Checking for equality breaks the infinite loop.
                    if self.offset != newOffset { self.offset = newOffset }
                    // for some reason with EmptyView or any other View the GeometryReader never got called, but with empty ForEach it does
                    return ForEach(0..<0) { _ -> EmptyView in
                        assertionFailure()
                        return EmptyView()
                    }
                }
                content
            }
    }
}


final class Store: BindableObject {
    let didChange = PassthroughSubject<(), Never>()
    var offset: CGFloat = 0 {
        didSet {
            print(offset)
            didChange.send(())
        }
    }
}

struct ContentView : View {
    var body: some View {
         MyView()
        .environmentObject(Store())
    }
}

struct MyView: View {
    @EnvironmentObject var store: Store
    var body: some View {
        
        return VStack {
            // even though initial value got written into the store, this view never rerendered, so this Text says 0 until a scroll happens
            Text("\(store.offset)")
            ScrollView(alwaysBounceVertical: true) {
                //             !!! we need to wrap the modified in something. VStack or single element ForEach work just fine.
                // If we use the text directly as the root of the ScrollView's content, it doesnt render.
                // Whats super strange is that we cant do this wrapping inside the Modifier.
                VStack {
                    ScrollContent(store: store)
                        .modifier(OnScroll(offset: self.$store.offset))
                    //                        .modifier(OnScroll { self.store.offset = $0 })
                }
            }
        }
        
    }
}

struct ScrollContent: View {
    //wtf EnvironmentObject wasnt getting injected here even though it worked fine for the other views
//    @EnvironmentObject var store: Store
    @ObjectBinding var store: Store
    var body: some View {
        
        // !!! make sure not to change the content on scroll, else scrolling gets interrupted
        
        //you can print it all you want
        print("I can print offset: \(store.offset)")
        //but you cant do this, else scrolling breaks
//        return Text("\(store.offset)")
        return Text("foo")
    }
}

#if DEBUG
struct ContentView_Previews : PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}
#endif

thanks for the code snippet, it's a bit outdated but the logic is there 👍

Yes, very outdated. I havent used this recently, but I worry if setting the binding value from the GeometryReader might cause a "setting state from body" warning. If you have any trouble, you should use a custom PreferenceKey (I didnt no this at the time)

I got a version working with preferences here: https://gist.github.com/khanlou/112cbb13ee2c776aa343bfc204f78259