ProgramaticScrolling.swift

import SwiftUI
import ComposableArchitecture

@Reducer
struct Scroll {
  @ObservableState
  struct State {
    var scrollPosition: Double
    var trackerId: String?
  }

  enum Action {
    case scrollPosition(Double)
    case setTracker(String?)
  }

  var body: some ReducerOf<Self> {
    Reduce { state, action in
      switch action {
      case let .scrollPosition(position):
        state.scrollPosition = position
        return .run { send in
          // Set the tracker id which causes the view to scroll,
          // then unset it so it can be set again next time
          await send(.setTracker(ScrollTracker.id))
          await send(.setTracker(nil))
        }
      case let .setTracker(tracker):
        state.trackerId = tracker
        return .none
      }
    }
  }
}

struct ContentView: View {
  @Bindable var store = Store(
    initialState: Scroll.State(scrollPosition: 0.0),
    reducer: { Scroll() }
  )

  var body: some View {
    Button("Down") {
      store.send(.scrollPosition(store.scrollPosition + 30))
      store.send(.setTracker(ScrollTracker.id))
    }
    ScrollView {
      ZStack {
        VStack {
          ForEach(0..<100) { _ in
            Rectangle()
              .fill(Color.green.gradient)
              .frame(height: 50)
          }
        }
        ScrollTracker(offset: $store.scrollPosition.sending(\.scrollPosition))
      }.scrollTargetLayout()
    }
    .scrollPosition(id: $store.trackerId.sending(\.setTracker))

  }
}

struct ScrollTracker: View {
  static let id = "tracker"
  @Binding var offset: Double

  var body: some View {
    VStack {
      Spacer(minLength: offset)
      GeometryReader { geometry in
        Rectangle()
          .frame(height: 10)
          .foregroundStyle(Color.red)
          .id(ScrollTracker.id)
          .task(id: geometry.frame(in: .scrollView(axis: .vertical))) {
            let scrollGeometry = geometry.frame(in: .scrollView(axis: .vertical))
            let verticalOffset = scrollGeometry.origin.y
            print("scroll", scrollGeometry)
            print("vertical offset", verticalOffset)
            offset -= verticalOffset
            offset = max(offset, 0)
            print("offset", offset)
          }
      }
      Spacer()
        .layoutPriority(1)
    }
  }
}

#Preview {
  ContentView()
}