SwiftUI - Two Directional SnapList

TwoWaySnapList.swift

// Checkout the explanation article here - https://sameer-syd.medium.com/swiftui-two-directional-snaplist-95cb852957be

import SwiftUI
import Combine

struct HomeView: View {
    
    @StateObject var viewModel: HomeViewModel
    
    var body: some View {
        GeometryReader { geo in
            ZStack {
                ScrollViewReader { reader in
                    ScrollView(.vertical, showsIndicators: false) {
                        LazyVStack(spacing: 0) {
                            ForEach(0..<(viewModel.messages.count), id: \.self) { i in
                                TabViewCell(media: viewModel.messages[i].media)
                                    .frame(width: geo.size.width, height: geo.size.height)
                                    .id(i)
                            }
                        }
                        .background(GeometryReader {
                            Color.clear.preference(key: ScrollOffsetKey.self,
                                value: -$0.frame(in: .named("scroll")).origin.y)
                        })
                        .onPreferenceChange(ScrollOffsetKey.self) { viewModel.scrollDetector.send($0) }
                    }
                    .coordinateSpace(name: "scroll")
                    .onReceive(viewModel.scrollPublisher) {
                        var index = $0/geo.size.height
                        // Check if next item is near
                        let value = index < 1 ? index : index.truncatingRemainder(dividingBy: CGFloat(Int(index)))
                        if value > 0.5 { index += 1 }
                        else { index = CGFloat(Int(index)) }
                        // Scroll to index
                        withAnimation { reader.scrollTo(Int(index), anchor: .center) }
                    }
                }   
            }.edgesIgnoringSafeArea(.all)
        }.edgesIgnoringSafeArea(.all)
    }
}


// To Get Scroll Offset
fileprivate struct ScrollOffsetKey: PreferenceKey {
    typealias Value = CGFloat
    static var defaultValue = CGFloat.zero
    static func reduce(value: inout Value, nextValue: () -> Value) {
        value += nextValue()
    }
}


// -----------------------------------------


class HomeViewModel: ObservableObject {
    
    let messages: [Message]
    let scrollDetector: CurrentValueSubject<CGFloat, Never>
    let scrollPublisher: AnyPublisher<CGFloat, Never>
    
    init(messages: [Message]) {
        self.messages = messages
        // detect when scroll ends
        let detector = CurrentValueSubject<CGFloat, Never>(0)
        self.scrollPublisher = detector
            .debounce(for: .seconds(0.2), scheduler: DispatchQueue.main)
            .dropFirst().eraseToAnyPublisher()
        self.scrollDetector = detector
    }
}