A View Modifier for a Paged Index View like, using ScrollToTargetBehavior

PagedIndexViewModifier.swift

//
//  PagedIndexViewModifier.swift
//
//  Copyright© 2023 ApiumHub
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
//  THE SOFTWARE.


import SwiftUI

@available(iOS 17.0, *)
struct PagedIndexViewModifier: ViewModifier {
    @Binding var index: Int
    @State private var offset: CGPoint = .zero
    
    func body(content: Content) -> some View {
        GeometryReader { contentProxy in
            ScrollViewReader { svReaderProxy  in
                ScrollView {
                    LazyVStack(spacing: 0) {
                        content
                            .frame(width: contentProxy.size.width, height: contentProxy.size.height)
                    }
                    .background(GeometryReader { geometry in
                        Color.clear
                            .preference(key: ScrollOffsetPreferenceKey.self, value: geometry.frame(in: .named("scroll")).origin)
                    })
                    .onPreferenceChange(ScrollOffsetPreferenceKey.self) { value in
                        self.offset = value
                        changeIndexIfNeeded(with: value, contentSize: contentProxy.size)
                    }
                }
                .scrollTargetBehavior(.paging)
                .scrollIndicators(.hidden)
                .onChange(of: index) { oldValue, newValue in
                    guard oldValue != newValue else {
                        return
                    }
                    svReaderProxy.scrollTo(newValue)
                }
            }
        }
    }
    
    private func changeIndexIfNeeded(with scrollValue: CGPoint, contentSize: CGSize) {
        let yPoint = Double(scrollValue.y)
        if yPoint == 0 && index == 0 {
            return
        }
        let height = contentSize.height
        guard yPoint.truncatingRemainder(dividingBy: height) == 0 else { return }
        let currentIndex = index
        let expectedIndex = Int(abs(yPoint / (height)))
        if expectedIndex == currentIndex { return }
        index = expectedIndex
    }
}

@available(iOS 17.0, *)
extension View {
    func indexedPageView(_ index: Binding<Int>) -> some View {
        modifier(PagedIndexViewModifier(index: index))
    }
}

fileprivate struct ScrollOffsetPreferenceKey: PreferenceKey {
    static var defaultValue: CGPoint = .zero
    
    static func reduce(value: inout CGPoint, nextValue: () -> CGPoint) { }
}