SwiftUI ScrollView Centered Items with ScrollTargetBehavior

CenteredItemsScrollTargetBehavior.swift

import SwiftUI

struct ContentView: View {

    private let colors: [Color] = [.red, .orange, .yellow, .green, .blue, .indigo, .purple]
    private let itemDimension = UIScreen.main.bounds.width * 0.7
    private let itemSpacing = CGFloat(16)
    private let scrollViewHorizontalInset = CGFloat(16)
    
    var body: some View {
        ScrollView(.horizontal, showsIndicators: false) {
            LazyHStack(spacing: itemSpacing) {
                ForEach(colors, id: \.self) { color in
                    VStack {
                        Rectangle()
                            .foregroundStyle(color)
                            .frame(width: itemDimension, height: itemDimension * 2/3)
                        Text(color.description.capitalized)
                    }
                }
            }
            .padding(.horizontal, scrollViewHorizontalInset)
        }
        .scrollTargetBehavior(CenteredBehavior(itemWidth: itemDimension,
                                               itemSpacing: itemSpacing,
                                               scrollViewHorizontalInset: scrollViewHorizontalInset))
    }
}

#Preview {
    ContentView()
}

struct CenteredBehavior: ScrollTargetBehavior {

    private let itemWidth: CGFloat
    private let itemSpacing: CGFloat
    private let scrollViewHorizontalInset: CGFloat
    private let widthOfItemWithSpacing: CGFloat

    init(itemWidth: CGFloat, itemSpacing: CGFloat, scrollViewHorizontalInset: CGFloat) {
        self.itemWidth = itemWidth
        self.itemSpacing = itemSpacing
        self.scrollViewHorizontalInset = scrollViewHorizontalInset
        widthOfItemWithSpacing = itemWidth + itemSpacing
    }

    func updateTarget(_ target: inout ScrollTarget, context: TargetContext) {
        let itemIndex = round((target.rect.origin.x - scrollViewHorizontalInset) / widthOfItemWithSpacing)
        let itemMinX = itemIndex * widthOfItemWithSpacing
        let amountToShiftLeft = (context.containerSize.width - itemWidth) / 2
        let newMinX = scrollViewHorizontalInset + itemMinX - amountToShiftLeft
        target.rect.origin.x = newMinX
    }

}