SwiftUI synchronized ScrollViews — based on code from Phil Zakharchenko's blog

SynchronizedScrollViews.swift

struct MultiColumnSyncedScrollDemo: View {

	let scrollViewCount: Int = 5

	@State private var scrollPositions: [ScrollPosition] = []	// Initialized in onAppear, so don't use until body prematurely!
	@State private var indexScrolledByUser: Int?

	@State private var colors: [Color] = []	// Initially empty

	let sampleArrayCount = 40
	@ViewBuilder func sampleContent() -> some View {
		ForEach(1..<sampleArrayCount, id: \.self) { n in
			Text("\(n)")
				.frame(width: 160, height: 50)
				.background(colors.count > 0 ? colors[n] : Color.clear)
		}
	}

	var body: some View {
		HStack {
			// This won't show any contents until scrollPositions
			// has been initialized (with same count as scrollViewCount)
			ForEach(0..<scrollPositions.count, id: \.self) { thisScrollIndex in
				ScrollView {
					sampleContent()
				}
				.scrollPosition($scrollPositions[thisScrollIndex])
				.onScrollGeometryChange(for: CGFloat.self) { geometry in
					geometry.contentOffset.y + geometry.contentInsets.top
				} action: { oldValue, newValue in
					guard oldValue != newValue else { return }
					for i in scrollPositions.indices where i != indexScrolledByUser {
						scrollPositions[i].scrollTo(y: newValue)
					}
				}
				.onScrollPhaseChange { _, newPhase in
					if newPhase.isScrolling {
						indexScrolledByUser = thisScrollIndex
					}
				}
			}
		}
		.onAppear {
			scrollPositions = Array(repeating: ScrollPosition(), count: scrollViewCount)
			for _ in 0..<sampleArrayCount {
				colors.append(Color.random)
			}
		}
	}
}