SwiftUI - Detect when ScrollView has finished scrolling?

Question

I need to find out the exact moment when my ScrollView stops moving. Is that possible with SwiftUI?

Here would be an equivalent for UIScrollView.

I have no idea after thinking a lot about it...

A sample project to test things out:

struct ContentView: View {
    
    var body: some View {
        ScrollView {
            VStack(spacing: 20) {
                ForEach(0...100, id: \.self) { i in
                    Rectangle()
                        .frame(width: 200, height: 100)
                        .foregroundColor(.green)
                        .overlay(Text("\(i)"))
                }
            }
            .frame(maxWidth: .infinity)
        }
    }
}

Thanks!

Answer 1

Here is a demo of possible approach - use publisher with changed scrolled content coordinates with debounce, so event reported only after coordinates stopped changing.

Tested with Xcode 12.1 / iOS 14.1

UPDATE: verified as worked with Xcode 13.3 / iOS 15.4

Note: you can play with debounce period to tune it for your needs.

import Combine

struct ContentView: View {
    let detector: CurrentValueSubject<CGFloat, Never>
    let publisher: AnyPublisher<CGFloat, Never>

    init() {
        let detector = CurrentValueSubject<CGFloat, Never>(0)
        self.publisher = detector
            .debounce(for: .seconds(0.2), scheduler: DispatchQueue.main)
            .dropFirst()
            .eraseToAnyPublisher()
        self.detector = detector
    }
    
    var body: some View {
        ScrollView {
            VStack(spacing: 20) {
                ForEach(0...100, id: \.self) { i in
                    Rectangle()
                        .frame(width: 200, height: 100)
                        .foregroundColor(.green)
                        .overlay(Text("\(i)"))
                }
            }
            .frame(maxWidth: .infinity)
            .background(GeometryReader {
                Color.clear.preference(key: ViewOffsetKey.self,
                    value: -$0.frame(in: .named("scroll")).origin.y)
            })
            .onPreferenceChange(ViewOffsetKey.self) { detector.send($0) }
        }.coordinateSpace(name: "scroll")
        .onReceive(publisher) {
            print("Stopped on: \($0)")
        }
    }
}

struct ViewOffsetKey: PreferenceKey {
    typealias Value = CGFloat
    static var defaultValue = CGFloat.zero
    static func reduce(value: inout Value, nextValue: () -> Value) {
        value += nextValue()
    }
}