SwiftUI RefreshableCompat iOS13

RefreshableCompat.swift

import SwiftUI

private struct RefreshableCompat: ViewModifier {

    let onRefresh: () async -> Void

    @State private var isRefreshing = false
    private let threshold: CGFloat = 50.0

    func body(content: Content) -> some View {
        GeometryReader { proxy in
            ZStack(alignment: .top) {
                ScrollView {
                    content
                        .background(Color.white)
                        .anchorPreference(key: OffsetPreferenceKey.self, value: .top) { proxy[$0].y }
                }
                .frame(maxWidth: .infinity)
                .padding(.top, isRefreshing ? threshold : 0)

                .zIndex(2)

                ProgressView()
                    .zIndex(1)
            }
            .onPreferenceChange(OffsetPreferenceKey.self) { offset in
                if offset > threshold, !isRefreshing {
                    isRefreshing = true
                    refresh()
                }
            }
        }
    }

    private func refresh() {
        Task {
            await onRefresh()
            isRefreshing = false
        }
    }

}

private struct OffsetPreferenceKey: PreferenceKey {
    static var defaultValue: CGFloat = 0

    static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
        value = nextValue()
    }
}

public extension View {
    @_disfavoredOverload
    @ViewBuilder
    func refreshable(_ perform: @escaping () async -> Void) -> some View {
        modifier(RefreshableCompat(onRefresh: perform))
    }
}

#if DEBUG && !TESTING

private struct DummyView: View {

    @State private var items = ["Item 1", "Item 2", "Item 3", "Item 4", "Item 5", "Item 6"]

    var body: some View {
        ForEach(items, id: \.self) { item in
            JadeText(item)
                .frame(maxWidth: .infinity)
        }
        .refreshable {
            try? await Task.sleep(nanoseconds: 1_000_000_000 * 3)
            items.shuffle()
        }
    }
}

struct RefreshableCompat_Previews: PreviewProvider {
    static var previews: some View {
        DummyView()
    }
}
#endif