Before: [SwiftUI] タブがタップされた時に上までスクロールする。あるいはそれを共通化する話。

ContentView.swift

extension Binding {
    func willSet(_ handler: @escaping (Value) -> ()) -> Binding<Value> {
        .init(
            get: { wrappedValue },
            set: { newValue in
                handler(newValue)
                wrappedValue = newValue
            }
        )
    }
}

enum Tab: String, Identifiable, CaseIterable {
    case first
    case second

    var id: String { rawValue }

    var title: String {
        switch self {
        case .first:  return "First"
        case .second: return "Second"
        }
    }

    @ViewBuilder
    func tabItem() -> some View {
        switch self {
        case .first:  Label(title, systemImage: "1.circle")
        case .second: Label(title, systemImage: "2.circle")
        }
    }
}

/// イベントのトリガー
struct Trigger {
    private(set) var key: Bool = false

    mutating func fire() {
        key.toggle()
    }
}

extension View {
    func onTrigger(of trigger: Trigger?, perform: @escaping () -> Void) -> some View {
        onChange(of: trigger?.key) { _ in
            perform()
        }
    }
}

/// ルートの View
struct ContentView: View {
    @State private var selectedTab: Tab = .first

    @State private var tabTappedTwices: [SelectionValue: Trigger] = .init(
        uniqueKeysWithValues: SelectionValue.allCases.map { ($0, .init()) }
    )

    var body: some View {
        TabView(selection: $selectedTab.willSet {
            if selectedTab == $0 {
                tabTappedTwice[selectedTab]?.fire()
            }
        }) {
            ForEach(Tab.allCases) { tab in
                SampleView(
                    title: tab.title,
                    tabTappedTwice: tabTappedTwices[tab]!
                )
                .tabItem(tab.tabItem)
                .tag(tab)
            }
        }
    }
}

/// タブに表示する View
struct SampleView: View {
    var title: String
    var tabTappedTwice: Trigger

    var body: some View {
        NavigationView {
            ScrollViewReader { proxy in
                List {
                    ForEach(items) { item in
                        Text(item.title)
                            .id(items.first?.id == item.id ? "top" : nil)
                    }
                }
                .listStyle(.plain)
                .onTrigger(of: tabTappedTwice) { _ in
                    withAnimation {
                        proxy.scrollTo("top", anchor: .top)
                    }
                }
            }
            .navigationTitle(title)
            .navigationBarTitleDisplayMode(.inline)
        }
    }
}

/// 要素
struct Item: Identifiable {
    var id: Int
    var title: String { "Item \(id)" }

    init(_ number: Int) {
        id = number
    }
}

/// ダミーデータ
private let items: [Item] = (1..<100).map(Item.init)