A solution to SwiftUI Challenge #3: Tab Selection (https://twitter.com/objcio/status/1230145263202758658)

ContentView.swift

struct TabRectsKey: PreferenceKey {
    static var defaultValue: [Int: CGRect] = [:]
    static func reduce(value: inout [Int : CGRect], nextValue: () -> [Int : CGRect]) {
        value.merge(nextValue(), uniquingKeysWith: { r1, r2 in r1 })
    }
}

struct TabBar: View {
    var items: [(Image, Text)]
    @State var selectedIndex: Int = 0
    @State var rects: [Int: CGRect] = [:]
    
    var body: some View {
        VStack(alignment: .leading, spacing: 0) {
            HStack {
                ForEach(0..<items.count, id: \.self) { index in
                    VStack {
                        self.items[index].0
                        self.items[index].1
                    }
                    .background(
                        GeometryReader { proxy in
                            Rectangle().hidden()
                                .preference(key: TabRectsKey.self, value: [index: proxy.frame(in: .named("TabContainer"))])
                        }
                    )
                        .foregroundColor(self.selectedIndex == index ? .blue : .black)
                        .onTapGesture {
                            self.selectedIndex = index
                    }
                }
            }
            .coordinateSpace(name: "TabContainer")
            Rectangle()
                .frame(width: rects[selectedIndex]?.width, height: 2)
                .foregroundColor(.blue)
                .offset(x: rects[selectedIndex]?.minX ?? 0, y: 0)
                .animation(.default)
        }
        .onPreferenceChange(TabRectsKey.self) { value in
            self.rects = value
        }
    }
}

struct ContentView: View {
    var body: some View {
        return TabBar(items: [
            (Image(systemName: "tray"), Text("Inbox")),
            (Image(systemName: "archivebox"), Text("Archive")),
            (Image(systemName: "doc.text"), Text("Drafts"))
        ])
    }
}