import SwiftUI

struct TimestampedRect: Equatable {
    var date: Date
    var rect: CGRect
}

struct MyTextPreferenceKey: PreferenceKey {
    typealias Value = TimestampedRect

    static var defaultValue: TimestampedRect = TimestampedRect(date: Date(), rect: .zero)

    static func reduce(value: inout TimestampedRect, nextValue: () -> TimestampedRect) {
        let next = nextValue()

        if (next.date > value.date) {
            value = next
        }
    }
}

struct ContentView : View {
    @State private var rect: CGRect = .zero

    var body: some View {
        ZStack(alignment: .topLeading) {
            RoundedRectangle(cornerRadius: 15)
                .stroke(lineWidth: 3.0)
                .foregroundColor(Color.green)
                .frame(width: rect.width, height: rect.height)
                .offset(x: rect.minX, y: rect.minY)
                .animation(.easeInOut(duration: 0.5))

            VStack {
                Spacer()

                HStack {
                    MonthView("January")
                    MonthView("February")
                    MonthView("March")
                    MonthView("April")
                }

                Spacer()

                HStack {
                    MonthView("May")
                    MonthView("June")
                    MonthView("July")
                    MonthView("August")
                }

                Spacer()

                HStack {
                    MonthView("September")
                    MonthView("October")
                    MonthView("November")
                    MonthView("December")
                }

                Spacer()
            }
                .onPreferenceChange(MyTextPreferenceKey.self) { preference in
                    rect = preference.rect
                }
        }
            .coordinateSpace(name: "main")
    }
}

struct MonthView: View {
    @State var date: Date = .distantPast

    let label: String

    init(_ label: String) {
        self.label = label
    }

    var body: some View {
        let preferenceView = MyPreferenceViewSetter(date: date)

        Text(label)
            .padding(10)
            .background(preferenceView)
            .onTapGesture {
                date = Date()
            }
    }
}

struct MyPreferenceViewSetter: View {
    let date: Date

    var body: some View {
        GeometryReader { geometry in
            Color.clear
                .preference(key: MyTextPreferenceKey.self,
                            value: TimestampedRect(date: date, rect: geometry.frame(in: .named("main"))))
        }
    }
}