Example of how to tell when a TabView changes which tab is selected, or when the same tab is tapped when it is already selected.

gistfile1.txt

//
//  ContentView.swift
//  TestTabs
//
//  Created by Kem Mason on 24/02/23.
//

import SwiftUI

enum TopNavTag: Hashable {
  case tab1, tab2
}

struct Tab1: View {
  var body: some View {
    Text("Tab1")
  }
}

struct Tab2: View {
  var body: some View {
    Text("Tab2")
  }
}

/* method 1
extension Binding {
// long name primaril
func onUpdatedValue(_ closure: @escaping () -> Void) -> Binding<Value> {
    Binding(get: {
        wrappedValue
    }, set: { newValue in
        wrappedValue = newValue
        closure()
    })
}}

struct ContentView: View {
  @State var previouslySelectedTab = TopNavTag.tab1
  @State var selectedTab: TopNavTag = .tab1
    var body: some View {
      TabView(selection: $selectedTab.onUpdatedValue {
        if (previouslySelectedTab != selectedTab) {
          previouslySelectedTab = selectedTab
          print("new tab selected: \(selectedTab)")
        } else {
          print("current tab selected again: \(selectedTab)")
        }
      })
      {
        Tab1().tabItem {
          Label("Tab1", systemImage: "star")
        }.tag(TopNavTag.tab1)
        Tab2().tabItem {
          Label("Tab2", systemImage: "chart.bar")
        }.tag(TopNavTag.tab2)
      }
    }
}
 */

// method 2
 struct ContentView: View {
   @State var previouslySelectedTab = TopNavTag.tab1
   @State var selectedTab: TopNavTag = .tab1
   private var selection: Binding<TopNavTag> { Binding(
    get: { selectedTab },
    set: { newValue in
      selectedTab = newValue
      if newValue == previouslySelectedTab {
        print("selected again: \(newValue)")
      } else {
        previouslySelectedTab = newValue
        print("selected first time: \(newValue)")
      }
    }
   )}
     var body: some View {
       TabView(selection: selection)
       {
         Tab1().tabItem {
           Label("Tab1", systemImage: "star")
         }.tag(TopNavTag.tab1)
         Tab2().tabItem {
           Label("Tab2", systemImage: "chart.bar")
         }.tag(TopNavTag.tab2)
       }
     }
 }

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}