TabViewBadge.swift

import SwiftUI

final class BadgeUpdater {
    var badgeNumber: Int { ///update on change of badge value
        didSet {
            self.tabViewController?.viewControllers?[badgeIndex].tabBarItem.badgeValue = "\(self.badgeNumber)"
        }
    }
    var badgeIndex: Int { ///update on change of index
        didSet {
            self.tabViewController?.viewControllers?[badgeIndex].tabBarItem.badgeValue = "\(self.badgeNumber)"
        }
    }
    
    weak var tabViewController: UITabBarController?
    var observar: NSKeyValueObservation?
    init(badgeNumber: Int, badgeIndex: Int) {
        self.badgeNumber = badgeNumber
        self.badgeIndex = badgeIndex
    }
    
    func onViewAppear() {
        if let rootTabVC =  UIApplication.shared.windows.filter({$0.isKeyWindow}).first?.rootViewController?.children.first as? UITabBarController { //get your tab controller from window
            self.tabViewController = rootTabVC
            self.tabViewController?.viewControllers?[badgeIndex].tabBarItem.badgeValue = "\(self.badgeNumber)"
            self.observar = self.tabViewController?.viewControllers?[badgeIndex].observe(\.tabBarItem, options: .new, changeHandler: { [weak self] (vc, valueWrapper) in
                guard let self = self else { return }
                if (self.badgeNumber != 0) && valueWrapper.newValue??.badgeValue == nil {
                    //force block tab badgeNumber reset
                    vc.tabBarItem.badgeValue = "\(self.badgeNumber)"
                }
            })
        }
    }
}

struct ContentView: View {
    let badgeUpdater = BadgeUpdater(badgeNumber: 10, badgeIndex: 1)
    @State private var selectedTab = 0
    @State private var name = ""
    var body: some View {
        TabView(selection: $selectedTab) {
            TextField("Enter your name", text: $name)
                .padding()
            .tabItem {
                Label("Menu", systemImage: "list.dash")
            }.tag(0)

            Text("Second View")
              .tabItem {
                Label("Order", systemImage: "square.and.pencil")
              }.tag(1)
        }
        .onAppear() {
            badgeUpdater.onViewAppear()
        }
    }
}

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