ContentTransition

Timer.swift

import SwiftUI

struct ContentView: View {
    let timer = Timer.publish(every: 1, on: .main, in: .common).autoconnect()
    @State private var totalSeconds = 30
    var body: some View {
        Text(timeStringFromSeconds(totalSeconds))
            .monospaced()
            .contentTransition(.numericText())
            .font(.system(size: 60))
            .bold()
            .onReceive(timer) { input in
                withAnimation {
                    totalSeconds -= 1
                }
            }
    }
    
    func timeStringFromSeconds(_ totalSeconds: Int) -> String {
        let minutes = totalSeconds / 60
        let seconds = totalSeconds % 60
            return String(format: "%01d:%02d", minutes, seconds)
    }
}

#Preview {
    ContentView()
}