ビューモデルにカウンタ更新用のコマンドを追加した

MainViewModel.cs

using System.ComponentModel;
using System.Windows.Input;

namespace MauiApp1 {
    internal class MainViewModel : INotifyPropertyChanged {

        public event PropertyChangedEventHandler PropertyChanged;
        public ICommand CounterCommand { private set; get; }
        private int Counter;

        #region コンストラクタ
        public MainViewModel() {
            CounterText = $"Current count: 0";
            CounterCommand = new Command(execute: () => CounterText = $"Current count: {++Counter}");
        }
        #endregion

        #region プロパティ
        string _CounterText;
        public string CounterText {
            get => _CounterText;
            private set {
                if (_CounterText == value) return;
                _CounterText = value;
                OnPropertyChanged(nameof(CounterText));
            }
        }
        #endregion

        protected virtual void OnPropertyChanged(string propertyName) {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}