Clock/Models/Timer.cs

namespace Clock.Models
{
    using System;
    using System.Collections.Generic;

    // ReSharper disable once ClassNeverInstantiated.Global
    [ExcludeFromCodeCoverage]
    internal class Timer : ITimer, IDisposable
    {
        private readonly System.Threading.Timer _timer;
        private readonly List<IObserver<Tick>> _observers = new List<IObserver<Tick>>();

        // ReSharper disable once MemberCanBePrivate.Global
        public Timer(TimeSpan period) => _timer = new System.Threading.Timer(Tick, null, TimeSpan.Zero, period);

        public IDisposable Subscribe(IObserver<Tick> observer)
        {
            _observers.Add(observer);
            return new Token(() => _observers.Remove(observer));
        }

        public void Dispose() => _timer.Dispose();

        private void Tick(object state)
        {
            foreach (var observer in _observers)
            {
                observer.OnNext(Models.Tick.Shared);
            }
        }

        private class Token: IDisposable
        {
            private readonly Action _action;

            public Token(Action action) => _action = action ?? throw new ArgumentNullException(nameof(action));

            public void Dispose() => _action();
        }
    }
}