Clock/ViewModels/ClockViewModel.cs

namespace Clock.ViewModels
{
    using System;
    using Models;

    // ReSharper disable once ClassNeverInstantiated.Global
    internal class ClockViewModel : ViewModel, IClockViewModel, IDisposable, IObserver<Tick>
    {
        private readonly IClock _clock;
        private readonly IDisposable _timerToken;

        public ClockViewModel(
            IClock clock,
            ITimer timer,
            IDispatcher dispatcher = null)
            :base(dispatcher)
        {
            _clock = clock ?? throw new ArgumentNullException(nameof(clock));
            _timerToken = (timer ?? throw new ArgumentNullException(nameof(timer))).Subscribe(this);
        }

        public string Time => _clock.Now.ToString("T");

        public string Date => _clock.Now.ToString("d");

        void IObserver<Tick>.OnNext(Tick value)
        {
            OnPropertyChanged(nameof(Time));
            OnPropertyChanged(nameof(Date));
        }

        void IObserver<Tick>.OnError(Exception error) { }

        void IObserver<Tick>.OnCompleted() { }

        void IDisposable.Dispose() => _timerToken.Dispose();
    }
}