common/Ui/Common.UI/Controls/Clock.xaml.cs

namespace Common.UI.Controls
{
    using System;
    using System.Timers;
    using System.Windows;
    using System.Windows.Controls;

    public partial class Clock : Label
    {
        const string DEFAULT_FORMAT = "hh\\:mm\\:ss";

        public static readonly DependencyProperty TimerProperty = DependencyProperty.Register(
               name: nameof(Timer)
             , propertyType: typeof(Timer)
             , ownerType: typeof(Clock)
             , typeMetadata: new FrameworkPropertyMetadata(TimerPropertyChanged));

        public static readonly DependencyProperty FormatProperty = DependencyProperty.Register(
               name: nameof(Format)
             , propertyType: typeof(string)
             , ownerType: typeof(Clock)
             , typeMetadata: new FrameworkPropertyMetadata(DEFAULT_FORMAT));

        public Clock() => this.InitializeComponent();

        public Timer Timer
        {
            get => GetValue(TimerProperty) as Timer;
            set => SetValue(TimerProperty, value);
        }

        public string Format
        {
            get => GetValue(FormatProperty) as string;
            set => SetValue(FormatProperty, value);
        }

        private DateTime? startTime;
        private void Timer_Elapsed(Object sender, ElapsedEventArgs args)
        {
            App.InvokeAsync(() =>
            {
                if (this.startTime is null)
                {
                    this.Opacity = 1;
                    this.startTime = args.SignalTime;
                }

                var timespan = args.SignalTime - this.startTime.Value;
                this.Content = timespan.ToString(this.Format ?? DEFAULT_FORMAT);
            });
        }

        private void BindTimer(object value)
        {
            App.InvokeAsync(() =>
            {
                if (value is Timer timer)
                {
                    timer.Elapsed += this.Timer_Elapsed;
                }

                this.startTime = null;
                this.Opacity = 0.3;
                this.Content = new TimeSpan().ToString(this.Format ?? DEFAULT_FORMAT);
            });
        }

        private void UnbindTimer(object value)
        {
            App.InvokeAsync(() =>
            {
                if (value is Timer timer)
                {
                    timer.Elapsed -= this.Timer_Elapsed;
                }

                this.startTime = null;
                this.Opacity = 0.3;
                this.Content = new TimeSpan().ToString(this.Format ?? DEFAULT_FORMAT);
            });
        }

        private static void TimerPropertyChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs args)
        {
            if (dependencyObject is Clock clock)
            {
                clock.UnbindTimer(args.OldValue);
                clock.BindTimer(args.NewValue);
            }
        }
    }
}