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 * 
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 *     following disclaimer. 
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 *     copyright notice, this list of conditions and the 
 *     following disclaimer in the documentation and/or other 
 *     materials provided with the distribution. 
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND 
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 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF 
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using System;
using System.Threading;
using XenAPI;

namespace XenAdmin.Utils
{
    /// <summary>
    /// Get the UTC server time in an async fashion
    /// 
    /// - Updated server time is not guaranteed
    /// - No retry
    /// - Default return time is the time at which the class is constructed
    /// </summary>
    public class AsyncServerTime
    {
        /// <summary>
        /// Event triggered when the server time is obtained
        /// </summary>
        public event Action ServerTimeObtained;
        private static readonly log4net.ILog log = log4net.LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);

        /// <summary>
        /// Server time (UTC) - updated by thread and user is informed via the ServerTimeEventHandler
        /// Default = DateTime.UtcNow
        /// </summary>
        public DateTime ServerTime { get; private set; }

        /// <summary>
        /// Constructs the class but doesn't trigger the fetch of the server time
        /// </summary>
        public AsyncServerTime()
        {
            ServerTime = DateTime.UtcNow;
        }

        /// <summary>
        /// Get the server time for given host on a background thread
        /// 
        /// Connect to the ServerTimeEventHandler for updates otherwise you'll get the current UTC time
        /// and it won't be updated
        /// </summary>
        /// <param name="host"></param>
        public void Fetch(Host host)
        {
            ThreadPool.QueueUserWorkItem(GetServerTime, host);
        }

        private void TriggerEvent()
        {
            if (ServerTimeObtained != null)
                ServerTimeObtained();
        }

        private void GetServerTime(object state)
        {
            Host host = state as Host;
            if (host == null || host.Connection == null)
            {
                log.ErrorFormat("Will not fetch server time: host or connection could not be resolved");
                return;
            }

            try
            {
                //Note we're using the get_servertime call which returns the UTC time
                ServerTime = Host.get_servertime(host.Connection.Session, host.opaque_ref);
                TriggerEvent();
            }
            catch (Exception e)
            {
                log.ErrorFormat("Failed to fetch server time for host {0}: {1}", host.name_label, e);
            }
        }
    }
}