DP44/Common/DTS.Common.Core/EventManager/EventManager.cs

using System;
using System.Collections.Generic;
using System.Reflection;


namespace DTS.Common.Core.EventManager
{
    /// <summary>
    /// Delegate used by event listeners
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="item"></param>
    public delegate void SubscriberCallbackDelegate<in T>(T item) where T : class;

    public delegate void DiagnosticCallbackDelegate(EventDiagnosticType eventType, Type t, object eventData, string listener);

    /// <summary>
    /// Allows components to publish events without knowing who is listening
    /// </summary>
    public static class EventManager
    {
        /// <summary>
        /// Tracks listeners
        /// </summary>
        private static readonly Dictionary<Type, List<object>> SubscriberList = new Dictionary<Type, List<object>>();

        /// <summary>
        /// Tracks diagnostic listeners
        /// </summary>
        private static readonly List<DiagnosticCallbackDelegate> DiagnosticList = new List<DiagnosticCallbackDelegate>();


        /// <summary>
        /// Publish an event
        /// </summary>
        /// <typeparam name="T">type of event</typeparam>
        /// <param name="eventData">event listener</param>
        public static void Publish<T>(T eventData) where T : class
        {
            if (!SubscriberList.ContainsKey(typeof(T))) return;

            var listeners = SubscriberList[typeof(T)];

            foreach (var listener in listeners)
            {
                var metaData = listener as EventMetaData<T>;
                if (metaData == null) continue;

                var triggerCallback = true;

                if (metaData.EventFilter != null)
                {
                    triggerCallback = metaData.EventFilter(eventData);
                }

                if (triggerCallback)
                {
                    metaData.Callback(eventData);
                }

                SendDiagnosticEvent(EventDiagnosticType.PublishEvent, typeof(T), eventData, metaData.EventFilter?.Method);
            }
        }

        /// <summary>
        /// Subscribe to an event
        /// </summary>
        /// <typeparam name="T">type of event</typeparam>
        /// <param name="listener">event listener</param>
        public static void Subscribe<T>(SubscriberCallbackDelegate<T> listener) where T : class
        {
            Subscribe(listener, null);
        }

        /// <summary>
        /// Subscribe to an event
        /// </summary>
        /// <typeparam name="T">type of event</typeparam>
        /// <param name="listener">event listener</param>
        /// <param name="eventFilter">predicate to filter events sent to event listener</param>
        public static void Subscribe<T>(SubscriberCallbackDelegate<T> listener, Predicate<T> eventFilter) where T : class
        {
            if (!SubscriberList.ContainsKey(typeof(T)))
            {
                SubscriberList.Add(typeof(T), new List<object>());
            }

            var listeners = SubscriberList[typeof(T)];

            var metaData = new EventMetaData<T> { Callback = listener, EventFilter = eventFilter };

            listeners.Add(metaData);

            SendDiagnosticEvent(EventDiagnosticType.AddListener, typeof(T), null, listener.Method);
        }

        /// <summary>
        /// Allows a subscriber to unsubscribe
        /// </summary>
        /// <typeparam name="T">type of event</typeparam>
        /// <param name="listener">event listener</param>
        public static void UnSubscribe<T>(SubscriberCallbackDelegate<T> listener) where T : class
        {
            if (!SubscriberList.ContainsKey(typeof(T))) return;

            var listeners = SubscriberList[typeof(T)];

            listeners.RemoveAll(p =>
            {
                return p is EventMetaData<T> eventMetaData && eventMetaData.Callback == listener;
            });

            SendDiagnosticEvent(EventDiagnosticType.RemoveListener, typeof(T), null, listener.Method);
        }

        /// <summary>
        /// Clears all listeners
        /// </summary>
        public static void Clear()
        {
            SubscriberList.Clear();
            SendDiagnosticEvent(EventDiagnosticType.RemoveListener, null, null, null);
        }

        /// <summary>
        /// Add diagnostic listener
        /// </summary>
        /// <param name="listener">event listener</param>
        public static void SubscribeToDiagnosticEvents(DiagnosticCallbackDelegate listener)
        {
            DiagnosticList.Add(listener);
            SendDiagnosticEvent(EventDiagnosticType.AddListenerDiagnostic, null, null, listener.Method);
        }

        /// <summary>
        /// Clear diagnostic event
        /// </summary>
        /// <param name="listener">listener to unsubscribe</param>
        public static void UnSubscribeToDiagnosticEvents(DiagnosticCallbackDelegate listener)
        {
            DiagnosticList.RemoveAll(p => p == listener);
            SendDiagnosticEvent(EventDiagnosticType.RemoveListenerDiagnostic, null, null, listener.Method);
        }

        /// <summary>
        /// Clear all diagnostic events
        /// </summary>
        public static void ClearDiagnosticEvents()
        {
            DiagnosticList.Clear();
            SendDiagnosticEvent(EventDiagnosticType.RemoveListenerDiagnostic, null, null, null);
        }

        /// <summary>
        /// Sends a diagnostic event
        /// </summary>
        /// <param name="diagnosticEventType"></param>
        /// <param name="eventType"></param>
        /// <param name="eventData"></param>
        /// <param name="listenerMethod"></param>
        private static void SendDiagnosticEvent(EventDiagnosticType diagnosticEventType, Type eventType, object eventData, MemberInfo listenerMethod)
        {
            foreach (var diagnosticEvent in DiagnosticList)
            {
                string listener = null;

                if (listenerMethod != null && listenerMethod.DeclaringType != null)
                {
                    listener = $"{listenerMethod.DeclaringType.FullName}.{listenerMethod.Name} , {listenerMethod.DeclaringType.Assembly.FullName}";
                }

                diagnosticEvent(diagnosticEventType, eventType, eventData, listener);
            }
        }

    }

    public enum EventDiagnosticType
    {
        AddListener = 0,
        AddListenerDiagnostic = 1,
        PublishEvent = 2,
        RemoveListenerDiagnostic = 3,
        RemoveListener = 4
    }

    internal class EventMetaData<T> where T : class
    {
        public Predicate<T> EventFilter { get; set; }
        public SubscriberCallbackDelegate<T> Callback { get; set; }
    }

}