Umbraco-CMS/src/Umbraco.Core/Events/EventNameExtractor.cs

using System;
using System.Collections.Concurrent;
using System.Linq;
using System.Reflection;
using System.Text.RegularExpressions;

namespace Umbraco.Core.Events
{
    /// <summary>
    /// There is actually no way to discover an event name in c# at the time of raising the event. It is possible
    /// to get the event name from the handler that is being executed based on the event being raised, however that is not
    /// what we want in this case. We need to find the event name before it is being raised - you would think that it's possible
    /// with reflection or anything but that is not the case, the delegate that defines an event has no info attached to it, it
    /// is literally just an event.
    ///
    /// So what this does is take the sender and event args objects, looks up all public/static events on the sender that have
    /// a generic event handler with generic arguments (but only) one, then we match the type of event arguments with the ones
    /// being passed in. As it turns out, in our services this will work for the majority of our events! In some cases it may not
    /// work and we'll have to supply a string but hopefully this saves a bit of magic strings.
    ///
    /// We can also write tests to validate these are all working correctly for all services.
    /// </summary>
    internal class EventNameExtractor
    {

        /// <summary>
        /// Finds the event name on the sender that matches the args type
        /// </summary>
        /// <param name="senderType"></param>
        /// <param name="argsType"></param>
        /// <param name="exclude">
        /// A filter to exclude matched event names, this filter should return true to exclude the event name from being matched
        /// </param>
        /// <returns>
        /// null if not found or an ambiguous match
        /// </returns>
        public static Attempt<EventNameExtractorResult> FindEvent(Type senderType, Type argsType, Func<string, bool> exclude)
        {
            var found = MatchedEventNames.GetOrAdd(new Tuple<Type, Type>(senderType, argsType), tuple =>
            {
                var events = CandidateEvents.GetOrAdd(senderType, t =>
                {
                    return t.GetEvents(BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.FlattenHierarchy)
                        //we can only look for events handlers with generic types because that is the only
                        // way that we can try to find a matching event based on the arg type passed in
                        .Where(x => x.EventHandlerType.IsGenericType)
                        .Select(x => new EventInfoArgs(x, x.EventHandlerType.GetGenericArguments()))
                        //we are only looking for event handlers that have more than one generic argument
                        .Where(x =>
                        {
                            if (x.GenericArgs.Length == 1) return true;

                            //special case for our own TypedEventHandler
                            if (x.EventInfo.EventHandlerType.GetGenericTypeDefinition() == typeof(TypedEventHandler<,>) && x.GenericArgs.Length == 2)
                            {
                                return true;
                            }

                            return false;
                        })
                        .ToArray();
                });

                return events.Where(x =>
                {
                    if (x.GenericArgs.Length == 1 && x.GenericArgs[0] == tuple.Item2)
                        return true;

                    //special case for our own TypedEventHandler
                    if (x.EventInfo.EventHandlerType.GetGenericTypeDefinition() == typeof(TypedEventHandler<,>)
                        && x.GenericArgs.Length == 2
                        && x.GenericArgs[1] == tuple.Item2)
                    {
                        return true;
                    }

                    return false;
                }).Select(x => x.EventInfo.Name).ToArray();
            });

            var filtered = found.Where(x => exclude(x) == false).ToArray();

            if (filtered.Length == 0)
                return Attempt.Fail(new EventNameExtractorResult(EventNameExtractorError.NoneFound));

            if (filtered.Length == 1)
                return Attempt.Succeed(new EventNameExtractorResult(filtered[0]));

            //there's more than one left so it's ambiguous!
            return Attempt.Fail(new EventNameExtractorResult(EventNameExtractorError.Ambiguous));
        }

        /// <summary>
        /// Finds the event name on the sender that matches the args type
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="args"></param>
        /// <param name="exclude">
        /// A filter to exclude matched event names, this filter should return true to exclude the event name from being matched
        /// </param>
        /// <returns>
        /// null if not found or an ambiguous match
        /// </returns>
        public static Attempt<EventNameExtractorResult> FindEvent(object sender, object args, Func<string, bool> exclude)
        {
            return FindEvent(sender.GetType(), args.GetType(), exclude);
        }

        /// <summary>
        /// Return true if the event is named with an ING name such as "Saving" or "RollingBack"
        /// </summary>
        /// <param name="eventName"></param>
        /// <returns></returns>
        internal static bool MatchIngNames(string eventName)
        {
            var splitter = new Regex(@"(?<!^)(?=[A-Z])");
            var words = splitter.Split(eventName);
            if (words.Length == 0)
                return false;
            return words[0].EndsWith("ing");
        }

        /// <summary>
        /// Return true if the event is not named with an ING name such as "Saving" or "RollingBack"
        /// </summary>
        /// <param name="eventName"></param>
        /// <returns></returns>
        internal static bool MatchNonIngNames(string eventName)
        {
            var splitter = new Regex(@"(?<!^)(?=[A-Z])");
            var words = splitter.Split(eventName);
            if (words.Length == 0)
                return false;
            return words[0].EndsWith("ing") == false;
        }

        private class EventInfoArgs
        {
            public EventInfo EventInfo { get; private set; }
            public Type[] GenericArgs { get; private set; }

            public EventInfoArgs(EventInfo eventInfo, Type[] genericArgs)
            {
                EventInfo = eventInfo;
                GenericArgs = genericArgs;
            }
        }

        /// <summary>
        /// Used to cache all candidate events for a given type so we don't re-look them up
        /// </summary>
        private static readonly ConcurrentDictionary<Type, EventInfoArgs[]> CandidateEvents = new ConcurrentDictionary<Type, EventInfoArgs[]>();

        /// <summary>
        /// Used to cache all matched event names by (sender type + arg type) so we don't re-look them up
        /// </summary>
        private static readonly ConcurrentDictionary<Tuple<Type, Type>, string[]> MatchedEventNames = new ConcurrentDictionary<Tuple<Type, Type>, string[]>();
    }
}