using System;
using System.IO;
using System.Runtime.Caching;

namespace wasSharpNET.IO
{
    /// <summary>
    ///     Filesystem watcher that ensures events are not fired twice
    /// </summary>
    /// <remarks>derived from Ben Hall @ benhall.io</remarks>
    public class FileSystemWatcher : IDisposable
    {
        public event FileSystemEventHandler FilesytemEvent;
        private readonly CacheItemPolicy _cacheItemPolicy;
        private readonly int _cacheTimeMilliseconds = 1000;
        private readonly MemoryCache _memCache;

        public FileSystemWatcher(string path, NotifyFilters notifyFilters, string filter)
        {
            _memCache = MemoryCache.Default;

            var watcher = new System.IO.FileSystemWatcher
            {
                Path = path,
                NotifyFilter = notifyFilters,
                Filter = filter
            };

            _cacheItemPolicy = new CacheItemPolicy
            {
                RemovedCallback = OnRemovedFromCache
            };

            watcher.Changed += OnChanged;
            watcher.EnableRaisingEvents = true;
        }

        public FileSystemWatcher(string path, NotifyFilters notifyFilters, string filter, int timeout) : this(path,
            notifyFilters, filter)
        {
            _cacheTimeMilliseconds = timeout;
        }

        // Add file event to cache for CacheTimeMilliseconds
        private void OnChanged(object source, FileSystemEventArgs e)
        {
            _cacheItemPolicy.AbsoluteExpiration =
                DateTimeOffset.Now.AddMilliseconds(_cacheTimeMilliseconds);

            // Only add if it is not there already (swallow others)
            _memCache.AddOrGetExisting(e.Name, e, _cacheItemPolicy);
        }

        // Handle cache item expiring
        private void OnRemovedFromCache(CacheEntryRemovedArguments args)
        {
            if (args.RemovedReason != CacheEntryRemovedReason.Expired) return;

            // Now actually handle file event.
            OnFileSystemEvent((FileSystemEventArgs)args.CacheItem.Value);
        }

        protected virtual void OnFileSystemEvent(FileSystemEventArgs e)
        {
           FilesytemEvent?.Invoke(this, e);
        }

        public void Dispose()
        {
            _memCache.Dispose();
        }
    }
}