greenshot/GreenshotPlugin/Core/Cache.cs

/*
 * Greenshot - a free and open source screenshot tool
 * Copyright (C) 2007-2016 Thomas Braun, Jens Klingen, Robin Krom
 * 
 * For more information see: http://getgreenshot.org/
 * The Greenshot project is hosted on GitHub https://github.com/greenshot/greenshot
 * 
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 1 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
using System;
using System.Collections.Generic;
using System.Timers;
using log4net;

namespace GreenshotPlugin.Core {
	/// <summary>
	/// Cache class 
	/// </summary>
	/// <typeparam name="TK">Type of key</typeparam>
	/// <typeparam name="TV">Type of value</typeparam>
	public class Cache<TK, TV> {
		private static readonly ILog Log = LogManager.GetLogger(typeof(Cache<TK, TV>));
		private readonly IDictionary<TK, TV> _internalCache = new Dictionary<TK, TV>();
		private readonly object _lockObject = new object();
		private readonly int _secondsToExpire = 10;
		private readonly CacheObjectExpired _expiredCallback;
		public delegate void CacheObjectExpired(TK key, TV cacheValue);

		/// <summary>
		/// Initialize the cache
		/// </summary>
		public Cache() {
		}
		/// <summary>
		/// Initialize the cache
		/// </summary>
		/// <param name="expiredCallback"></param>
		public Cache(CacheObjectExpired expiredCallback) : this() {
			_expiredCallback = expiredCallback;
		}

		/// <summary>
		/// Initialize the cache with a expire setting
		/// </summary>
		/// <param name="secondsToExpire"></param>
		public Cache(int secondsToExpire) : this() {
			_secondsToExpire = secondsToExpire;
		}

		/// <summary>
		/// Initialize the cache with a expire setting
		/// </summary>
		/// <param name="secondsToExpire"></param>
		/// <param name="expiredCallback"></param>
		public Cache(int secondsToExpire, CacheObjectExpired expiredCallback) : this(expiredCallback) {
			_secondsToExpire = secondsToExpire;
		}

		/// <summary>
		/// Enumerable for the values in the cache
		/// </summary>
		public IEnumerable<TV> Elements {
			get {
				List<TV> elements = new List<TV>();

				lock (_lockObject)
				{
					foreach (TV element in _internalCache.Values) {
						elements.Add(element);
					}
				}
				foreach (TV element in elements) {
					yield return element;
				}
			}
		}

		/// <summary>
		/// Get the value by key from the cache
		/// </summary>
		/// <param name="key"></param>
		/// <returns></returns>
		public TV this[TK key] {
			get {
				TV result = default(TV);
				lock (_lockObject) {
					if (_internalCache.ContainsKey(key)) {
						result = _internalCache[key];
					}
				}
				return result;
			}
		}

		/// <summary>
		/// Contains
		/// </summary>
		/// <param name="key"></param>
		/// <returns>true if the cache contains the key</returns>
		public bool Contains(TK key)
		{
			lock (_lockObject)
			{
				return _internalCache.ContainsKey(key);
			}
		}

		/// <summary>
		/// Add a value to the cache
		/// </summary>
		/// <param name="key"></param>
		/// <param name="value"></param>
		public void Add(TK key, TV value) {
			Add(key, value, null);
		}

		/// <summary>
		/// Add a value to the cache
		/// </summary>
		/// <param name="key"></param>
		/// <param name="value"></param>
		/// <param name="secondsToExpire">optional value for the seconds to expire</param>
		public void Add(TK key, TV value, int? secondsToExpire) {
			lock (_lockObject) {
				var cachedItem = new CachedItem(key, value, secondsToExpire ?? _secondsToExpire);
				cachedItem.Expired += delegate(TK cacheKey, TV cacheValue) {
					if (_internalCache.ContainsKey(cacheKey)) {
						Log.DebugFormat("Expiring object with Key: {0}", cacheKey);
						_expiredCallback?.Invoke(cacheKey, cacheValue);
						Remove(cacheKey);
					} else {
						Log.DebugFormat("Expired old object with Key: {0}", cacheKey);
					}
				};

				if (_internalCache.ContainsKey(key)) {
					_internalCache[key] = value;
					Log.DebugFormat("Updated item with Key: {0}", key);
				} else {
					_internalCache.Add(key, cachedItem);
					Log.DebugFormat("Added item with Key: {0}", key);
				}
			}
		}

		/// <summary>
		/// Remove item from cache
		/// </summary>
		/// <param name="key"></param>
		public void Remove(TK key) {
			lock (_lockObject) {
				if (!_internalCache.ContainsKey(key)) {
					throw new ApplicationException($"An object with key ‘{key}’ does not exists in cache");
				}
				_internalCache.Remove(key);
				Log.DebugFormat("Removed item with Key: {0}", key);
			}
		}

		/// <summary>
		/// A cache item
		/// </summary>
		private class CachedItem {
			public event CacheObjectExpired Expired;
			private readonly int _secondsToExpire;
			private readonly Timer _timerEvent;

			public CachedItem(TK key, TV item, int secondsToExpire) {
				if (key == null) {
					throw new ArgumentNullException(nameof(key));
				}
				Key = key;
				Item = item;
				_secondsToExpire = secondsToExpire;
				if (secondsToExpire <= 0)
				{
					return;
				}
				_timerEvent = new Timer(secondsToExpire * 1000) { AutoReset = false };
				_timerEvent.Elapsed += timerEvent_Elapsed;
				_timerEvent.Start();
			}

			private void ExpireNow() {
				_timerEvent.Stop();
				if (_secondsToExpire > 0) {
					Expired?.Invoke(Key, Item);
				}
			}

			private void timerEvent_Elapsed(object sender, ElapsedEventArgs e) {
				ExpireNow();
			}

			public TK Key { get; private set; }
			public TV Item { get; private set; }

			public static implicit operator TV(CachedItem a) {
				return a.Item;
			}
		}
	}
}