Added "itemEvicted" callback, fixes #6 fixes #11

Author: alex-jitbit

FastCache/FastCache.cs

@@ -15,13 +15,22 @@ public class FastCache<TKey, TValue> : IEnumerable<KeyValuePair<TKey, TValue>>,
 		private readonly ConcurrentDictionary<TKey, TtlValue> _dict = new ConcurrentDictionary<TKey, TtlValue>();
 
 		private readonly Timer _cleanUpTimer;
+		private readonly EvictionCallback _itemEvicted;
+
+		/// <summary>
+		/// Callback (RUNS ON THREAD POOL!) when an item is evicted from the cache.
+		/// </summary>
+		/// <param name="key"></param>
+		public delegate void EvictionCallback(TKey key);
 
 		/// <summary>
 		/// Initializes a new empty instance of <see cref="FastCache{TKey,TValue}"/>
 		/// </summary>
 		/// <param name="cleanupJobInterval">cleanup interval in milliseconds, default is 10000</param>
-		public FastCache(int cleanupJobInterval = 10000)
+		/// <param name="itemEvicted">Optional callback (RUNS ON THREAD POOL!) when an item is evicted from the cache</param>
+		public FastCache(int cleanupJobInterval = 10000, EvictionCallback itemEvicted = null)
 		{
+			_itemEvicted = itemEvicted;
 			_cleanUpTimer = new Timer(s => { _ = EvictExpiredJob(); }, null, cleanupJobInterval, cleanupJobInterval);
 		}
 
@@ -65,7 +74,10 @@ public void EvictExpired()
 					foreach (var p in _dict)
 					{
 						if (p.Value.IsExpired(currTime)) //call IsExpired with "currTime" to avoid calling Environment.TickCount64 multiple times
+						{
 							_dict.TryRemove(p);
+							OnEviction(p.Key);
+						}
 					}
 				}
 				finally
@@ -151,6 +163,8 @@ public bool TryGet(TKey key, out TValue value)
 				 * 
 				 * */
 
+				OnEviction(key);
+
 				return false;
 			}
 
@@ -188,7 +202,8 @@ private TValue GetOrAddCore(TKey key, Func<TValue> valueFactory, TimeSpan ttl)
 			//since TtlValue is a reference type we can update its properties in-place, instead of removing and re-adding to the dictionary (extra lookups)
 			if (!wasAdded) //performance hack: skip expiration check if a brand item was just added
 			{
-				ttlValue.ModifyIfExpired(valueFactory, ttl);
+				if (ttlValue.ModifyIfExpired(valueFactory, ttl))
+					OnEviction(key);
 			}
 
 			return ttlValue.Value;
@@ -259,6 +274,22 @@ IEnumerator IEnumerable.GetEnumerator()
 			return this.GetEnumerator();
 		}
 
+		private void OnEviction(TKey key)
+		{
+			if (_itemEvicted == null) return;
+
+			Task.Run(() => //run on thread pool to avoid blocking
+			{
+				try
+				{
+					_itemEvicted(key);
+				}
+				catch {
+					var i = 0;
+				} //to prevent any exceptions from crashing the thread
+			});
+		}
+
 		private class TtlValue
 		{
 			public TValue Value { get; private set; }
@@ -278,14 +309,17 @@ public TtlValue(TValue value, TimeSpan ttl)
 			/// <summary>
 			/// Updates the value and TTL only if the item is expired
 			/// </summary>
-			public void ModifyIfExpired(Func<TValue> newValueFactory, TimeSpan newTtl)
+			/// <returns>True if the item expired and was updated, otherwise false</returns>
+			public bool ModifyIfExpired(Func<TValue> newValueFactory, TimeSpan newTtl)
 			{
 				var ticks = Environment.TickCount64; //save to a var to prevent multiple calls to Environment.TickCount64
 				if (IsExpired(ticks)) //if expired - update the value and TTL
 				{
 					TickCountWhenToKill = ticks + (long)newTtl.TotalMilliseconds; //update the expiration time first for better concurrency
 					Value = newValueFactory();
+					return true;
 				}
+				return false;
 			}
 		}
 

UnitTests/EvictionCallbackTests.cs

@@ -0,0 +1,80 @@
+using System;
+using Jitbit.Utils;
+
+namespace UnitTests;
+
+[TestClass]
+public class EvictionCallbackTests
+{
+    private List<string> _evictedKeys;
+    private FastCache<string, string> _cache;
+
+    [TestInitialize]
+    public void Setup()
+    {
+        _evictedKeys = new List<string>();
+        _cache = new FastCache<string, string>(
+            cleanupJobInterval: 100, 
+            itemEvicted: key => _evictedKeys.Add(key));
+    }
+
+    [TestMethod]
+    public async Task WhenItemExpires_EvictionCallbackFires()
+    {
+        // Arrange
+        var key = "test-key";
+        _cache.AddOrUpdate(key, "value", TimeSpan.FromMilliseconds(1));
+
+        // Act
+        await Task.Delay(110); // Wait for expiration
+
+        // Assert
+        Assert.AreEqual(1, _evictedKeys.Count);
+        Assert.AreEqual(key, _evictedKeys[0]);
+    }
+
+    [TestMethod]
+    public async Task WhenMultipleItemsExpire_CallbackFiresForEach()
+    {
+        // Arrange
+        var keys = new[] { "key1", "key2", "key3" };
+        foreach (var key in keys)
+        {
+            _cache.AddOrUpdate(key, "value", TimeSpan.FromMilliseconds(1));
+        }
+
+        // Act
+		await Task.Delay(5); // Wait for 1ms expiration
+        _cache.EvictExpired();
+		await Task.Delay(5); // Wait for callback to finish on another thread
+
+        // Assert
+        CollectionAssert.AreEquivalent(keys, _evictedKeys);
+    }
+
+    [TestMethod]
+    public void WhenItemNotExpired_CallbackDoesNotFire()
+    {
+        // Arrange
+        _cache.AddOrUpdate("key", "value", TimeSpan.FromMinutes(1));
+
+        // Act
+        _cache.EvictExpired();
+
+        // Assert
+        Assert.AreEqual(0, _evictedKeys.Count);
+    }
+
+    [TestMethod]
+    public async Task AutomaticCleanup_FiresCallback()
+    {
+        // Arrange
+        _cache.AddOrUpdate("key", "value", TimeSpan.FromMilliseconds(1));
+
+        // Act
+        await Task.Delay(110); // Wait for cleanup job
+
+        // Assert
+        Assert.AreEqual(1, _evictedKeys.Count);
+    }
+}