Feature: event system and impression data events

README.md

@@ -71,6 +71,27 @@ When your application shuts down, remember to dispose the unleash instance.
 unleash?.Dispose()
 ```
 
+### Handling events
+Currently supported events:
+- [Impression data events](https://docs.getunleash.io/advanced/impression-data#impression-event-data)
+
+```csharp
+
+var settings = new UnleashSettings()
+{
+    // ...
+};
+
+var unleash = new DefaultUnleash(settings);
+
+// Set up handling of impression events
+unleash.ConfigureEvents(cfg =>
+{
+    cfg.ImpressionEvent = evt => { Console.WriteLine($"{evt.FeatureName}: {evt.Enabled}"); };
+});
+
+```
+
 ### Configuring projects in unleash client
 
 If you're organizing your feature toggles in `Projects` in Unleash Enterprise, you can specify the `ProjectId` on the `UnleashSettings` to select which project to fetch feature toggles for.

src/Unleash/DefaultUnleash.cs

@@ -78,6 +78,8 @@ public DefaultUnleash(UnleashSettings settings, bool overrideDefaultStrategies,
         /// <inheritdoc />
         public ICollection<FeatureToggle> FeatureToggles => services.ToggleCollection.Instance.Features;
 
+        private EventCallbackConfig EventConfig { get; set; }
+
         /// <inheritdoc />
         public bool IsEnabled(string toggleName)
         {
@@ -125,6 +127,9 @@ private bool CheckIsEnabled(string toggleName, UnleashContext context, bool defa
             }
 
             RegisterCount(toggleName, enabled);
+
+            if (featureToggle?.ImpressionData ?? false) EmitImpressionEvent("isEnabled", context, enabled, featureToggle.Name);
+
             return enabled;
         }
 
@@ -146,6 +151,9 @@ public Variant GetVariant(string toggleName, UnleashContext context, Variant def
             var variant = enabled ? VariantUtils.SelectVariant(toggle, context, defaultValue) : defaultValue;
 
             RegisterVariant(toggleName, variant);
+
+            if (toggle?.ImpressionData ?? false) EmitImpressionEvent("getVariant", context, enabled, toggle.Name, variant.Name);
+
             return variant;
         }
 
@@ -235,6 +243,52 @@ private IEnumerable<Constraint> ResolveConstraints(ActivationStrategy activation
             }
         }
 
+        public void ConfigureEvents(Action<EventCallbackConfig> callback)
+        {
+            if (callback == null)
+            {
+                Logger.Error($"UNLEASH: Unleash->ConfigureEvents parameter callback is null");
+                return;
+            }
+
+            try
+            {
+                var evtConfig = new EventCallbackConfig();
+                callback(evtConfig);
+                EventConfig = evtConfig;
+            }
+            catch (Exception ex)
+            {
+                Logger.Error($"UNLEASH: Unleash->ConfigureEvents executing callback threw exception: {ex.Message}");
+            }
+        }
+
+        private void EmitImpressionEvent(string type, UnleashContext context, bool enabled, string name, string variant = null)
+        {
+            if (EventConfig.ImpressionEvent == null)
+            {
+                Logger.Error($"UNLEASH: Unleash->ImpressionData callback is null, unable to emit event");
+                return;
+            }
+
+            try
+            {
+                EventConfig.ImpressionEvent(new ImpressionEvent
+                {
+                    Type = type,
+                    Context = context,
+                    EventId = Guid.NewGuid().ToString(),
+                    Enabled = enabled,
+                    FeatureName = name,
+                    Variant = variant
+                });
+            }
+            catch (Exception ex)
+            {
+                Logger.Error($"UNLEASH: Emitting impression event callback threw exception: {ex.Message}");
+            }
+        }
+
         public void Dispose()
         {
             services?.Dispose(); 

src/Unleash/IUnleash.cs

@@ -82,5 +82,7 @@ public interface IUnleash : IDisposable
         /// /// <param name="context">The Unleash context to evaluate the toggle state against.</param>
         /// <returns>A list of available variants.</returns>
         IEnumerable<VariantDefinition> GetVariants(string toggleName, UnleashContext context);
+
+        void ConfigureEvents(Action<EventCallbackConfig> config);
     }
 }

src/Unleash/Internal/EventCallbackConfig.cs

@@ -0,0 +1,11 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Unleash.Internal
+{
+    public class EventCallbackConfig
+    {
+        public Action<ImpressionEvent> ImpressionEvent { get; set; }
+    }
+}

src/Unleash/Internal/FeatureToggle.cs

@@ -5,25 +5,28 @@ namespace Unleash.Internal
 {
     public class FeatureToggle
     {
-        public FeatureToggle(string name, string type, bool enabled, List<ActivationStrategy> strategies, List<VariantDefinition> variants = null)
+        public FeatureToggle(string name, string type, bool enabled, bool impressionData, List<ActivationStrategy> strategies, List<VariantDefinition> variants = null)
         {
             Name = name;
             Type = type;
             Enabled = enabled;
+            ImpressionData = impressionData;
             Strategies = strategies;
             Variants = variants ?? new List<VariantDefinition>();
         }
 
         public string Name { get; }
         public string Type { get; }
         public bool Enabled { get; }
+        public bool ImpressionData { get; }
+
         public List<ActivationStrategy> Strategies { get; }
 
         public List<VariantDefinition> Variants { get; }
 
         public override string ToString()
         {
-            return $"FeatureToggle{{name=\'{Name}{'\''}, enabled={Enabled}, strategies=\'{Strategies}{'\''}{'}'}";
+            return $"FeatureToggle{{name=\'{Name}{'\''}, enabled={Enabled}, impressionData={ImpressionData}, strategies=\'{Strategies}{'\''}{'}'}";
         }
     }
 }

src/Unleash/Internal/ImpressionEvent.cs

@@ -0,0 +1,16 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Unleash.Internal
+{
+    public class ImpressionEvent
+    {
+        public string Type { get; set; }
+        public string EventId { get; set; }
+        public UnleashContext Context { get; set; }
+        public bool Enabled { get; set; }
+        public string FeatureName { get; set; }
+        public string Variant { get; set; }
+    }
+}

src/Unleash/Serialization/JsonSerializerTester.cs

@@ -14,14 +14,14 @@ public static class JsonSerializerTester
     {
         private static readonly ToggleCollection Toggles = new ToggleCollection(new List<FeatureToggle>
         {
-            new FeatureToggle("Feature1", "release", true, new List<ActivationStrategy>()
+            new FeatureToggle("Feature1", "release", true, false, new List<ActivationStrategy>()
             {
                 new ActivationStrategy("remoteAddress", new Dictionary<string, string>()
                 {
                     {"IPs", "127.0.0.1"}
                 })
             }),
-            new FeatureToggle("feature2", "release", false, new List<ActivationStrategy>()
+            new FeatureToggle("feature2", "release", false, false, new List<ActivationStrategy>()
             {
                 new ActivationStrategy("userWithId", new Dictionary<string, string>()
                 {

tests/Unleash.Tests/App_Data/impressiondata-v2.json

@@ -0,0 +1,30 @@
+{
+  "version": 2,
+  "features": [
+    {
+      "name": "Tests only",
+      "description": "Where name has test in it",
+      "enabled": true,
+      "impressionData": true,
+      "strategies": [
+        {
+          "name": "default",
+          "parameters": {},
+          "segments": [ 1 ]
+        }
+      ]
+    }
+  ],
+  "segments": [
+    {
+      "id": 1,
+      "constraints": [
+        {
+          "contextName": "name",
+          "operator": "STR_CONTAINS",
+          "value": "test"
+        }
+      ]
+    }
+  ]
+}

tests/Unleash.Tests/DefaultUnleashTests.cs

@@ -0,0 +1,31 @@
+using FluentAssertions;
+using NUnit.Framework;
+using System;
+
+namespace Unleash.Tests
+{
+    public class DefaultUnleashTests
+    {
+        [Test]
+        public void ConfigureEvents_should_invoke_callback()
+        {
+            // Arrange
+            var settings = new UnleashSettings
+            {
+                AppName = "testapp",
+            };
+
+            var unleash = new DefaultUnleash(settings);
+            var callbackCalled = false;
+
+            // Act
+            unleash.ConfigureEvents(cfg =>
+            {
+                callbackCalled = true;
+            });
+
+            // Assert
+            callbackCalled.Should().BeTrue();
+        }
+    }
+}

tests/Unleash.Tests/Internal/CachedFilesLoader_Bootstrap_Tests.cs

@@ -17,7 +17,7 @@ private static ToggleCollection GetTestToggles()
         {
             return new ToggleCollection(new List<FeatureToggle>
             {
-                new FeatureToggle("one-enabled",  "release", true, new List<ActivationStrategy>()
+                new FeatureToggle("one-enabled",  "release", true, false, new List<ActivationStrategy>()
                 {
                     new ActivationStrategy("userWithId", new Dictionary<string, string>(){
                         {"userIds", "userA" }
@@ -29,7 +29,7 @@ private static ToggleCollection GetTestToggles()
                     new VariantDefinition("Ab", 34, null, new List<VariantOverride>{ new VariantOverride("context", new[] { "a", "b"}) }),
                 }
                 ),
-                new FeatureToggle("one-disabled",  "release", false, new List<ActivationStrategy>()
+                new FeatureToggle("one-disabled",  "release", false, false, new List<ActivationStrategy>()
                 {
                     new ActivationStrategy("userWithId", new Dictionary<string, string>()
                     {