feat: add an topic event enumerable

src/Momento.Sdk/Internal/ScsTopicClient.cs

@@ -198,7 +198,7 @@ public async Task Subscribe()
             _subscribed = true;
         }
 
-        public async ValueTask<TopicMessage?> GetNextRelevantMessageFromGrpcStreamAsync(
+        public async ValueTask<ITopicEvent?> GetNextRelevantMessageFromGrpcStreamAsync(
             CancellationToken cancellationToken)
         {
             while (!cancellationToken.IsCancellationRequested)

src/Momento.Sdk/Responses/Topic/ITopicEvent.cs

@@ -0,0 +1,12 @@
+namespace Momento.Sdk.Responses;
+
+/// <summary>
+/// Represents an event that can be published to a topic.
+/// 
+/// This includes not just topic mesasges, but also system events
+/// such as discontinuities and heartbeats.
+/// </summary>
+public interface ITopicEvent
+{
+
+}

src/Momento.Sdk/Responses/Topic/TopicMessage.cs

@@ -1,5 +1,6 @@
 using Momento.Protos.CacheClient.Pubsub;
 using Momento.Sdk.Exceptions;
+using Momento.Sdk.Internal.ExtensionMethods;
 
 namespace Momento.Sdk.Responses;
 
@@ -31,7 +32,7 @@ namespace Momento.Sdk.Responses;
 /// }
 /// </code>
 /// </summary>
-public abstract class TopicMessage
+public abstract class TopicMessage : ITopicEvent
 {
     /// <summary>
     /// A topic message containing a text value.
@@ -63,6 +64,12 @@ public Text(_TopicValue topicValue, long topicSequenceNumber, string? tokenId =
         /// This can be used to securely identify the sender of a message.
         /// </summary>
         public string? TokenId { get; }
+
+        /// <inheritdoc />
+        public override string ToString()
+        {
+            return $"{base.ToString()}: Value: \"{this.Value.Truncate()}\" SequenceNumber: {this.TopicSequenceNumber} TokenId: \"{this.TokenId}\"";
+        }
     }
 
     /// <summary>
@@ -95,6 +102,12 @@ public Binary(_TopicValue topicValue, long topicSequenceNumber, string? tokenId
         /// This can be used to securely identify the sender of a message.
         /// </summary>
         public string? TokenId { get; }
+
+        /// <inheritdoc />
+        public override string ToString()
+        {
+            return $"{base.ToString()}: Value: \"{Value.ToPrettyHexString().Truncate()}\" SequenceNumber: {this.TopicSequenceNumber} TokenId: \"{this.TokenId}\"";
+        }
     }
 
     /// <include file="../../docs.xml" path='docs/class[@name="Error"]/description/*' />

src/Momento.Sdk/Responses/Topic/TopicSubscribeResponse.cs

@@ -43,14 +43,14 @@ public abstract class TopicSubscribeResponse
     /// </summary>
     public class Subscription : TopicSubscribeResponse, IDisposable, IAsyncEnumerable<TopicMessage?>
     {
-        private readonly Func<CancellationToken, ValueTask<TopicMessage?>> _moveNextFunction;
+        private readonly Func<CancellationToken, ValueTask<ITopicEvent?>> _moveNextFunction;
         private CancellationTokenSource _subscriptionCancellationToken = new();
         private readonly Action _disposalAction;
 
         /// <summary>
         /// Constructs a Subscription with a wrapped topic iterator and an action to dispose of it.
         /// </summary>
-        public Subscription(Func<CancellationToken, ValueTask<TopicMessage?>> moveNextFunction, Action disposalAction)
+        public Subscription(Func<CancellationToken, ValueTask<ITopicEvent?>> moveNextFunction, Action disposalAction)
         {
             _moveNextFunction = moveNextFunction;
             _disposalAction = disposalAction;
@@ -78,12 +78,12 @@ public void Dispose()
 
     private class TopicMessageEnumerator : IAsyncEnumerator<TopicMessage?>
     {
-        private readonly Func<CancellationToken, ValueTask<TopicMessage?>> _moveNextFunction;
+        private readonly Func<CancellationToken, ValueTask<ITopicEvent?>> _moveNextFunction;
         private readonly CancellationToken _subscriptionCancellationToken;
         private readonly CancellationToken _enumeratorCancellationToken;
 
         public TopicMessageEnumerator(
-            Func<CancellationToken, ValueTask<TopicMessage?>> moveNextFunction,
+            Func<CancellationToken, ValueTask<ITopicEvent?>> moveNextFunction,
             CancellationToken subscriptionCancellationToken,
             CancellationToken enumeratorCancellationToken)
         {
@@ -107,10 +107,10 @@ public async ValueTask<bool> MoveNextAsync()
             {
                 case TopicMessage.Text:
                 case TopicMessage.Binary:
-                    Current = nextMessage;
+                    Current = (TopicMessage)nextMessage;
                     return true;
                 case TopicMessage.Error:
-                    Current = nextMessage;
+                    Current = (TopicMessage)nextMessage;
                     return false;
                 default:
                     Current = null;

src/Momento.Sdk/Responses/Topic/TopicSystemEvent.cs

@@ -0,0 +1,56 @@
+namespace Momento.Sdk.Responses;
+
+/// <summary>
+/// Represents a system event that can be published to a topic.
+/// </summary>
+public abstract class TopicSystemEvent : ITopicEvent
+{
+    /// <summary>
+    /// Represents a heartbeat event.
+    /// </summary>
+    public class Heartbeat : TopicSystemEvent
+    {
+        /// <summary>
+        /// Constructs a new heartbeat event.
+        /// </summary>
+        public Heartbeat()
+        {
+
+        }
+
+        /// <inheritdoc/>
+        public override string ToString() => base.ToString() ?? "Heartbeat";
+    }
+
+    /// <summary>
+    /// Represents a discontinuity event.
+    /// </summary>
+    public class Discontinuity : TopicSystemEvent
+    {
+        /// <summary>
+        /// Constructs a new discontinuity event.
+        /// </summary>
+        /// <param name="lastKnownSequenceNumber">The last known sequence number before the discontinuity.</param>
+        /// <param name="sequenceNumber">The sequence number of the discontinuity.</param>
+        public Discontinuity(long lastKnownSequenceNumber, long sequenceNumber)
+        {
+            LastKnownSequenceNumber = lastKnownSequenceNumber;
+            SequenceNumber = sequenceNumber;
+        }
+
+        /// <summary>
+        /// The last known sequence number before the discontinuity.
+        /// </summary>
+        public long LastKnownSequenceNumber { get; }
+        /// <summary>
+        /// The sequence number of the discontinuity.
+        /// </summary>
+        public long SequenceNumber { get; }
+
+        /// <inheritdoc/>
+        public override string ToString()
+        {
+            return $"{base.ToString()}: LastKnownSequenceNumber: {LastKnownSequenceNumber} SequenceNumber: {SequenceNumber}";
+        }
+    }
+}