Writing Event Handlers

Writing Event Handlers

The heart of an autogen application are the event handlers. Agents select a ```TopicSubscription``` to listen for events on a specific topic. When an event is received, the agent's event handler is called with the event data.

Within that event handler you may optionally *emit* new events, which are then sent to the event bus for other agents to process. The EventTypes are declared gRPC ProtoBuf messages that are used to define the schema of the event.  The default protos are available via the ```Microsoft.AutoGen.Contracts;``` namespace and are defined in [autogen/protos](/autogen/protos). The EventTypes are registered in the agent's constructor using the ```IHandle``` interface.

```csharp
TopicSubscription("HelloAgents")]
public class HelloAgent(
    iAgentWorker worker,
    [FromKeyedServices("AgentsMetadata")] AgentsMetadata typeRegistry) : ConsoleAgent(
        worker,
        typeRegistry),
        ISayHello,
        IHandle
,
        IHandle

{
    public async Task Handle(NewMessageReceived item, CancellationToken cancellationToken = default)
    {
        var response = await SayHello(item.Message).ConfigureAwait(false);
        var evt = new Output
        {
            Message = response
        }.ToCloudEvent(this.AgentId.Key);
        await PublishEventAsync(evt).ConfigureAwait(false);
        var goodbye = new ConversationClosed
        {
            UserId = this.AgentId.Key,
            UserMessage = "Goodbye"
        }.ToCloudEvent(this.AgentId.Key);
        await PublishEventAsync(goodbye).ConfigureAwait(false);
    }
```