Events

Lifecycle Events

Overview

Four event interfaces let you react to client lifecycle changes. Multiple handlers can be registered for each event; they are all invoked sequentially.

Interface	Builder method	Triggered when
IMqttClientConnectedEvent	.OnClientConnected<T>()	A client successfully connects
IMqttClientDisconnectedEvent	.OnClientDisconnected<T>()	A client disconnects
IMqttClientSubscribedTopicEvent	.OnClientSubscribedTopic<T>()	A client subscribes to a topic filter
IMqttClientUnsubscribedTopicEvent	.OnClientUnsubscribedTopic<T>()	A client unsubscribes from a topic filter

---

IMqttClientConnectedEvent

public interface IMqttClientConnectedEvent
{
    Task OnClientConnectedAsync(string clientId);
}

Example — notify a presence service:

public class PresenceConnectedHandler(IPresenceService presence) : IMqttClientConnectedEvent
{
    public Task OnClientConnectedAsync(string clientId)
        => presence.SetOnlineAsync(clientId);
}

---

IMqttClientDisconnectedEvent

public interface IMqttClientDisconnectedEvent
{
    Task OnClientDisconnectedAsync(string clientId);
}

Example — clean up resources:

public class PresenceDisconnectedHandler(IPresenceService presence) : IMqttClientDisconnectedEvent
{
    public Task OnClientDisconnectedAsync(string clientId)
        => presence.SetOfflineAsync(clientId);
}

---

IMqttClientSubscribedTopicEvent

public interface IMqttClientSubscribedTopicEvent
{
    Task OnClientSubscribedTopicAsync(string clientId, string topicFilter);
}

Example — audit log:

public class SubscribeAuditHandler(IAuditLog audit) : IMqttClientSubscribedTopicEvent
{
    public Task OnClientSubscribedTopicAsync(string clientId, string topicFilter)
        => audit.RecordAsync($"SUBSCRIBE {clientId} → {topicFilter}");
}

---

IMqttClientUnsubscribedTopicEvent

public interface IMqttClientUnsubscribedTopicEvent
{
    Task OnClientUnsubscribedTopicAsync(string clientId, string topicFilter);
}

---

Registration

Chain all the handlers you need:

builder.Services
    .AddMqttServer(configuration)
    .WithControllers<GeneratedMqttControllerRegistration>()
    .WithAuthentication<MyAuthProvider>()
    .OnClientConnected<PresenceConnectedHandler>()
    .OnClientConnected<AuditConnectedHandler>()         // two handlers for the same event
    .OnClientDisconnected<PresenceDisconnectedHandler>()
    .OnClientSubscribedTopic<SubscribeAuditHandler>()
    .OnClientUnsubscribedTopic<UnsubscribeAuditHandler>();

All handlers are registered as scoped services and resolved from a new scope per event.