Event System

Overview

Lemmego includes a simple pub/sub event system that allows you to hook into the application lifecycle and communicate between components without tight coupling.

Using the EventEmitter

The App instance implements the EventEmitter interface:

type EventEmitter interface {
    On(event string, listener EventListener)
    Dispatch(event string, payload ...any) error
}

type EventListener func(payload any) error

Listening to Events

func (p *MyProvider) Provide(a app.App) error {
    a.On(app.ServerStarted, func(payload any) error {
        slog.Info("Server is now listening!")
        return nil
    })
    return nil
}

Built-in Lifecycle Events

The framework dispatches events during bootstrapping:

Event Constant	When It Fires
`app.MiddlewareRegistering`	Before middleware is registered
`app.MiddlewareRegistered`	After middleware is registered
`app.RoutesRegistering`	Before routes are registered
`app.RoutesRegistered`	After routes are registered
`app.CommandsRegistering`	Before CLI commands are registered
`app.CommandsRegistered`	After CLI commands are registered
`app.ServicesRegistering`	Before providers run
`app.ServicesRegistered`	After all providers have run
`app.ServerStarted`	After the HTTP server starts listening

Dispatching Custom Events

// Dispatch a custom event
a.Dispatch("user.registered", user)

// Listen for custom events
a.On("user.registered", func(payload any) error {
    user := payload.(*User)
    return sendWelcomeEmail(user.Email)
})

Multiple Listeners

Multiple listeners can register for the same event. They all receive the payload when the event is dispatched:

a.On("user.registered", sendWelcomeEmail)
a.On("user.registered", logRegistration)
a.On("user.registered", notifyAdmins)

Shutdown Behavior

When the application shuts down gracefully:

New event registrations are blocked
Pending event dispatches complete
All ShutdownProvider instances are notified

a.On(app.ServerStarted, func(payload any) error {
    // This won't run if shutdown has begun
    return nil
})

Error Handling

If a listener returns an error, the dispatch continues to other listeners. The error is not propagated:

a.On("user.registered", func(payload any) error {
    // Even if this fails, other listeners still run
    return someOperation()
})

Use Cases

Plugin hooks — let plugins react to application events
Cache invalidation — clear caches when data changes
Metrics — record metrics at lifecycle points
Logging — audit log important events
Notifications — send emails, push notifications

Dependency Injection