Attribute Macro handler 

#[handler]

Creates an event handler for use with event subscriptions

The #[evento::handler(AggregateType)] attribute macro transforms a function into an event handler that can be used with [evento::subscribe]. The macro generates the necessary boilerplate to integrate with the subscription system.

Syntax

#[evento::handler(AggregateType)]
async fn handler_name<E: evento::Executor>(
    context: &evento::Context<'_, E>,
    event: EventDetails<EventType>,
) -> anyhow::Result<()> {
    // Handler logic
    Ok(())
}

Parameters

• AggregateType: The aggregate type this handler is associated with

Function Requirements

The decorated function must:

• Be async
• Take &evento::Context<'_, E> as first parameter where E: evento::Executor
• Take EventDetails<SomeEventType> as second parameter
• Return anyhow::Result<()>

Examples

use evento::{Context, EventDetails, Executor};
use bincode::{Encode, Decode};


#[evento::handler(User)]
async fn on_user_created<E: Executor>(
    context: &Context<'_, E>,
    event: EventDetails<UserCreated>,
) -> anyhow::Result<()> {
    println!("User created: {}", event.data.name);
     
    // Can trigger side effects, call external services, etc.
     
    Ok(())
}

// Use with subscription
evento::subscribe("user-handlers")
    .aggregator::<User>()
    .handler(on_user_created())
    .run(&executor)
    .await?;

Generated Code

The macro generates:

• A struct implementing [evento::SubscribeHandler]
• A constructor function returning an instance of that struct
• Type-safe event filtering based on the event type