EventBus Trait

pub struct Event {
    pub id: String,              // Unique event ID (UUID)
    pub topic: String,           // "sensor.temperature", "device.status"
    pub data: HashMap<String, String>,  // Flexible key-value payload
    pub source: String,          // Source of the event
    pub priority: EventPriority, // Low, Normal, High, Critical
    pub timestamp: DateTime<Utc>, // When event was created
}

pub enum EventPriority {
    Low,
    Normal,    // default
    High,
    Critical,
}

pub trait EventBus: Send + Sync {
    fn publish(&self, event: Event) -> Result<()>;
    fn subscribe(&self, topic_pattern: &str, handler: EventHandler) -> Result<String>;
    fn unsubscribe(&self, subscription_id: &str) -> Result<bool>;
    fn pending_count(&self) -> usize;
    fn drain(&self) -> Result<usize>;
    fn recent_events(&self, limit: usize) -> Result<Vec<Event>>;
}

bus.publish(Event::new("sensor.temperature", "sensor-1")
    .with_data("value", "42.5")
    .with_priority(EventPriority::High))?;

let sub_id = bus.subscribe("sensor.*", Box::new(|event| {
    println!("Sensor event: {}", event.topic);
    None  // No response event
}))?;

pub type EventHandler = Box<dyn Fn(&Event) -> Option<Event> + Send + Sync>;

bus.unsubscribe(&sub_id)?;

let count = bus.pending_count();

let processed = bus.drain()?;

let events = bus.recent_events(10)?;  // Last 10 events

let event = Event::new("sensor.temperature", "sensor-1")
    .with_data("value", "42.5")
    .with_data("unit", "celsius")
    .with_priority(EventPriority::High);

let bus = NoopEventBus::new();
// All operations succeed but do nothing
bus.publish(event)?;  // OK, ignored
assert_eq!(bus.pending_count(), 0);
assert_eq!(bus.drain()?, 0);