hive-events

Async event system for Clojure - re-frame patterns for JVM.

Overview

hive-events brings re-frame's elegant event-driven architecture to Clojure/JVM with first-class async support via core.async. It provides:

• Interceptor chain - Composable middleware for event processing
• Effects system - Declarative side effects (:db, :dispatch, custom)
• Coeffects system - Dependency injection for pure handlers
• Async dispatch - Event queuing with core.async channels
• Advanced patterns - Debounce, throttle, retry with backoff, sagas

Installation

Add to your deps.edn:

{:deps {io.github.BuddhiLW/hive-events {:git/tag "v0.1.0" :git/sha "..."}}}

Quick Start

(require '[hive.events :as ev])

;; Initialize with your app state atom
(ev/init! (atom {:count 0}))

;; Register a simple event handler
(ev/reg-event-db :counter/inc
  (fn [db [_ amount]]
    (update db :count + (or amount 1))))

;; Dispatch events
(ev/dispatch [:counter/inc 5])
(ev/dispatch-sync [:counter/inc 1])  ; synchronous

Event Handlers

Two types of handlers are supported:

reg-event-db

For handlers that only update the database:

(ev/reg-event-db :user/set-name
  (fn [db [_ name]]
    (assoc db :user-name name)))

reg-event-fx

For handlers that produce multiple effects:

(ev/reg-event-fx :user/fetch
  (fn [{:keys [db]} [_ user-id]]
    {:db (assoc db :loading? true)
     :http {:url (str "/api/users/" user-id)
            :on-success :user/fetch-success}}))

Effects (fx)

Register custom effect handlers:

(ev/reg-fx :http
  (fn [{:keys [url method on-success]}]
    ;; perform HTTP request
    ;; dispatch on-success event with response
    ))

(ev/reg-fx :local-storage
  (fn [{:keys [key value]}]
    (.setItem js/localStorage key value)))

Built-in effects:

• :db - Update application state
• :dispatch - Dispatch another event
• :dispatch-n - Dispatch multiple events
• :dispatch-later - Dispatch after delay

Coeffects (cofx)

Inject dependencies into handlers:

(ev/reg-cofx :now
  (fn [coeffects]
    (assoc coeffects :now (System/currentTimeMillis))))

(ev/reg-event-fx :log/timestamp
  [(ev/inject-cofx :now)]
  (fn [{:keys [db now]} _]
    {:db (assoc db :last-timestamp now)}))

Built-in coeffects:

• :now - Current timestamp
• :random - Random number
• :uuid - Random UUID string

Interceptors

Compose cross-cutting concerns:

(def validate-user
  (ev/->interceptor
    :id :validate-user
    :before (fn [ctx]
              (let [user (get-in ctx [:coeffects :db :user])]
                (if (valid? user)
                  ctx
                  (assoc ctx :effects {:dispatch [:error/invalid-user]}))))))

(ev/reg-event-fx :user/update
  [validate-user ev/debug]
  (fn [{:keys [db]} [_ changes]]
    {:db (update db :user merge changes)}))

Built-in interceptors:

• debug - Log event and effects
• trim-v - Remove event id from vector
• path - Focus handler on db path

Async Extensions (JVM only)

(require '[hive.events.async :as async])

;; Dispatch with timeout
(async/dispatch-with-timeout [:slow/operation] 5000)

;; Retry with exponential backoff
(async/dispatch-with-retry [:flaky/api-call]
  {:max-retries 3
   :initial-delay-ms 100
   :retry-pred (fn [ctx] (get-in ctx [:effects :error]))})

;; Debounced dispatch (search-as-you-type)
(async/dispatch-debounced :search [:search/query "foo"] 300)

;; Throttled dispatch (scroll handler)
(async/dispatch-throttled :scroll [:ui/position pos] 100)

;; Pub/sub event streams
(let [ch (async/subscribe-events :user/login)]
  (go-loop []
    (when-let [event (<! ch)]
      (println "Login:" event)
      (recur))))

;; Sagas for complex workflows
(async/saga [:order/created :payment/completed]
  (fn [event state]
    (case (first event)
      :order/created
      {:state (assoc state :order (second event))
       :dispatch [:payment/initiate (:id (second event))]}

      :payment/completed
      {:state (assoc state :paid? true)
       :dispatch [:fulfillment/start (:order-id state)]})))

Architecture

hive.events (facade)
├── hive.events.interceptor  ; Interceptor chain
├── hive.events.fx           ; Effects registry
├── hive.events.cofx         ; Coeffects registry
├── hive.events.router       ; Event dispatch
└── hive.events.async        ; Async patterns (JVM)

Design Principles

• Events as data - Events are vectors like [:event-id & args]
• Pure handlers - Handlers receive data, return data
• Declarative effects - Side effects described, not performed
• Composable interceptors - AOP-style middleware
• Async-first - Built on core.async for JVM

Related Projects

• re-frame - Original inspiration (ClojureScript)
• domino - Declarative data flow

License

MIT