re-frame/src/re_frame/router.cljc

251 lines
10 KiB
Clojure

(ns re-frame.router
(:require [re-frame.events :refer [handle]]
[re-frame.interop :refer [after-render empty-queue next-tick]]
[re-frame.loggers :refer [console]]))
;; -- Router Loop ------------------------------------------------------------
;;
;; A call to "re-frame.core/dispatch" places an event on a queue for processing.
;; A short time later, the handler registered to handle this event will be run.
;; What follows is the implemtation of this process.
;;
;; The task is to process queued events in a perpetual loop, one after
;; the other, FIFO, calling the registered event-handler for each, being idle when
;; there are no events, and firing up when one arrives.
;;
;; But browsers only have a single thread of control and we must be
;; careful to not hog the CPU. When processing events one after another, we
;; must regularly hand back control to the browser, so it can redraw, process
;; websockets, etc. But not too regularly! If we are in a de-focused browser
;; tab, our app will be CPU throttled. Each time we get back control, we have
;; to process all queued events, or else something like a bursty websocket
;; (producing events) might overwhelm the queue. So there's a balance.
;;
;; The processing/handling of an event happens "asynchronously" sometime after
;; that event was enqueued via "dispatch". The original implementation of this router loop
;; used `core.async`. As a result, it was fairly simple, and it mostly worked,
;; but it did not give enough control. So now we hand-roll our own,
;; finite-state-machine and all.
;;
;; In what follows, the strategy is this:
;; - maintain a FIFO queue of `dispatched` events.
;; - when a new event arrives, "schedule" processing of this queue using
;; goog.async.nextTick, which means it will happen "very soon".
;; - when processing events, one after the other, do ALL the those currently
;; queued. Don't stop. Don't yield to the browser. Hog that CPU.
;; - but if any new events are dispatched during this cycle of processing,
;; don't do them immediately. Leave them queued. Yield first to the browser,
;; and do these new events in the next processing cycle. That way we drain
;; the queue up to a point, but we never hog the CPU forever. In
;; particular, we handle the case where handling one event will beget
;; another event. The freshly begatted event will be handled next cycle,
;; with yielding in-between.
;; - In some cases, an event should not be handled until after the GUI has been
;; updated, i.e., after the next Reagent animation frame. In such a case,
;; the event should be dispatched with :flush-dom metadata like this:
;; (dispatch ^:flush-dom [:event-id other params])
;; Such an event will temporarily block all further processing because
;; events are processed sequentially: we handle one event completely
;; before we handle the ones behind it.
;;
;; Implementation notes:
;; - queue processing can be in a number of states: scheduled, running, paused
;; etc. So it is modeled as a Finite State Machine.
;; See "-fsm-trigger" (below) for the states and transitions.
;; - the scheduling is done via "goog.async.nextTick" which is pretty quick
;; - when the event has :flush-dom metadata we schedule via
;; "reagent.core.after-render"
;; which will run event processing after the next Reagent animation frame.
;;
;; Events can have metadata which says to pause event processing.
;; event metadata -> "run later" functions
(def later-fns
{:flush-dom (fn [f] (after-render #(next-tick f))) ;; one tick after the end of the next annimation frame
:yield next-tick}) ;; almost immediately
;; Event Queue Abstraction
(defprotocol IEventQueue
;; -- API
(push [this event])
(add-post-event-callback [this id callack])
(remove-post-event-callback [this f])
;; -- Implementation via a Finite State Machine
(-fsm-trigger [this trigger arg])
;; -- Finite State Machine actions
(-add-event [this event])
(-process-1st-event-in-queue [this])
(-run-next-tick [this])
(-run-queue [this])
(-exception [this ex])
(-pause [this later-fn])
(-resume [this])
(-call-post-event-callbacks[this event]))
;; Concrete implementation of IEventQueue
(deftype EventQueue [#?(:cljs ^:mutable fsm-state :clj ^:volatile-mutable fsm-state)
#?(:cljs ^:mutable queue :clj ^:volatile-mutable queue)
#?(:cljs ^:mutable post-event-callback-fns :clj ^:volatile-mutable post-event-callback-fns)]
IEventQueue
;; -- API ------------------------------------------------------------------
(push [this event] ;; presumably called by dispatch
(-fsm-trigger this :add-event event))
;; register a callback function which will be called after each event is processed
(add-post-event-callback [_ id callback-fn]
(if (contains? post-event-callback-fns id)
(console :warn "re-frame: overwriting existing post event call back with id: " id))
(->> (assoc post-event-callback-fns id callback-fn)
(set! post-event-callback-fns)))
(remove-post-event-callback [_ id]
(if-not (contains? post-event-callback-fns id)
(console :warn "re-frame: could not remove post event call back with id: " id)
(->> (dissoc post-event-callback-fns id)
(set! post-event-callback-fns))))
;; -- FSM Implementation ---------------------------------------------------
(-fsm-trigger
[this trigger arg]
;; The following "case" impliments the Finite State Machine.
;; Given a "trigger", and the existing FSM state, it computes the
;; new FSM state and the tranistion action (function).
(let [[new-fsm-state action-fn]
(case [fsm-state trigger]
;; You should read the following "case" as:
;; [current-FSM-state trigger] -> [new-FSM-state action-fn]
;;
;; So, for example, the next line should be interpreted as:
;; if you are in state ":idle" and a trigger ":add-event"
;; happens, then move the FSM to state ":scheduled" and execute
;; that two-part "do" fucntion.
[:idle :add-event] [:scheduled #(do (-add-event this arg)
(-run-next-tick this))]
;; State: :scheduled (the queue is scheduled to run, soon)
[:scheduled :add-event] [:scheduled #(-add-event this arg)]
[:scheduled :run-queue] [:running #(-run-queue this)]
;; State: :running (the queue is being processed one event after another)
[:running :add-event ] [:running #(-add-event this arg)]
[:running :pause ] [:paused #(-pause this arg)]
[:running :exception ] [:idle #(-exception this arg)]
[:running :finish-run] (if (empty? queue) ;; FSM guard
[:idle]
[:scheduled #(-run-next-tick this)])
;; State: :paused (:flush-dom metadata on an event has caused a temporary pause in processing)
[:paused :add-event] [:paused #(-add-event this arg)]
[:paused :resume ] [:running #(-resume this)]
(throw (ex-info (str "re-frame: router state transition not found. " fsm-state " " trigger)
{:fsm-state fsm-state, :trigger trigger})))]
;; The "case" above computed both the new FSM state, and the action. Now, make it happen.
(set! fsm-state new-fsm-state)
(when action-fn (action-fn))))
(-add-event
[_ event]
(set! queue (conj queue event)))
(-process-1st-event-in-queue
[this]
(let [event-v (peek queue)]
(try
(handle event-v)
(set! queue (pop queue))
(-call-post-event-callbacks this event-v)
(catch #?(:cljs :default :clj Exception) ex
(-fsm-trigger this :exception ex)))))
(-run-next-tick
[this]
(next-tick #(-fsm-trigger this :run-queue nil)))
;; Process all the events currently in the queue, but not any new ones.
;; Be aware that events might have metadata which will pause processing.
(-run-queue
[this]
(loop [n (count queue)]
(if (zero? n)
(-fsm-trigger this :finish-run nil)
(if-let [later-fn (some later-fns (-> queue peek meta keys))] ;; any metadata which causes pausing?
(-fsm-trigger this :pause later-fn)
(do (-process-1st-event-in-queue this)
(recur (dec n)))))))
(-exception
[_ ex]
(set! queue empty-queue) ;; purge the queue
(throw ex))
(-pause
[this later-fn]
(later-fn #(-fsm-trigger this :resume nil)))
(-call-post-event-callbacks
[_ event-v]
(doseq [callback (vals post-event-callback-fns)]
(callback event-v queue)))
(-resume
[this]
(-process-1st-event-in-queue this) ;; do the event which paused processing
(-run-queue this))) ;; do the rest of the queued events
;; ---------------------------------------------------------------------------
;; Event Queue
;; When "dispatch" is called, the event is added into this event queue. Later,
;; the queue will "run" and the event will be "handled" by the registered function.
;;
(def event-queue (->EventQueue :idle empty-queue {}))
;; ---------------------------------------------------------------------------
;; Dispatching
;;
(defn dispatch
"Queue the given event for processing by the registered event handler.
Just to be clear: the event handler is not run immediately - it is not run
synchronously. It will likely be run 'very soon', although it may be
added to the end of a FIFO queue which already contain events.
Usage:
(dispatch [:delete-item 42])"
[event]
(if (nil? event)
(throw (ex-info "re-frame: you called \"dispatch\" without an event vector." {}))
(push event-queue event))
nil) ;; Ensure nil return. See https://github.com/Day8/re-frame/wiki/Beware-Returning-False
(defn dispatch-sync
"Sychronously (immediaetly!) process the given event using the registered handler.
Generally, you shouldn't use this - you should use `dispatch` instead. It
is an error to use `dispatch-sync` within an event handler.
Usage:
(dispatch-sync [:delete-item 42])"
[event-v]
(handle event-v)
(-call-post-event-callbacks event-queue event-v) ;; slightly ugly hack. Run the registered post event callbacks.
nil) ;; Ensure nil return. See https://github.com/Day8/re-frame/wiki/Beware-Returning-False