How to evaluate foo when mouse is down and moving using core/async?
Whilst attempting to learn the concepts behind core/async I have worked through the ClojureScript 101 tutorial (but I suspect this question applies to clojure to).
I create a channel where mouse movement events are placed using the following:
;; helper to get a channel where a dom event type will be put
(defn listen [el type]
(let [out (chan)]
(events/listen el type
(fn [e] (put! out e)))
out))
;; create a channel for mouse moves, take the values
;; and pass them to the console
(let [moves (listen (dom/getElement "canvas") "mousemove")]
(go (while true
(foo (<! moves)))))
This works, foo is evaluated when the mouse moves. But how can this be done only when the mouse is down?
My first guess would be to use an atom and two new channels for mousedown and mouseup. Then update atom with the mouse state and test against this in the go block. But I suspect this is wrong due to the use of an atom; hence the question.
Answering my own question, here is the closest I have got. Appears to work.
;; Util for create DOM channels
(defn listen [el type]
"Takes a DOM element and an event type. Returns a channel for the event"
;; out is a new channel
(let [out (chan (sliding-buffer 1))]
;; attach an event listener
(events/listen el type
;; the handler/callback of the listener takes the
;; event and put! in on the channel. We are using
;; put because we are not in a go block
(fn [e] (put! out e)))
;; return the channel
out))
(def canvas-el (dom/getElement "canvas"))
(def mouse-up (listen canvas-el "mouseup"))
(def mouse-down (listen canvas-el "mousedown"))
(def mouse-move (listen canvas-el "mousemove"))
(go (while true
(<! mouse-down)
(loop []
(let [[v ch] (alts! [mouse-move mouse-up])]
(when (= ch mouse-move)
(do
(.log js/console "move" (.-clientX v) (.-clientY v))
(recur)))))))
Related
I have a channel where I am putting values into inside a doseq loop.
This code reads from a list of isbns and for each isbn, does an amazon search to return contents of a book, and then calls another function to get the title and rank
(def book_channel (chan 10))
make sure you use clojure.core.async/into rather than clojure.core/into. Here is an example of a round trip from collection to channel and back to collection:
user> (require '[clojure.core.async :as async :refer [<! <!! >!! >! chan go]])
nil
user> (def book-chan (async/to-chan [:book1 :book2 :book3]))
#'user/book-chan
user> (<!! (clojure.core.async/into [] book-chan))
[:book1 :book2 :book3]
clojure.core.async/into returns a channel that will have exactly one item written to it. That one item will be written once it's input channel closes. This keeps the whole thing asynchronous and it does require that the code putting things into the book-channel close the chan to signal that all the books are there.
You need to do some type of coordination to determine when all of your work is finished. You can pull that coordination out into the main thread fairly easily:
(def book_channel (chan 10))
(defn concurrency_test
[list_of_isbns]
(doseq [isbn list_of_isbns]
(go (>! book_channel
(get_title_and_rank_for_one_isbn
(amazon_search isbn)))))
(prn (loop [results []]
(if (= (count results) (count list_of_isbns))
results
(recur (conj results (<!! book_channel)))))))
Here, I used a loop that keeps waiting for results and adding them to the vector until we have as many results as we do isbns. You'll want to make sure that get_title_and_rank_for_one_isbn always generates a result that can be put on a channel, otherwise the loop will wait forever.
You should close! the book_channel after you finish pushing stuff into it. Per async/into documentation - "ch must close before into produces a result."
(let [book> (chan)]
(go
(doseq [e (range 8)]
(>! book> e))
(close! book>))
(<!! (async/into [] book>)))
Alternatively, you can use async/onto-chan which will close the channel for you:
(let [book> (chan)]
(async/onto-chan book> (range 8))
(<!! (async/into [] book>)))
Is there an elegant way to stop a running go block?
(without introducing a flag and polluting the code with checks/branches)
(ns example
(:require-macros [cljs.core.async.macros :refer [go]])
(:require [cljs.core.async :refer [<! timeout]]))
(defn some-long-task []
(go
(println "entering")
; some complex long-running task (e.g. fetching something via network)
(<! (timeout 1000))
(<! (timeout 1000))
(<! (timeout 1000))
(<! (timeout 1000))
(println "leaving")))
; run the task
(def task (some-long-task))
; later, realize we no longer need the result and want to cancel it
; (stop! task)
Sorry, this is not possible with core.async today. What you get back from creating a go block is a normal channel what the result of the block will be put on, though this does not give you any handle to the actual block itself.
As stated in Arthur's answer, you cannot terminate a go block immediately, but you since your example indicates a multi-phased task (using sub-tasks), an approach like this might work:
(defn task-processor
"Takes an initial state value and number of tasks (fns). Puts tasks
on a work queue channel and then executes them in a go-loop, with
each task passed the current state. A task's return value is used as
input for next task. When all tasks are processed or queue has been
closed, places current result/state onto a result channel. To allow
nil values, result is wrapped in a map:
{:value state :complete? true/false}
This fn returns a map of {:queue queue-chan :result result-chan}"
[init & tasks]
(assert (pos? (count tasks)))
(let [queue (chan)
result (chan)]
(async/onto-chan queue tasks)
(go-loop [state init, i 0]
(if-let [task (<! queue)]
(recur (task state) (inc i))
(do (prn "task queue finished/terminated")
(>! result {:value state :complete? (== i (count tasks))}))))
{:queue queue
:result result}))
(defn dummy-task [x] (prn :task x) (Thread/sleep 1000) (inc x))
;; kick of tasks
(def proc (apply task-processor 0 (repeat 100 dummy-task)))
;; result handler
(go
(let [res (<! (:result proc))]
(prn :final-result res)))
;; to stop the queue after current task is complete
;; in this example it might take up to an additional second
;; for the terminated result to be delivered
(close! (:queue proc))
You may want to use future and future-cancel for such task.
(def f (future (while (not (Thread/interrupted)) (your-function ... ))))
(future-cancel f)
Why do cancelled Clojure futures continue using CPU?
When I evaluate the following core.async clojurescript code I get an error: "Uncaught Error: <! used not in (go ...) block"
(let [chans [(chan)]]
(go
(doall (for [c chans]
(let [x (<! c)]
x)))))
What am I doing wrong here? It definitely looks like the <! is in the go block.
because go blocks can't cross function boundaries I tend to fall back on loop/recur for a lot of these cases. the (go (loop pattern is so common that it has a short-hand form in core.async that is useful in cases like this:
user> (require '[clojure.core.async :as async])
user> (async/<!! (let [chans [(async/chan) (async/chan) (async/chan)]]
(doseq [c chans]
(async/go (async/>! c 42)))
(async/go-loop [[f & r] chans result []]
(if f
(recur r (conj result (async/<! f)))
result))))
[42 42 42]
Why dont you use alts! from Core.Async?
This function lets you listen on multiple channels and know which channel you read from on each data.
For example:
(let [chans [(chan)]]
(go
(let [[data ch] (alts! chans)]
data)))))
You can ask of the channel origin too:
...
(let [slow-chan (chan)
fast-chan (chan)
[data ch] (alts! [slow-chan fast-chan])]
(when (= ch slow-chan)
...))
From the Docs:
Completes at most one of several channel operations. Must be called
inside a (go ...) block. ports is a vector of channel endpoints,
which can be either a channel to take from or a vector of
[channel-to-put-to val-to-put], in any combination. Takes will be
made as if by !. Unless
the :priority option is true, if more than one port operation is
ready a non-deterministic choice will be made. If no operation is
ready and a :default value is supplied, [default-val :default] will
be returned, otherwise alts! will park until the first operation to
become ready completes. Returns [val port] of the completed
operation, where val is the value taken for takes, and a
boolean (true unless already closed, as per put!) for put
Doumentation ref
(ns example.asyncq
(:require
[cljs.core.async :as async]
goog.async.AnimationDelay)
(:require-macros
[cljs.core.async.macros :refer [go go-loop]]))
(defn loop-fx-on-atm-when-pred?-is-true [atm fx pred?]
(let [c (async/chan)
f (goog.async.AnimationDelay. #(async/put! c :loop))
a (atom false)]
(add-watch
atm
:loop-fx-on-atm-when-pred?-is-true
(fn [_ _ _ _]
(when-not #a
(async/put! c :initial-loop))))
(go-loop []
(async/<! c)
(reset! a true)
(swap! atm #(if (pred? %) (do (.start f) (fx %)) %))
(reset! a false)
(recur))))
(def the-final-countdown (atom 4))
(loop-fx-on-atm-when-pred?-is-true
the-final-countdown
#(do (js/console.log %) (dec %))
pos?)
(swap! the-final-countdown inc)
;; prints "5" "4" "3" "2" "1", each on a seperate frame (hearbeat)
The purpose of loop-fx-on.... is to register fx to be called on atm (per heartbeat) whenever pred? #atm is true.
I would like the following properties:
This code should only execute when pred? might return true. Otherwise, the state machine sits idle.
This should not register more than once per frame. Modifications of the atom after the AnimationDelay has been called but before the next actual frame are effectively noops. That is, the atom is modified, but it does not cause f to be called again.
From the point of view of the person using the atom, all they need do is modify the atom. They do not need to concern themselves with registering or unregistering the atom for AnimationFrame processing, it just works. The atom will continue to be processed every frame until pred? returns false.
My biggest gripe with the above code is (reset! a true) and (reset! a false) around the swap!. That is really ugly, and in a non single threaded environment would probably be a bug. Is there any way to improve this code so that I don't end up using a ?
My second concern is that 2 is not being met. Currently, if you modify the same atom twice between a frame, it will actually result in 2 calls to (f); or 2 callbacks on the next frame. I could use another atom to record whether I have actually registered for this frame or not, but this is already getting messy. Anything better?
Something like this might work for you:
(defn my-fn [a f pred]
(let [pending? (atom false)
f (fn []
(reset! pending? false)
(f #a))]
(add-watch a (gensym)
(fn [_k _a _old-val new-val]
(when (and (not #pending?) (pred new-val))
(reset! pending? true)
(if (exists? js/requestAnimationFrame)
(js/requestAnimationFrame f)
(js/setTimeout f 16)))))))
What's best way in clojure to implement something like an actor or agent (asynchronously updated, uncoordinated reference) that does the following?
gets sent messages/data
executes some function on that data to obtain new state; something like (fn [state new-msgs] ...)
continues to receive messages/data during that update
once done with that update, runs the same update function against all messages that have been sent in the interim
An agent doesn't seem quite right here. One must simultaneously send function and data to agents, which doesn't leave room for a function which operates on all data that has come in during the last update. The goal implicitly requires a decoupling of function and data.
The actor model seems generally better suited in that there is a decoupling of function and data. However, all actor frameworks I'm aware of seem to assume each message sent will be processed separately. It's not clear how one would turn this on it's head without adding extra machinery. I know Pulsar's actors accept a :lifecycle-handle function which can be used to make actors do "special tricks" but there isn't a lot of documentation around this so it's unclear whether the functionality would be helpful.
I do have a solution to this problem using agents, core.async channels, and watch functions, but it's a bit messy, and I'm hoping there is a better solution. I'll post it as a solution in case others find it helpful, but I'd like to see what other's come up with.
Here's the solution I came up with using agents, core.async channels, and watch functions. Again, it's a bit messy, but it does what I need it to for now. Here it is, in broad strokes:
(require '[clojure.core.async :as async :refer [>!! <!! >! <! chan go]])
; We'll call this thing a queued-agent
(defprotocol IQueuedAgent
(enqueue [this message])
(ping [this]))
(defrecord QueuedAgent [agent queue]
IQueuedAgent
(enqueue [_ message]
(go (>! queue message)))
(ping [_]
(send agent identity)))
; Need a function for draining a core async channel of all messages
(defn drain! [c]
(let [cc (chan 1)]
(go (>! cc ::queue-empty))
(letfn
; This fn does all the hard work, but closes over cc to avoid reconstruction
[(drainer! [c]
(let [[v _] (<!! (go (async/alts! [c cc] :priority true)))]
(if (= v ::queue-empty)
(lazy-seq [])
(lazy-seq (cons v (drainer! c))))))]
(drainer! c))))
; Constructor function
(defn queued-agent [& {:keys [buffer update-fn init-fn error-handler-builder] :or {:buffer 100}}]
(let [q (chan buffer)
a (agent (if init-fn (init-fn) {}))
error-handler-fn (error-handler-builder q a)]
; Set up the queue, and watcher which runs the update function when there is new data
(add-watch
a
:update-conv
(fn [k r o n]
(let [queued (drain! q)]
(when-not (empty? queued)
(send a update-fn queued error-handler-fn)))))
(QueuedAgent. a q)))
; Now we can use these like this
(def a (queued-agent
:init-fn (fn [] {:some "initial value"})
:update-fn (fn [a queued-data error-handler-fn]
(println "Receiving data" queued-data)
; Simulate some work/load on data
(Thread/sleep 2000)
(println "Done with work; ready to queue more up!"))
; This is a little warty at the moment, but closing over the queue and agent lets you requeue work on
; failure so you can try again.
:error-handler-builder
(fn [q a] (println "do something with errors"))))
(defn -main []
(doseq [i (range 10)]
(enqueue a (str "data" i))
(Thread/sleep 500) ; simulate things happening
; This part stinks... have to manually let the queued agent know that we've queued some things up for it
(ping a)))
As you'll notice, having to ping the queued-agent here every time new data is added is pretty warty. It definitely feels like things are being twisted out of typical usage.
Agents are the inverse of what you want here - they are a value that gets sent updating functions. This easiest with a queue and a Thread. For convenience I am using future to construct the thread.
user> (def q (java.util.concurrent.LinkedBlockingDeque.))
#'user/q
user> (defn accumulate
[summary input]
(let [{vowels true consonents false}
(group-by #(contains? (set "aeiouAEIOU") %) input)]
(-> summary
(update-in [:vowels] + (count vowels))
(update-in [:consonents] + (count consonents)))))
#'user/accumulate
user> (def worker
(future (loop [summary {:vowels 0 :consonents 0} in-string (.take q)]
(if (not in-string)
summary
(recur (accumulate summary in-string)
(.take q))))))
#'user/worker
user> (.add q "hello")
true
user> (.add q "goodbye")
true
user> (.add q false)
true
user> #worker
{:vowels 5, :consonents 7}
I came up with something closer to an actor, inspired by Tim Baldridge's cast on actors (Episode 16). I think this addresses the problem much more cleanly.
(defmacro take-all! [c]
`(loop [acc# []]
(let [[v# ~c] (alts! [~c] :default nil)]
(if (not= ~c :default)
(recur (conj acc# v#))
acc#))))
(defn eager-actor [f]
(let [msgbox (chan 1024)]
(go (loop [f f]
(let [first-msg (<! msgbox) ; do this so we park efficiently, and only
; run when there are actually messages
msgs (take-all! msgbox)
msgs (concat [first-msg] msgs)]
(recur (f msgs)))))
msgbox))
(let [a (eager-actor (fn f [ms]
(Thread/sleep 1000) ; simulate work
(println "doing something with" ms)
f))]
(doseq [i (range 20)]
(Thread/sleep 300)
(put! a i)))
;; =>
;; doing something with (0)
;; doing something with (1 2 3)
;; doing something with (4 5 6)
;; doing something with (7 8 9 10)
;; doing something with (11 12 13)