re-pollsive

"Everywhere I look, I see the repulsive sight of hundreds, thousands of revolting little children."

• Grand High Witch, The Witches

re-pollsive is a library that handles polling events for re-frame applications.

[re-pollsive "0.1.0"]

And in your ns:

(ns your-ns
  (:require [re-pollsive.core :as poll]))

Note: For now, this library should be considered alpha quality, as the api is still settling.

The Problem

When you want to poll something, you will likely reach for javascript's setInterval. However, if you aren't careful, it is easy to add a bunch of setIntervals scattered throughout your application. When these setIntervals collide, this can lead to unexpected and hard to debug behavior.

In addition, if you are using a hot-reloading tool (such as figwheel), you will need to proactively defend against unintended setIntervals starting on reload.

Re-pollsive's solution

With re-pollsive, you only set up one setInterval when your application starts, by dispatching ::poll/init. This will start a counter that counts up every second.

Re-pollsive allows you to dynamically define polling rules by dispatching ::poll/set-rules. An interval is defined for each rule, and will be tracked against the aforementioned counter.

In addition, because you are anchoring off of only one setInterval, you can freely use hot-reloading tools (such as figwheel) without any problems.

API

::poll/init

::poll/init starts a counter for your polling rules to hang off of.

Note: This needs to be dispatched only once, when the application first loads.

(re-frame/dispatch-sync [::poll/init])

::poll/set-rules

::poll/set-rules takes a vector of hash-maps with the following shape:

:poll-when is a re-frame subscription vector (e.g. [:should-i-be-polling?]), and its value should be a boolean. :poll-when can be used to effectively start and stop the poller. If you do not supply :poll-when, then the poller will always run.

:dispatch-event-on-start? is a way to dispatch the event at time 0. Say you have an interval of 30, by setting :dispatch-event-on-start? to true, then the event will dispatch at 0 seconds, 30 seconds, 60 seconds, etc. If :dispatch-event-on-start? is false, then the event will dispatch at 30 seconds, 60 seconds, etc. (and not at time 0).

Here is an example:

(re-frame/dispatch
 [::poll/set-rules
  [;; rule #1
   {:interval                 4
    :event                    [::events/log "POLL (every 4 seconds)"]
    :poll-when                [::subs/poll?]
    :dispatch-event-on-start? true}

   ;; rule #2
   {:interval                 6
    :event                    [::events/log "POLL (every 6 seconds)"]
    :poll-when                [::subs/poll?]
    :dispatch-event-on-start? false}
   ]])

Usage

Create a new re-frame application.

lein new re-frame foo

Add the following to the :dependencies vector of your project.clj file.

[re-pollsive "0.1.0"]

Then require re-pollsive in the core namespace, and add the ::poll/init event.

(ns foo.core
  (:require [reagent.core :as reagent]
            [re-frame.core :as re-frame]

            ;; Add this (1 of 2)
            [re-pollsive.core :as poll]

            [foo.events :as events]
            [foo.views :as views]
            [foo.config :as config]
            ))

(defn dev-setup []
  (when config/debug?
    (enable-console-print!)
    (println "dev mode")))

(defn mount-root []
  (re-frame/clear-subscription-cache!)
  (reagent/render [views/main-panel]
                  (.getElementById js/document "app")))

(defn ^:export init []
  (re-frame/dispatch-sync [::events/initialize-db])

  ;; And this (2 of 2)
  (re-frame/dispatch-sync [::poll/init])

  (dev-setup)
  (mount-root))

Next, you will need to dispatch a ::poll/set-rules event somewhere. Personally, I like dispatching this in my routes file (because I may want to handle polling events differently on each page).

Non-goals

• A perfectly accurate counter. The internal counter is not intended to be perfectly accurate. For example, if you set an interval to 30, expect the event to be dispatched roughly every 30 seconds, but not exactly. In other words, don't use re-pollsive for a clock.
• Handling really quick events. The choice of having the :interval be measured in seconds as opposed to milliseconds was intentional. If you want to dispatch events with millisecond granularity, you are likely better off using a one-off setTimeout.

Questions

If you have questions, I can usually be found hanging out in the clojurians #reagent slack channel (my handle is @gadfly361).

License

Copyright © 2018 Matthew Jaoudi

Distributed under the The MIT License (MIT).