2021 0007 0009

Building a Personal Dashboard in ClojureScript Part 2

Following the previous installment in my series on building a dashboard in ClojureScript, I’ll be diving into the weather card.

Weather card

Like any re-frame application, this comes in two major pieces: consuming from the API to update the application state, and rendering the state on the page. Before showing how this is wired up, however, let’s first dive into the external weather API itself.

Weather API

There are several different weather APIs with a free tier that can handle the minimal traffic of a single dashboard. I landed on Open Weather Map API, which has both a free tier and an easy-to-use one call endpoint containing all the weather granularity (current and day/hour/minute-level) needed for a reasonable dashboard.

A sample request (with lots of fields omitted):

> curl 'http://api.openweathermap.org/data/2.5/onecall?lat=<latitude>&lon=<longitude>&units=imperial&appid=<apikey>' | jq .

  "current": {
    "dt": 1625517908,
    "sunrise": 1625477417,
    "sunset": 1625531408,
    "temp": 82.31,
    "feels_like": 84.31,
    "pressure": 1017,
    "humidity": 57,
    "weather": [
        "id": 800,
        "main": "Clear",
        "description": "clear sky",
        "icon": "01d"
  "daily": [
      "dt": 1625504400,
      "sunrise": 1625477417,
      "sunset": 1625531408,
      "temp": {
        "day": 83.12,
        "min": 66.2,
        "max": 83.82,
        "night": 75.74,
        "eve": 81.82,
        "morn": 67.89
      "feels_like": {
        "day": 83.86,
        "night": 76.21,
        "eve": 83.5,
        "morn": 68.29
      "humidity": 49,
      "weather": [
          "id": 500,
          "main": "Rain",
          "description": "light rain",
          "icon": "10d"
      "rain": 0.53,
  "minutely": [...],
  "hourly": [...],
  "alerts": [...]

In addition, we’ll want to tie the payload to a set of weather icons supplied by the Weather Icons font together using this mapping (represented below as id->icon).

API Client

In re-frame parlance, we use an “effects handler” to make http calls, which is helpfully provided by re-frame-http-fx. This allows us to define a ::fetch-weather event analogous to the curl command above:

 (fn [_ _]
    {:method :get
     :uri    "http://api.openweathermap.org/data/2.5/onecall"
     :params {:lat   (:lat config/home)
              :lon   (:lon config/home)
              :units "imperial"
              :appid config/open-weather-api-key}
     :response-format (ajax/json-response-format {:keywords? true})
     :on-success      [::events/http-success [:weather]]
     :on-failure      [::events/http-fail [:weather]]}}))

where the success and fail events are defined as:

 (fn [db [_ key-path result]]
   (assoc-in db key-path result)))

 (fn [db [_ key-path]]
   (assoc-in db key-path {})))

We can trigger this event at regular intervals, similar to the clock card:

(defn init []
    [{:interval                 900 ; 15 minutes
      :event                    [::weather/fetch-weather]
      :dispatch-event-on-start? true}]])

The 15 minute interval is set such that the API’s free tier daily request limit is apportioned throughout the day with some headroom remaining.

Finally, it is customary to create a “level 2” extractor subscription to pull the payload back out of the application state even though it is largely a trivial subscription:

 (fn [db _]
   (:weather db)))

Getting the weather payload ensconced in re-frame.db/app-db with a basic extractor is but our first step. It would be awkward for our view to consume directly from the full API payload as it contains many elements that would need to be filtered out or ignored; it also has the disadvantage that re-frame would have to re-render the weather element every time the payload is fetched even for UI elements that do not need to change. Enter the “level 3” materialized view, which filters down the payload into meaningful units of work. In this case, these units are:

  • Sunrise and sunset time
  • Current conditions
  • 6 day forecast

The sunrise/sunset subscription is easy once we’ve defined the epoch->local-date helper (that uses cljs-time internally) to parse the times into an object:

 :<- [::weather]
 (fn [{{:keys [sunrise sunset]} :current} _]
   {:sunrise (-> sunrise epoch->local-date .toUsTimeString)
    :sunset  (-> sunset epoch->local-date .toUsTimeString)}))

The current conditions subscription is also relatively simple, involving some light formatting (some of which could arguably be pushed down to the view layer):

 :<- [::weather]
 (fn [{{humidity                :humidity
        feels-like              :feels_like
        current-temp            :temp
        [{:keys [description]}] :weather} :current
       [{:keys                [rain snow]
         {low :min high :max} :temp}]     :daily} _]
   {:humidity    (-> humidity (str "%"))
    :feels-like  (-> feels-like int (str "°"))
    :description (some-> description str/capitalize)
    :rain        (some-> rain mm->in (round-nonzero 2) (str "\""))
    :snow        (some-> snow mm->in (round-nonzero 2) (str "\""))
    :temp        (some-> current-temp int (str "°"))
    :low         (some-> low int (str "°"))
    :high        (some-> high int (str "°"))}))

This subscription plucks the current weather conditions from the payload (using the fancy destructuring that makes Clojure so effective) and returns a new, sparser map with the values formatted and ready to be used in a view.

The most complex subscription is the forecast, which involves processing the :daily list of elements and returning a new list of ready-to-template maps:

 :<- [::weather]
 (fn [{forecast :daily} _]
   (->> forecast
        rest                            ; skip today
        (map (fn [{date                 :dt
                   {low :min high :max} :temp
                   rain                 :rain
                   snow                 :snow
                   [{icon-id :id} & _]  :weather}]
               {:epoch   date
                :weekday (-> date
                :icon    (id->icon icon-id)
                :high    (some-> high int (str "°"))
                :low     (some-> low int (str "°"))
                :rain    (some-> rain mm->in (round-nonzero 1) (str "\""))
                :snow    (some-> snow mm->in (round-nonzero 1) (str "\""))}))
        (take 6))))

This is similar to the current conditions subscription above; the major change here is that we are maping over the list of forecasts and taking only a fixed number of them.

This wraps up the event/subscription handling; with this code, we now ingest from the API and have defined a graph of subscriptions that whittles the payload down into filtered chunks that are ready to be placed into our view.

Weather Card

Creating views like the weather card is as much as art as it is an engineering effort, and I don’t expect I’d win any awards for either aspect.

Like any normal Clojure function, breaking our view into smaller pieces will greatly aid readability:

(defn weather []
  [:> Card
   [:> CardContent

Like the previous installment, the view uses uses the Material-UI react framework (i.e., the Card, CardContent components and many more) which comes with much saner style defaults than any CSS I could cook up.

(defn weather-conditions []
  [:> Grid {:container true :justify "center"}
   [:> Grid {:item true :xs 3}
    [:> Typography {:variant "h1"}
     ;; Display a large icon of current conditions
     [:i {:class (str "wi wi-"
                      @(re-frame/subscribe [::weather/icon]))}]]]
   [:> Grid {:item true :xs 5}
    [:> Typography {:align "center" :variant "h1"
                    :display "inline"}
     ;; Large view of the current temperature
     (:temp @(re-frame/subscribe [::weather/conditions]))]]
   [:> Grid {:item true :xs 2}
    (let [{:keys [low high]}
	     @(re-frame/subscribe [::weather/conditions])]
      [:> Typography {:align "right" :variant "h4"}
       high [:br] low])]])

With some minor extra styling, we end up with a nice, large display of the current temperature:

Current weather

Like the companion subscription, the forecast view maps over the individual days in the subscribed output to produce, in this case, Grid items to fill the card:

(defn weather-forecast []
  [:> Grid {:container true}
    (fn [{:keys [epoch weekday icon high low rain snow]}]
      ^{:key epoch}
      [:> Grid {:item true :xs 2}
       [:> Typography {:key epoch
                       :variant "body1"
                       :align "center"}
       [:> Typography {:align "center" :variant "h5"}
        [:i {:class (str "wi wi-" icon)}]]
       [:> Typography {:align "center" :variant "subtitle2"}
        (gstring/unescapeEntities "&#8194;")
	    (when rain
          [:<> [:br] (list " " rain)])
        (when snow
            (list " " snow)])]])
    @(re-frame/subscribe [::weather/forecast]))])

When generating view elements dynamically, specifying the key is important for re-frame (and React under-the-hood) to reliably match up elements that must be re-rendered when the payload changes. This gives us our 6-day forecast (which is all I could fit on the card even though the API returns more data):

Weather forecast

Last but not least, having a general text description of the weather is handy to capture leftover details that do not appear elsewhere in the UI:

(defn weather-description []
  (let [{:keys [humidity feels-like description rain snow]}
        @(re-frame/subscribe [::weather/conditions])]
    (->> [{:content description :render? description}
          {:prefix "Feels like " :content feels-like :render? true}
          {:content humidity :render? true}
          {:postfix " rain" :content rain :render? rain}
          {:postfix " snow" :content snow :render? snow}]
         (map (fn [{:keys [prefix postfix content render?]}]
                (if render?
                  (->> [prefix content postfix] (remove nil?) vec)
         (remove empty?)
         (interpose [" | "])
         (apply concat [:> Typography {:align "center"
                                       :color "textSecondary"
                                       :variant "body1"}])

This function is more elaborate than it needs to be, but is handy for adding new things to appear in the description–it first converts the individual datapoints into a vector of maps that (depending on the value of the :render? key) are subsequently concatenated into a |-separated series of descriptions:

Weather description

The full working code is available in weather.cljs and views.cljs which include a few extra visual tweaks and custom React components. Also omitted from the code in this post are a few visual details from the screenshot above, including the “refresh” button that triggers the ::fetch-weather event on-demand and the timer in the corner showing how much time has elapsed since the last fetch–not essential features for everyday use, but valuable for debugging.

With luck, the next post in this series will get to my favorite part of the dashboard: the transit card.

Part 1 Part 2