September 24, 2015

Here is a simple Reagent terminal emulator written in Clojurescript. It is basically a Lojban blog, and you can type ‘bavla’i’ (Lojban ‘next’) to get an older article.

In case you are not familiar with Reagent, it is a React.js wrapper for Clojurescript. For example, the following codes create a “terminal page” and mount it to the document:

(def outputs (atom []))

(defn terminal-page []
    (for [output @outputs]

(reagent/render-component [terminal-page]
  (.getElementById js/document "app"))

Reagent uses a similar syntax to Hiccup, which is at the same time, valid Clojure codes, so that you can have whole power of Clojure while creating a template.

The terminal-page function above queries an atom called outputs, and wrap all items in the output list to a div of class line. It also contains a prompt-view, which I will talk about later.

Output to the Terminal

To output new texts to the terminal, you simply modify the output atom. The below function outputs a new line:

(defn outln [line]
  (swap! outputs conj line))

The Prompt View

The prompt view works with a hidden text input, which is always focused. It reads keyboard input and change an atom called prompt-value accordingly. A separate span is used to actually display the value.

(defn prompt-view []
   [:span {:style {:font-weight 700}}
    "  "]
   [:span @prompt-value]
   [:span.prompt "\u00a0"]
   [:input {:style {:opacity 0}
            :auto-focus true
            :value @prompt-value
            :on-blur #(let [target (-> % .-target)]
                        (.setTimeout js/window
                          (fn [] (.focus target)) 0))
            :on-change #(reset! prompt-value
                          (-> % .-target .-value))
            :on-key-down #(case (.-which %)
                            37 (.preventDefault %)
                            39 (.preventDefault %)
                            27 (reset! prompt-value "")
                            13 (do
                                 (execute @prompt-value)
                                 (reset! prompt-value ""))

Note that when outputting from the prompt, we also need to output the current state of the prompt since it will not be saved automatically:

(outln [:div
        [:span {:style {:font-weight 700}} " "]
        [:span prompt]])

Scroll to Bottom Automatically

Another problem with the above code is that the view will not be automatically scrolled to the bottom after each output. The solution is to create a scroll to bottom function:

(defn scroll-to-bottom []
  (println "Scrolling ...")
  (let [height (-> js/document .-body .-scrollHeight)]
    (println height)
    (.scrollTo js/window 0 height)))

And we also need to change the render function with a new scrollable view:

(def terminal-page-scroll
  (with-meta terminal-page
    {:component-did-mount #(do (.log js/console "did-mount")
     :component-did-update #(do (.log js/console "did-update")

(reagent/render-component [terminal-page-scroll]
  (.getElementById js/document "app"))