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A prototype for a ClojureScript/Om/core.async based in-browser UI.

This is work in progress. I just started with Om (beginning of Nov'14).

A running demo is hosted here.


I prefer a strict separation between presentation logic and markup. The Om examples I'm aware of tend to mix both aspects a bit, however, IMO using channels as proposed by @swannodette is the key to strong decoupling.

I'm interested in boring enterprise style forms-over-data UIs. I like to succinctly specify the visual content of views without mixing it up with presentation logic. However, things like state, actions, validation, rules, inter-component and remote communication need a place. I prefer to implement these using ordinary language features like atoms and pure functions, perhaps augmented with access to channels, if necessary.

My hope is to show how the combination of Om and core.async enables drastically simpler enterprise-style UI development.


This is how I like the code for boring UIs to look alike:

;; ============================================================================
;; Address Details

(def fields [:private :name :company :street :city :birthday])

(def address-constraints
   [[:name :value]]
   c/required (c/min-length 3)
   [[:private :value] [:company :value]]
   (fn [p? c]
     (if p?
       (if (> (count c) 0)
         "Company must be empty")
       (if (empty? c)
         "Company must not be empty")))))

;; ----------------------------------------------------------------------------
;; Actions are functions [state event -> state]

(defn details-add!
  [state event]
  (let [a  (get-all state :value fields)
        i  (-> state :edit-index)]
    (put-view! "addressbook" {:type :action
                              :id "add"
                              :address a
                              :index i})
    (-> state
        (assoc-in  [:edit-index] nil)
        (update-all :value fields nil))))

(defn details-reset
  [state event]
  (-> state
      (assoc-in  [:edit-index] nil)
      (update-all :value fields nil)
      (update-all :message fields nil)))

(defn details-edit
  [state {:keys [address index]}]
  (-> state
      (assoc-in  [:edit-index] index)
      (update-all :value fields address)
      (update-all :message fields nil)))

;; ----------------------------------------------------------------------------
;; All rules are represented by one function [state -> state]

(defn addressdetails-rules
  (let [invalid?  (->> state (e/validate address-constraints) (e/has-errors?))
        edit?     (-> state :edit-index)
        private?  (-> state :private :value)]
    (-> state
        (assoc-in [:add :text]         (if edit? "Update"))
        (assoc-in [:title]             (if edit? "Edit Details" "Details"))
        (assoc-in [:company :disabled] private?)
        (update-in [:company :value]   #(if private? "" %))
        (assoc-in [:add :disabled]     (if invalid? true)))))

;; ----------------------------------------------------------------------------
;; A concise "model" of the Addressbook view

(defn addressdetails-view
  (view "details"
        [(panel "fields" :layout :two-columns
                :elements [(checkbox "private")
                           (textfield "name" :label "Full name")
                           (textfield "company")
                           (textfield "street")
                           (selectbox "city")
                           (datepicker "birthday")])
         (panel "actions" :elements
                [(button "add" :text "Add Address") (button "reset")])]
        :actions {:add     details-add!
                  :edit    details-edit
                  :reset   details-reset}
        :rules addressdetails-rules
        :constraints address-constraints))

;; ============================================================================
;; Addressbook

;; ----------------------------------------------------------------------------
;; Actions are functions [state event -> state]

(defn addressbook-add
  [state {:keys [address index]}]
  (update-in state [:addresses :items] add-or-replace index address))

(defn addressbook-edit!
  [state event]
  (when-let [i (first (get-in state [:addresses :selection]))]
    (let [a (get-in state [:addresses :items i])]
      (put-view! "details" {:type :action
                            :id "edit"
                            :address a
                            :index i})))

(defn <addressbook-delete
  [state event]
  (let [question (str "You're about to delete item "
                      (-> state :addresses :selection first)
                      ". Are you sure?")]
    (go (if (= :ok (<! (<ask question)))
          (remove-selected state [:addresses])

(defn <addressbook-reload
  [state event]
  (go (if (= :ok (<! (<ask "You will loose all your local changes. Are you sure?")))
        (let [{s :status addresses :body} (<! (http/get "/addresses"))]
          (if (= 200 s)
            (assoc-in state [:addresses :items] addresses)
            (Message. "Error loading addresses")))

;; ----------------------------------------------------------------------------
;; Rules are represented by one function [state -> state]

(defn addressbook-rules
  (let [none-sel?  (-> state :addresses :selection empty?)]
    (-> state
        (assoc-in [:edit :disabled]   none-sel?)
        (assoc-in [:delete :disabled] none-sel?))))

;; ----------------------------------------------------------------------------
;; A concise "model" of the Addressbook view

(defn addressbook-view
  (view id
         (table "addresses"
                :columns [(column "name")
                          (column "company")
                          (column "street")
                          (column "city")
                          (column "birthday")]
                :actions-fn (fn [item]
                              [(action-link "edit" :image "images/pencil.png")
                               (action-link "delete" :image "images/cross.png")]))
         (button "reload")]
        :actions {:add       addressbook-add
                  :edit      addressbook-edit!
                  :delete    <addressbook-delete
                  :reload    <addressbook-reload}
        :rules addressbook-rules))

As you can see there is almost no access to technical APIs left, I added a thin DSL layer on top of the Om components, you may consider it a means to succinctly parameterize components. However, it's likely that a concrete project will have to invent it's own DSL to suit its needs. The advantage of a DSL layer is that it becomes almost trivial to create and understand a UI like this.

Open questions

Here are some points that I have to make up my mind about:

In my setup each view gets its own channel, and all channels are registered in a central map. Are there any general rules that determine the process+channel topology?

How can input focus be controlled? The decision, where to take the focus to, could be part of an action or, more general, in an event-handler. React basically supports this: I've seen that Reagents TodoMVC uses .focus within did-mount.

My ideas and decisions so far

Application state is kept in a global atom, according to Om's pgm model. Local component state is almost not used, so even "transient" information like currently selected items or enabled/disabled state of a component is part of global application state. This makes possible to directly influence this data in action functions that are not part of the component to be controlled.

A view bundles the specification of visual contents, actions, rules and validation constraints.

View contents is specified as data using a bunch of functions that create nested maps (see zackzack.specs namespace).

There is a generic view component that starts a generic CSP-style controller process and builds components by interpretation of the spec-fn result.

Each view component has one input channel that takes every event submitted by JS event listeners. Any user input is routed as :update event through this channel and the application state is updated immediately with the result of a field specific parser function [string -> anything] application.

After processing an :update, validation with respect to the updated field is applied. This results in an update of all :message values stored in the state maps of input fields.

If an input field loses the input focus a formatter is applied to show the user input properly formatted (not yet implemented).

Actions (the stuff that happens for example upon button clicks) are triggered by :action events and should ideally be pure functions of the form [state event -> (U state channel message)]. If they communicate with another component they use the foreign components channel via put-view!. They usually return the new state of the view, but can alternatively return a channel (see also Remote Communication) or an error message.

Remote communication is done asynchronously. An action body can be wrapped in a go block to access a remote service operation. In this case, the action returns a channel. When this channel emits a message it is used as new view state in an :update message processed by the responsible view. (This is a bit dangerous, because meantime the user might apply changes somewhere else which are then overwritten by the message. The piece of state that gets updated must be narrowed, alternatively the UI has to be blocked.)

After processing of an :update or :action event a rules function [state -> state] is called that ensures that invariants regarding the components state are re-established.

To enable communication among controller processes each view has its own input channel which is created when the view components state is initialised (IInitState). Upon mounting, the channel is registered under the views id in a global map. The channel is removed from this map when it is unmounted. A view used as child of a parent view contains the channel of its parent in the :parent-ch entry in its spec. An action that wishes to send an event to a different process would use type :action, which, in turn, triggers the execution of an arbitrary action function.


  • Component access to global data like user, roles, rights (can this be done through Om's global, immutable shared state?)
  • Formatting / parsing of values
  • Controlling input focus


Clone this repo. Make sure you're on Java 1.7 or higher and have at least Leiningen 2.5 installed.

To enter interactive development

  • If you have just worked with cljsbuild auto and switch to interactive mode make sure to delete resources/public/js before starting the REPL.
  • Create a REPL session, load zackzack.backend namespace.
  • (start!) starts http-kit web server.
  • (cljs-repl) starts a browser-connected ClojureScript REPL. Wait for an output like << started Weasel server on ws:// >>. Now the REPL is ready for the browser to connect.
  • Load the page http://localhost:8080/testindex.html, it will connect to the waiting REPL.
  • Open a cljs source file. Re-evaluate some code. If you changed data, not just functions, then you also have to re-evaluate the om/root expression (I wrapped it in a function, so (refresh) does the trick if you changed the views layout, to improve interactivity see Emacs/CIDER tip below).
  • You should be able to see the effect in the browser without reloading anything. In fact, if you reload in the browser all your prior evaluations will be gone.

With Emacs/CIDER you can trigger a (refresh) by sending a form to a REPL using cider-interactive-eval:

(defun refresh-zackzack ()
  (cider-interactive-eval "("))

If you bind this function to a keyboard shortcut you can redraw your browser by just one keystroke.

To use cljsbuild auto and develop Cljs without REPL connection

lein with-profile auto do, cljsbuild auto or execute ./

You'll find the JS build results in resources/public. Use the testindex.html to start the frontend without the backend. If the backend is started you can use http://localhost:8080/testindex.html.

To start the backend from a terminal

lein ring server will start a process listening on http://localhost:8080/.

To produce a Cljs frontend to publish

lein do clean, jar or execute ./

You'll find the JS build results in resources/public. Use the index.html to start the frontend without the backend. If the backend is started you can use http://localhost:8080/index.html.

To produce an Uberjar

lein uberjar will create a self-contained Jar that can be run using java -jar target/zackzack.jar and starts a web server waiting on http://localhost:8080/.

To execute automated tests

Clojure based tests use Selenium webdriver with Firefox. Unfortunately, ClojureScript tests are not properly executed due to a bug either in my project.clj or in one of the ClojureScript deps.

Give it a try with lein test.

To run it in a Docker container

Suppose your working dir is the project root dir and you have an Ubuntu-based Docker image zackzack with just openjdk-7-jdk installed, then the following command will create and run a fresh container:

sudo docker run -d -p -v $(pwd):/root zackzack java -jar /root/zackzack.jar

From the browser you reach this instance with http://localhost.


Copyright © 2014, 2015 F.Riemenschneider

Distributed under the Eclipse Public License version 1.0.


A Om+core.async prototype of enterprise-style browser UIs.



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