A while back we did a small introductory talk on angular.js with @brutasse. Our talk was aimed at backend developers looking for a solution to build simple interfaces for REST services. The app used exposed a simplistic job board with no persistence and a mere three views.

Our main user-facing app at exoscale is an angular one and we also have a large internal one. Most of our backend work is done in clojure and python, with the occasional guest language. While not discontent with angular, we kept a close eye on the promise of fast updates with react’s approach.

om came at a time when we were looking for alternatives for building interfaces with workloads involving plenty of updates and interacting with server sent events.

While very happy with om so far, most of the introductory material out there focuses on complex apps and features, and when starting out, we fell like simple introductions were missing. Hopefully the following can help bridge that gap, assuming a familiarity with the clojure language.

Om in 3 minutes.

om provides a thin layer of abstraction on top of facebook’s react.js in clojurescript. om relies on atoms to provide application state and schedules renders on atom changes. This article does not dive in clojure specifics. If you’re new to the language past the basic language elements, to get an understanding of what is going on you will need to understand:

How atoms work and how to operate on them

How protocols work and how they may be implemented on the fly with reify.

When the global state stored in an atom is changed, the render phase walks through a tree of components which are given all or part of the state as input.

A very simple example would be:

( ns omg.frontend ( :require [ om.core :as om :include-macros true ] [ om.dom :as dom :include-macros true ])) ( def app-state ( atom { :messages [ "hello" "hello again" "bye" ]})) ( defn message-list [ app owner ] ( reify om/IRender ( render [ this ] ( apply dom/ul {} ( for [ m ( :messages app )] ( dom/li {} m )))))) ( om/root message-list app-state { :target ( . js/document ( getElementById "app" ))})

Three things happen in the above:

The app state is initialized to a list of messages.

A component is created which displays messages

The component is bound to the app state and mounted on the DOM.

The render phase uses available functions to create DOM elements, the use of apply does not help make the component explicit, we will see how to improve this later on.

The accompanying HTML can be as simple as:

<!doctype html> < html lang = "en" > < body > < div id = "app" > </ body > < script src = "//cdnjs.cloudflare.com/ajax/libs/react/0.11.2/react.min.js" ></ script > < script src = "/js/out/goog/base.js" ></ script > < script src = "/js/app.js" ></ script > < script type = "text/javascript" > goog . require ( "omg.frontend" );</ script > </ html >

Compared to angular, om does not provide any standard way to structure apps. This is in part due to the fact that om is fairly recent, but also to the fact that the clojure community puts a bigger emphasis on libraries than framework, which do not impose as much on their consumers.

Fortunately, there are already plenty of librarie which help dealing with common tasks. This project will use:

sablono: provides a form based DSL to generate DOM elements

cljs-ajax: a simple AJAX client

secretary: a library to help with routes within the frontend application

om-tools: a library which provides syntactic sugar for om components

A note on building

In the clojure world, projects are built with leiningen and configured with a project.clj file. To build the above project you would have a project structure like this:

. ├── project.clj ├── resources │ └── public │ └── index.html └── src └── omg └── frontend.cljs 4 directories, 3 files

Using the following project.clj file:

( defproject omg "0.1.0" :description "demo om application" :dependencies [[ org.clojure/clojure "1.6.0" ] [ org.clojure/clojurescript "0.0-2371" :scope "provided" ] [ om "0.7.3" ]] :plugins [[ lein-cljsbuild "1.0.3" ]] :cljsbuild { :builds { :app { :source-paths [ "src" ] :compiler { :output-to "resources/public/js/app.js" :output-dir "resources/public/js/out" :source-map "resources/public/js/out.js.map" :optimizations :none :pretty-print true }}}})

Extending our minimal app

To get a feel of the libraries we will be working with, let’s dwell on this example for a while and make it use some of the available libraries. These can be added to the :dependencies vector in the project.clj file:

[ secretary "1.2.1" ] [ sablono "0.2.22" ] [ cljs-ajax "0.3.3" ] [ prismatic/om-tools "0.3.6" ]

The first thing we can do is use sablono, which gives a familiar DSL for building DOM elements, similar to hiccup:

( ns omg.frontend ( :require [ om.core :as om :include-macros true ] [ sablono.core :as html :refer-macros [ html ]])) ( def app-state ( atom { :messages [ "hello" "hello again" "bye" ]})) ( defn message-list [ app owner ] ( reify om/IRender ( render [ this ] ( html [ :ul ( for [ m ( :messages app )] [ :li m ])])))) ( om/root message-list app-state { :target ( . js/document ( getElementById "app" ))})

The next step is to use the syntactic sugar provided by om-tools to create components:

( ns omg.frontend ( :require [ om.core :as om ] [ om-tools.core :refer-macros [ defcomponent ]] [ sablono.core :as html :refer-macros [ html ]])) ( def app-state ( atom { :messages [ "hello" "hello again" "bye" ]})) ( defcomponent message-list [ app owner ] ( render [ this ] ( html [ :ul ( for [ m ( :messages app )] [ :li m ])]))) ( om/root message-list app-state { :target ( . js/document ( getElementById "app" ))})

Our last step in this short introduction will be to build on the fact that components can be nested and build on sub elements of the application state (referred to as cursors):

( ns omg.frontend ( :require [ om.core :as om ] [ om-tools.core :refer-macros [ defcomponent ]] [ sablono.core :as html :refer-macros [ html ]])) ( def app-state ( atom { :messages [ "hello" "hello again" "bye" ]})) ( defcomponent message [ m owner ] ( render [ this ] ( html [ :li m ]))) ( defcomponent message-list [ app owner ] ( render [ this ] ( html [ :ul ( om/build-all message ( :messages app ))]))) ( om/root message-list app-state { :target ( . js/document ( getElementById "app" ))})

Structure of the app

The basic idea behind this article was to re-build the simple job board we built in angular.js (https://github.com/exoscale/angular-jobs) with om. The following principles were applied:

Show-case the use of JSON for interaction.

for interaction. Assume we’re talking to a simple RESTish API, not publishing app state diffs.

Try to separate concerns between views, model and router cleanly to recreate a known frontend app structure.

The app is built around 3 simple views:

A job listing view which can be filtered.

A job detail view.

A job post view.

To build this, the following will need to be built

A router to correctly dispatch based on location.

A service to listen to model changes.

Appropriate views to display contents.

As far as routes in the REST api are concerned, the following endpoints are provided:

GET /jobs : retrieves a map of job id (a UUID) to a map containing a title , company and desc key to provide details on a job.

: retrieves a map of job id (a UUID) to a map containing a , and key to provide details on a job. POST /jobs : expects a JSON body containing a map with the title , company and desc keys and will yield the augmented full map, as for the GET call.

: expects a JSON body containing a map with the , and keys and will yield the augmented full map, as for the call. DELETE /jobs/:id : will delete the job at key id from the map and return the modified map as for the GET call.

A simple om router

One of the appealing things popular frontend frameworks offer is a simple way to bind routes to controllers and views through a router. This makes diving into an app explicit since the entry point for it contains a collection of routes to look at.

In our equivalent angular.js app, this is what we had:

app . config ( function ( $routeProvider ) { $routeProvider . when ( '/list' , { templateUrl : 'listing.html' , controller : 'Jobs' }) . when ( '/details/:id' , { templateUrl : 'details.html' , controller : 'Jobs' }) . when ( '/post' , { templateUrl : 'post.html' , controller : 'Jobs' }) . otherwise ( { redirectTo : '/list' }); });

Once you get past the syntax, the intent is clear. We are presented with three routes which point to different templates and a single controller.

The story with om is slightly different, since the concepts of controllers is totally separate from views and is only responsible for modifying the global state atom which will trigger re-renders on views.

A router’s duty will thus only be to map a route to the correct component, and provide a way to jump to a different location within the app programmaticaly.

secretary is a library which helps with routing and understands the ubiquitous keyword based routes. secretary provides two functions which we will be using:

add-route! : associates a function with a route path.

: associates a function with a route path. dispatch! : changes the location.

Since om’s re-renders are only triggered by changes in the state, a simple approach is to make the router be a component which dispatches to the appropriate component based on the parsed route.

We can base our approach on a vector to configure the router’s behavior:

( def routes [ "/" views/jobs "/job/:id" views/job "/post" views/job-post ]

If secretary handles setting a key in the application state designating the current view and params, it becomes simple to build a component which dispatches appropriately:

( defn init [ routes app ] ;; walk through provided routes, adding a callback ;; which updates the global state ( doseq [[ route view ] ( partition 2 routes )] ( add-route! route # ( swap! app assoc :router { :view view :params % }))) ;; Yield a component which dispatches to the appropriate ;; component previously stored by our route callback ( fn [ app owner ] ( reify om/IRender ( render [ this ] ( om/build ( get-in app [ :router :view ]) app )))))

Our job here is not completely done, we will also need to listen on history events. Since om builds on top of google closure, we can use the provided History object, here is the complete namespace with comments eluded:

( ns jobs.router ( :require [ goog.events :as events ] [ om.core :as om ] [ goog.history.EventType :as EventType ] [ secretary.core :refer [ add-route! dispatch! ]] [ sablono.core :refer-macros [ html ]]) ( :import goog.History )) ( defonce history ( History. )) ( defn init [ routes app ] ( doseq [[ route view ] ( partition 2 routes )] ( add-route! route # ( swap! app assoc :router { :view view :params % }))) ( goog.events/listen history EventType/NAVIGATE # ( -> % .-token dispatch! )) ( .setEnabled history true ) ( fn [ app owner ] ( reify om/IRender ( render [ this ] ( om/build ( get-in app [ :router :view ]) app ))))) ( defn redirect [ location ] ( .setToken history location ))

The added redirect function will trigger an event type of NAVIGATE which will dispatch to the appropriate route and thus update our global state.

To wire in our router, we can then just build our main namespace by using a router as the main component:

( ns jobs.frontend ( :require [ om.core :as om ] [ jobs.views :as views ] [ jobs.router :as router ])) ( defonce app-state ( atom {})) ( def routes [ "/" views/jobs "/job/:id" views/job "/post" views/job-post ]) ( let [ router ( router/init routes app-state ) target { :target ( . js/document ( getElementById "app" ))}] ( om/root router app-state target ))

With this it’s now trivial to organize the application logically accross several view components.

The model

With dispatching to the appropriate view out of the way, the next concern is building interaction with our API.

om build on top of core.async to provide a flexible API to avoid relying exclusively on callbacks, which we will leverage to build our model service. One of the core components of core.async is it’s channel interface which allows seemingly separate threads of execution to have a simple communication interface.

Our app only needs to be responsible for retrieval, creation and deletion of jobs.

To simplify dealing with queries, we will leverage the cljs-ajax library. Here are two simple examples of the API exposed by cljs-ajax:

( GET "/jobs" { :handler ( fn [ resp ] ... )}) ( POST "/jobs" { :params { :title "developer" :company "supercorpo" :desc "..." } :handler ( fn [ resp ] ... ) :format :json })

The simplest approach is to create a channel, looping on incoming messages and taking the appropriate action.

We will define two message types:

delete : based on an ID will delete from

: based on an ID will delete from post : will expect a map of params and create a job

A map is a great container for these messages and will take either of the following two forms:

;; delete message { :type :delete :id "307b630c-76d0-4cdb-af3f-79acd135f508" } ;; post message { :type :post :params { :title "developer" :company "foo" :desc "..." }}

A first version of the service can then be built like this:

( defn jobs [ app ] ( let [ in ( chan )] ;; Get an initial state when starting the service. ( GET "/jobs" { :handler # ( swap! app assoc :jobs % )}) ( go-loop [{ :keys [ type id params ]} ( <! in )] ( condp = type :delete ( DELETE ( str "/jobs/" id ) { :handler # ( swap app assoc :jobs % )}) :post ( POST "/jobs" { :params params :handler # ( swap app assoc :jobs % ) :format :json })) ( recur ( <! in ))) in ))

For each incoming message, based on the :type key within the message, the appropriate action is taken. We can take things a step further and prevent consumers of the model to have to deal with core.async by adding convenience functions within the app state:

( defn sanitize [ in [ id { :strs [ title company desc ]}]] { :id id :delete! ( fn [ & _ ] ( put! in { :type :delete :id id })) :title title :company company :desc desc }) ( defn updater [ in app jobs ] ( swap! app assoc :jobs ( mapv ( partial sanitize in ) jobs ))) ( defn jobs [ app ] ( let [ in ( chan ) update! ( partial updater in app )] ;; Get an initial state when starting the service. ( GET "/jobs" { :handler update! }) ( go-loop [{ :keys [ type id params ]} ( <! in )] ( condp = type :delete ( DELETE ( str "/jobs/" id ) { :handler update! }) :post ( POST "/jobs" { :params params :handler update! :format :json })) ( recur ( <! in ))) ( swap! app assoc :create! # ( put! in { :type :post :params % })) in ))

This way we get nice idiomatic clojure vector of maps for jobs. Each job map will contain keyword keys and contain a delete! function to kill the element.

Wiring up our model in the main namespace is now as simple as:

( ns jobs.frontend ( :require [ om.core :as om ] [ jobs.views :as views ] [ jobs.model :as model ] [ jobs.router :as router ])) ( defonce app-state ( atom {})) ( def routes [ "/" views/jobs "/job/:id" views/job "/post" views/job-post ]) ( model/jobs app-state ) ( let [ router ( router/init routes app-state ) target { :target ( . js/document ( getElementById "app" ))}] ( om/root router app-state target ))

View essentials

Let’s look at how to build view components now. The simplest one is the job detail view:

( defcomponent job [ app owner ] ( render [ this ] ( let [ id ( get-in app [ :router :params :id ]) job ( first ( filter ( comp ( partial = id ) :id ) ( :jobs app )))] ( html [ :h2 ( :title job ) " at " ( :company job )] [ :p ( :desc job )]))))

Likewise, the job list view can be kept simple:

( defcomponent job-line [{ :keys [ title company url ]} owner ] ( render [ this ] ( html [ :li [ :a { :href url } title " at " company ]]))) ( defcomponent jobs [ app owner ] ( render [ this ] ( html [ :ul ( om/build-all job-line ( :jobs app ))])))

This is unfortunately not too helpful, the first thing that can be added is a call to the delete function in job-line :

( defcomponent job-line [{ :keys [ title company url delete! ]} owner ] ( render [ this ] ( html [ :li [ :a { :href url } title " at " company ] [ :button { :on-click delete! } "delete" ]])))

Life without two-way bindings

One thing om does not (and cannot, given the re-render approach) is the oft-touted two-way bindings found in angular or ember. This means that to handle input, events will need to be listened on and appropriately propagated.

Let’s first look at a sample filter for the job list, in order to recreate the list filter mechanism found in angular.

It would be a shame to store state that is component-local in the global application state, fortunately, om provides a concept of local component state, the render method use must be changed to render-state and an optional init-state method may be provided.

We can then change our job list component to read:

( defn build-predicate [ owner event ] ( let [ v ( or ( -> event .-target .-value ) "" )] ( om/set-state! owner [ :keep? ] ( comp ( partial re-find ( re-pattern v )) :title )))) ( defcomponent jobs [ app owner ] ( init-state [ this ] { :keep? identity }) ( render-state [ this state ] ( html [ :div [ :input { :on-change ( partial build-predicate owner )}] [ :ul ( om/build-all job-line ( filter ( :keep? state ) ( :jobs app )))]])))

In the above, we keep a local predicate function in the component’s state which is initialized to identity to match all jobs.

When changes occur on the text input box, a predicate is built which will be applied on the list for future renders.

The same approach can be taken to implement the form for posting new job entries:

( defn ev->state "Helper function to set local component state" [ owner k event ] ( om/set-state! owner [ k ] ( -> event .-target .-value ))) ( defcomponent job-post [ app owner ] ( render-state [ this { :keys [ title company desc ] :as state }] ( let [ submit! ( fn [ & _ ] (( :create! @ app ) state ) ( redirect "/" ))] ( html [ :form [ :label "title" ] [ :input { :value title :on-change ( partial ev->state owner :title )}] [ :label "company" ] [ :input { :value company :on-change ( partial ev->state owner :company )}] [ :label "description" ] [ :textarea { :value desc :on-change ( partial ev->state owner :desc )}] [ :input { :type "submit" :on-click submit! }]]))))

Wrapping up

With this, we conclude our whirlwind tour of om, paving the way for structured apps. The complete application is available at https://github.com/pyr/om-jobs, with the only notable difference being views relying on twitter bootstrap.

While building on om definitely requires a bigger ramp-up than angular, we have been very happy with this simple approach for routing and model services. Once these basic building blocks are out of the way, we’ve found om to be rather more straightforward and composable than angular.

Our biggest public om application is now the frontend for warp, with more coming!