News in 0.6.0-alpha


Reagent 0.6.0-alpha contains new reactivity helpers, better integration with native React components, a new version of React (0.14.3), new React dependencies (react-dom and react-dom-server), better performance, and much more.

This is a quite big release, so it probably contains a fair amount of bugs as well…

Breaking changes

  • Reagent now depends on cljsjs/react-dom and cljsjs/react-dom-server (see below).
  • The javascript interop macros .' and .!, in the reagent.interop namespace are now called $ and $! respectively (the old names clashed with bootstrapped ClojureScript).
  • Reactions, i.e cursor called with a function, reagent.ratom/reaction, reagent.ratom/run! and reagent.ratom/make-reaction are now lazy and executed asynchronously. Previously, reactions used to execute immediately whenever the atoms they depended on changed. This could cause performance issues in code with expensive reactions and frequent updates to state. However, this change may break existing code that depends on the timing of side-effects from running reactions. flush can be used to force outstanding reactions to run at a given time.
  • Reactions now only trigger updates of dependent components and other reactions if they produce a new result, compared with =. Previously, identical? was used.
  • next-tick is now guaranteed to execute its argument before the next render (more on that below.)

track: Use any function as a reactive value

reagent.core/track takes a function, and optional arguments for that function, and gives a derefable (i.e ”atom-like”) value, containing whatever is returned by that function. If the tracked function depends on a Reagent atom, it is called again whenever that atom changes – just like a Reagent component function. If the value returned by track is used in a component, the component is re-rendered when the value returned by the function changes.

In other words, @(r/track foo x) gives the same result as (foo x) – but in the first case, foo is only called again when the atom(s) it depends on changes.

Here's an example:



  • John Smith
  • Maggie Johnson


(ns example.core
  (:require [reagent.core :as r]))
(defonce app-state (r/atom {:people
                            {1 {:name "John Smith"}
                             2 {:name "Maggie Johnson"}}}))

(defn people []
  (:people @app-state))

(defn person-keys []
  (-> @(r/track people)

(defn person [id]
  (-> @(r/track people)
      (get id)))

(defn name-comp [id]
  (let [p @(r/track person id)]
     (:name p)]))

(defn name-list []
  (let [ids @(r/track person-keys)]
     (for [i ids]
       ^{:key i} [name-comp i])]))

Here, the name-list component will only be re-rendered if the keys of the :people map changes. Every name-comp only renders again when needed, etc.

Use of track can improve performance in three ways:

  • It can be used as a cache for an expensive function, that is automatically updated if that function depends on Reagent atoms (or other tracks, cursors, etc).
  • It can also be used to limit the number of times a component is re-rendered. The user of track is only updated when the function’s result changes. In other words, you can use track as a kind of generalized, read-only cursor.
  • Every use of track with the same arguments will only result in one execution of the function. E.g the two uses of @(r/track people) in the example above will only result in one call to the people function (both initially, and when the state atom changes).

If you've been using reagent.ratom/reaction etc, track should be quite familiar. The main difference is that track uses named functions and variables, rather than depending on closures, and that you don’t have to manage their creation manually (since tracks are automatically cached and reused).

Note: The first argument to track should be a named function, i.e not an anonymous one. Also, beware of lazy data sequences: don’t use deref (i.e ”@”) with the for macro, unless wrapped in doall (just like in Reagent components).


track! is another new function. It works just like track, except that the function passed is invoked immediately, and continues to be invoked whenever any atoms used within it changes.

For example, given this function:


(defn log-app-state []
  (prn @app-state))

you could use (defonce logger (r/track! log-app-state)) to monitor changes to app-state. log-app-state would continue to run until you stop it, using (r/dispose! logger).

with-let: Handling destruction

Reagent now has a new way of writing components that need to do something when they are no longer around: the with-let macro. It looks just like let – but the bindings only execute once, and it takes an optional finally clause, that runs when the component is no longer rendered.

For example: here's a component that sets up an event listener for mouse moves, and stops listening when the component is removed.



Pointer moved to:


(defn mouse-pos-comp []
  (r/with-let [pointer (r/atom nil)
               handler #(swap! pointer assoc
                               :x (.-pageX %)
                               :y (.-pageY %))
               _ (.addEventListener js/document "mousemove" handler)]
     "Pointer moved to: "
     (str @pointer)]
      (.removeEventListener js/document "mousemove" handler))))

The same thing could of course be achieved with React lifecycle methods, but that would be a lot more verbose.

with-let can also be combined with track (and other reactive contexts). For example, the component above could be written as:



Pointer moved to:


(defn mouse-pos []
  (r/with-let [pointer (r/atom nil)
               handler #(swap! pointer assoc
                               :x (.-pageX %)
                               :y (.-pageY %))
               _ (.addEventListener js/document "mousemove" handler)]
      (.removeEventListener js/document "mousemove" handler))))

(defn tracked-pos []
   "Pointer moved to: "
   (str @(r/track mouse-pos))])

The finally clause will run when mouse-pos is no longer tracked anywhere, i.e in this case when tracked-posis unmounted.

with-let can also generally be used instead of returning functions from components that keep local state, and may be a bit easier to read.

Event handling with rswap!

rswap! is another new function in 0.6.0. It works like standard swap! except that it

  • always returns nil
  • allows recursive applications of rswap! on the same atom.

That makes rswap! especially suited for event handling.

Here’s an example that uses event handling with rswap! to edit the data introduced in the section about track above:



  • John Smith
  • Maggie Johnson


(defn event-handler [state [event-name id value]]
  (case event-name
    :set-name   (assoc-in state [:people id :name]
    :add-person (let [new-key (->> state :people keys (apply max) inc)]
                  (assoc-in state [:people new-key]
                            {:name ""}))

(defn emit [e]
  ;; (js/console.log "Handling event" (str e))
  (r/rswap! app-state event-handler e))

(defn name-edit [id]
  (let [p @(r/track person id)]
     [:input {:value (:name p)
              :on-change #(emit [:set-name id (.-target.value %)])}]]))

(defn edit-fields []
  (let [ids @(r/track person-keys)]
     (for [i ids]
       ^{:key i} [name-edit i])
     [:input {:type 'button
              :value "Add person"
              :on-click #(emit [:add-person])}]]))

All events are passed through the emit function, consisting of a trivial application of rswap! and some optional logging. This is the only place where application state actually changes – the rest is pure functions.

The actual event handling is done in event-handler, which takes state and event as parameters, and returns a new state (events are represented by vectors here, with an event name in the first position).

All the UI components have to do is then just to return some markup, and set up routing of events through the emit function.

This architecture basically divides the application into two logical functions:

  • The first takes state and an event as input, and returns the next state.
  • The other takes state as input, and returns a UI definition.

This simple application could probably just as well use the common swap! instead of rswap!, but using swap! in React’s event handlers may trigger warnings due to unexpected return values, and may cause severe headaches if an event handler called by emit itself emits a new event (that would result in lost events, and much confusion).

For a more structured version of a similar approach, see the excellent re-frame framework.

New React version and new namespaces

Reagent now depends on React version 0.14.3. React itself is now split into three parts, with separate packages for browser specific code, and HTML generation respectively.

To reflect that, two new namespaces have been introduced in Reagent as well: reagent.dom and reagent.dom.server. They contain functions that used to be in reagent.core.

reagent.dom contains:

  • render
  • unmount-component-at-node
  • dom-node
  • force-update-all

reagent.dom.server contains:

  • render-to-string
  • render-to-static-markup

These functions are still available in reagent.core in this release (for backward compatibility reasons), but they may be deprecated in the future.

The changes in React also mean that if you specify the React version to use in your project.clj, with cljsjs/react in the :dependencies section, you now have to specify cljsjs/react-dom and cljsjs/react-dom-server instead.

Better interop with native React

The output of create-class can now be used directly in JSX.

”Native React components” can now be used directly in Reagent’s hiccup forms, using this syntax: [:> nativeComp {:key "value"}]. This might sometimes be more convenient than using adapt-react-class.

Reagent should now also be a bit easier to use in node.js. If global React is not defined (i.e React, ReactDOM and ReactDOMServer), Reagent tries to use require instead, to get react, react-dom and react-dom/server from npm.

Better cursor

Cursors are now cached, which should make them a bit easier to use. Previously, every instance of cursor had its own state. Now cursors called with the same arguments share data, which means that components like this now make sense:



A person: John Smith


(defn cursor-example []
  (let [first-person (r/cursor app-state [:people 1])]
    [:p "A person: " (:name @first-person)]))

Previously cursors were really only useful (in the sense that unnecessary re-renderings were avoided) when passed as arguments to child components.

Tapping into the rendering loop

The next-tick function now has a more predictable timing. The function passed to next-tick is now invoked immediately before the next rendering (which is in turn triggered using requestAnimationFrame).

after-update works just like next-tick, except that the function given is invoked immediately after the next rendering.

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