reagent/README.md

# Reagent

A simple [ClojureScript](http://github.com/clojure/clojurescript) interface to [React](http://facebook.github.io/react/).

Reagent provides a way to write efficient React components using (almost) nothing but plain ClojureScript functions.

  * **[Detailed intro with live examples](http://reagent-project.github.io/)**
  * **[News](http://reagent-project.github.io/news/index.html)**
  * **[API Documentation](http://reagent-project.github.io/docs/master/)**
  * **[Tutorials and FAQ](https://github.com/reagent-project/reagent/tree/master/docs)**
  * **Community discussion and support channels**
    * **[#reagent](https://clojurians.slack.com/messages/reagent/)** channel in [Clojure Slack](http://clojurians.net/)
    * **[Reagent Project Mailing List](https://groups.google.com/forum/#!forum/reagent-project)**

### Prerequisites

* Java JDK
* [Leiningen](http://leiningen.org/)

### Usage

To create a new Reagent project simply run:

    lein new reagent myproject

If you wish to only create the assets for ClojureScript without a Clojure backend then do the following instead:

    lein new reagent-frontend myproject

This will setup a new Reagent project with some reasonable defaults, see here for more [details](https://github.com/reagent-project/reagent-template). 

To use Reagent in an existing project you add this to your dependencies in `project.clj`:

[![Clojars Project](http://clojars.org/reagent/latest-version.svg)](http://clojars.org/reagent) <br>
[![CircleCI](https://circleci.com/gh/reagent-project/reagent.svg?style=svg)](https://circleci.com/gh/reagent-project/reagent)

This is all you need to do if you want the standard version of React. If you want the version of React with addons, you'd use something like this instead:

    [reagent "0.7.0" :exclusions [cljsjs/react]]
    [cljsjs/react-with-addons "15.4.2-2"]

If you want to use your own build of React (or React from a CDN), you have to use `:exclusions` variant of the dependency, and also provide a file named "cljsjs/react.cljs", containing just `(ns cljsjs.react)`, in your project.


## Examples

Reagent uses [Hiccup-like](https://github.com/weavejester/hiccup) markup instead of React's sort-of html. It looks like this:

```clj
(defn some-component []
  [:div
   [:h3 "I am a component!"]
   [:p.someclass 
    "I have " [:strong "bold"]
    [:span {:style {:color "red"}} " and red"]
    " text."]])
```

Reagent extends standard Hiccup in one way: it is possible to "squeeze" elements together by using a `>` character.

```clj
[:div
  [:p
    [:b "Nested Element"]]]
```
      
can be written as:
      
```clj
[:div>p>b "Nested Element"]
```        

> **Since version 0.8:** The `:class` attribute also supports collections of classes, and nil values are removed:
> 
> ```clj
> [:div {:class ["a-class" (when active? "active") "b-class"]}]
> ```

You can use one component inside another:

```clj
(defn calling-component []
  [:div "Parent component"
   [some-component]])
```

And pass properties from one component to another:

```clj
(defn child [name]
  [:p "Hi, I am " name])

(defn childcaller []
  [child "Foo Bar"])
```

You mount the component into the DOM like this:

```clj
(defn mountit []
  (r/render [childcaller]
            (.-body js/document)))
```

assuming we have imported Reagent like this:

```clj
(ns example
  (:require [reagent.core :as r]))
```

State is handled using Reagent's version of `atom`, like this:

```clj
(defonce click-count (r/atom 0))

(defn state-ful-with-atom []
  [:div {:on-click #(swap! click-count inc)}
   "I have been clicked " @click-count " times."])
```

Any component that dereferences a `reagent.core/atom` will be automatically re-rendered.

If you want do some setting up when the component is first created, the component function can return a new function that will be called to do the actual rendering:

```clj
(defn timer-component []
  (let [seconds-elapsed (r/atom 0)]
    (fn []
      (js/setTimeout #(swap! seconds-elapsed inc) 1000)
      [:div
       "Seconds Elapsed: " @seconds-elapsed])))
```

This way you can avoid using React's lifecycle callbacks like `getInitialState` and `componentWillMount` most of the time.

But you can still use them if you want to, either using `reagent.core/create-class` or by attaching meta-data to a component function:

```clj
(defonce my-html (r/atom ""))

(defn plain-component []
  [:p "My html is " @my-html])

(def component-with-callback
  (with-meta plain-component
    {:component-did-mount
     (fn [this]
       (reset! my-html (.-innerHTML (reagent/dom-node this))))}))
```

See the examples directory for more examples.


## Performance

React is pretty darn fast, and so is Reagent. It should even be faster than plain old javascript React a lot of the time, since ClojureScript allows us to skip a lot of unnecessary rendering (through judicious use of React's `shouldComponentUpdate`).

The ClojureScript overhead is kept down, thanks to lots of caching.

Code size is a little bigger than React.js, but still quite small. The todomvc example clocks in at roughly 79K gzipped, using advanced compilation.


## About

The idea and some of the code for making components atom-like comes from [pump](https://github.com/piranha/pump). The reactive-atom idea (and some code) comes from [reflex](https://github.com/lynaghk/reflex).

The license is MIT.