Components DSL Overview

Hyperloop Components are implemented in the HyperComponent Gem.

Hyperloop Component DSL (Domain Specific Language) is a set of class and instance methods that are used to describe your React components.

The DSL has the following major areas:

  • The Hyperloop::Component class and the equivilent Hyperloop::Component::Mixin mixin.
  • Class methods or macros that describe component class level behaviors.
  • The four data accessors methods: params, state, mutate, and children.
  • The tag and component rendering methods.
  • Event handlers.
  • Miscellaneous methods.

To understand the DSL we will walk through an example that will cover each of these areas in detail.

class Clock < Hyperloop::Component

  param initial_mode: 12

  before_mount do
    mutate.mode params.initial_mode

  after_mount do
    @timer = every(1) { force_update! }

  before_unmount do

    12 => "%a, %e %b %Y %S %I:%M %p",
    24 => "%a, %e %b %Y %S %H:%M"

  render do
    DIV(class: 'time') do[state.mode]).span
      SELECT(style: {"margin-left" => 20}, value: state.mode)  do
        OPTION(value: 12) { "12 Hour Clock" }
        OPTION(value: 24) { "24 Hour Clock" }
      end.on(:change) do |e|


Hyperloop Components classes either include Hyperloop::Component::Mixin or are subclasses of Hyperloop::Component.

class Component < Hyperloop::Component

# if subclassing is inappropriate, you can mixin instead
class AnotherComponent
  include Hyperloop::Component::Mixin

At a minimum every component class must define a render macro which returns one single child element. That child may in turn have an arbitrarily deep structure.

class Component < Hyperloop::Component
  render do
    div # render an empty div

You may also include the top level element to be rendered:

class Component < Hyperloop::Component
  render(DIV) do
    # everything will be rendered in a dic

To render a component, you reference its class name in the DSL as a method call. This creates a new instance, passes any parameters proceeds with the component lifecycle.

class AnotherComponent < Hyperloop::Component
  render do
    Component() # ruby syntax requires either () or {} following the class name

Note that you should never redefine the new or initialize methods, or call them directly. The equivilent of initialize is the before_mount callback.

Macros (Class Methods)

Macros specify class wide behaviors. In our example we use the five most common macros.

class Clock < Hyperloop::Component
  param ...
  before_mount ...
  after_mount ...
  before_unmount ...
  render ...

The param macro describes the parameters the component expects.

The before_mount macro defines code to be run (a callback) when a component instance is first initialized.

The after_mount macro likewise runs after the instance has completed initialization, and is visible in the DOM.

The before_unmount macro provides any cleanup actions before the instance is destroyed.

The render macro defines the render method.

The available macros are: render, param, state, mutate, before_mount, after_mount, before_receive_props, before_update, after_update, before_unmount

Data Accessor Methods

The four data accessor methods - params, state, mutate, and children are instance methods that give access to a component's React specific instance data.

The params method gives (read only) access to each of the params passed to this instance, the state method allows state variables to be read and written, and children returns an enumerator of a component's children.

In our example we see

  before_mount do
    mutate.mode params.mode

params.mode will return the value of the mode parameter passed to this instance, and mutate.mode initializes (or updates) the mode state variable. State variables are like reactive instance variables. They can only be changed using the mutate method, and when they change they will cause a rerender.

More on the details of these methods can be found in the Component API section.

Tag and Component Rendering

    DIV(class: :time) do

HTML such as DIV, A, SELECT, OPTION etc. each have a corresponding instance method that will render that tag. For all the tags the method call looks like this:

tag_name(attribute1 => value1, attribute2 => value2 ...) do
  ...nested tags...

Each key-value pair in the parameter block is passed down as an attribute to the tag as you would expect, with the exception of the style attribute, which takes a hash that is translated to the corresponding style string.

The same rules apply for application defined components, except that the class constant is used to reference the component.

Clock(mode: 12)

Using Strings

Strings are treated specially as follows:

If a render method or a nested tag block returns a string, the string is automatically wrapped in a <span> tag.

The code SPAN { "hello" } can be shortened to "hello".SPAN, likewise for BR, PARA, TD, TH tags.

"some string".BR generates <span>some string<span><br/>[state.mode]).SPAN  # generates <span>...current time formatted...</span>
  OPTION(value: 12) { "12 Hour Clock" }      # generates <option value=12><span>12 Hour Clock</span></option>

HAML style class names

Any tag or component name can be followed by .class_name HAML style.

  # short for
div(class: "class1 class2")

In this style, CSS blends with HTML elements. For example, the following two lines are equivalent:

 BUTTON(class: 'btn-primary') { "Press me" }        
  # is the same as
 button.btn_primary { "Press me" }      

Note that underscores are translated to dashes. So .foo_bar will add the foo-bar class to the tag. If you need to use an underscore in a class name use a double underscore which will be translated to a single underscore in the class name.

Event Handlers

Event Handlers are attached to tags and components using the on method.

SELECT ... do
end.on(:change) do |e|

The on method takes the event name symbol (note that onClick becomes :click) and the block is passed the React.js event object.

Event handlers can be chained like so

INPUT ... do
  end.on(:key_up) do |e|
  end.on(:change) do |e|

Miscellaneous Methods

force_update! is a component instance method that causes the component to re-rerender.

as_node can be attached to a component or tag, and removes the element from the rendering buffer and returns it. This is useful when you need store an element in some data structure, or passing to a native JS component. When passing an element to another Hyperloop Component .as_node will be automatically applied so you normally don't need it.

render can be applied to the objects returned by as_node and children to actually render the node.

class Test < Hyperloop::Component
  param :node

  render do
    DIV do
      children.each do |child|

Ruby and Hyperloop

A key design goal of the DSL is to make it work seamlessly with the rest of Ruby. Notice in the above example, the use of constant declaration (FORMATS), regular instance variables (@timer), and other non-react methods like every (an Opal Browser method).

Component classes can be organized like any other class into a logical module hierarchy or even subclassed.

Likewise the render method can invoke other methods to compute values or even internally build tags.

DSL Gotchas

There are few gotchas with the DSL you should be aware of:

React has implemented a browser-independent events and DOM system for performance and cross-browser compatibility reasons. We took the opportunity to clean up a few rough edges in browser DOM implementations.

  • All DOM properties and attributes (including event handlers) should be snake_cased to be consistent with standard Ruby style. We intentionally break with the spec here since the spec is inconsistent. However, data-* and aria-* attributes conform to the specs and should be lower-cased only.
  • The style attribute accepts a Hash with camelCased properties rather than a CSS string. This is more efficient, and prevents XSS security holes.
  • All event objects conform to the W3C spec, and all events (including submit) bubble correctly per the W3C spec. See Event System for more details.
  • The onChange event (on(:change)) behaves as you would expect it to: whenever a form field is changed this event is fired rather than inconsistently on blur. We intentionally break from existing browser behavior because onChange is a misnomer for its behavior and React relies on this event to react to user input in real time.
  • Form input attributes such as value and checked, as well as textarea.

HTML Entities

If you want to display an HTML entity within dynamic content, you will run into double escaping issues as React.js escapes all the strings you are displaying in order to prevent a wide range of XSS attacks by default.

DIV {'First &middot; Second' }
  # Bad: It displays "First &middot; Second"

To workaround this you have to insert raw HTML.

DIV(dangerously_set_inner_HTML: { __html: "First &middot; Second"})

Custom HTML Attributes

If you pass properties to native HTML elements that do not exist in the HTML specification, React will not render them. If you want to use a custom attribute, you should prefix it with data-.

DIV("data-custom-attribute" => "foo")

Web Accessibility attributes starting with aria- will be rendered properly.

DIV("aria-hidden" => true)

Invoking Application Components

When invoking a custom component you must have a (possibly empty) parameter list or (possibly empty) block. This is not necessary with standard html tags.

MyCustomComponent()  # okay
MyCustomComponent {} # okay
MyCustomComponent    # breaks
br                   # okay