Renderer

The renderer is the most important utility providing methods to render your styles. It caches every single style that is rendered at any time. Therefore it always has an up-to-date snapshot of your current CSS environment.

You should only have a single renderer which handles all styles of your whole application. To create a new renderer instance, simply use the createRenderer method to actually get a renderer instance.

Methods

renderRule

Renders a rule using the props to resolve it.

Arguments

Argument Type Default Description
rule Function A function which satisfies the rule behavior.
It must return a valid style object.
props Object? {} An object containing properties to resolve dynamic rule values.

Returns

(string): The resolving CSS class name.

Example

import { createRenderer } from 'fela'

const renderer = createRenderer(mountNode)

const rule = props => ({
  backgroundColor: 'red',
  fontSize: props.size,
  color: 'blue'
})

renderer.renderRule(rule, { size: '12px' }) // => a b c
renderer.renderRule(rule) // => a c

Tips & Tricks

To write more advanced and/or simpler rules there are some helpful tips & tricks you might want to know and use:

  • Optional props
    Many rules define CSS declarations that semantically belong together e.g. alignItems and justifyContent. Still, you might not want to use both every time. Therefore, Fela supports optional props. If a value is not set and thus undefined or a string containing undefined it is simply removed by default.
const rule = props => ({
  justifyContent: props.justify,
  alignItems: props.align
})

renderer.renderRule(rule, { justify: 'center' }) // => a
// .a{justify-content:center}
  • Default declarations
    Sometimes you do not pass all props required to completely resolve all style declarations, but want to use a default value in order to not produce any invalid CSS markup. You can achieve this in two ways. Either with ECMA2015 default function parameters or with the logical OR (||) operator.
// default parameters
const rule = ({ color = 'red' } = {}) => ({
  color: color
})

// OR operator
const rule = props => ({
  color: props.color || 'red'
})

rule({ color: 'blue' }) // => { color: 'blue' }
rule({ }) // => { color: 'red' }
  • Conditional values
    Some values might only be applied, if a certain condition is fulfilled. Instead of complex and big if statements you can use the ternary operator.
const rule = ({ type }) => ({
  color: type === 'error' ? 'red' : 'green'
})

rule({ type: 'error' }) // => { color: 'red' }
rule({ }) // => { color: 'green' }

renderKeyframe`

Renders a keyframe using the props to resolve it.

Arguments

Argument Type Default Description
keyframe Function A function which satisfies the keyframe behavior.
It must return a valid keyframe object.
props Object? {} An object containing properties to resolve dynamic keyframe values.

Returns

(string): The resolving animation name.

Example

import { createRenderer } from 'fela'

const renderer = createRenderer(mountNode)

const keyframe = props => ({
  '0%': { color: props.initialColor },
  '33%': { color: 'red' },
  '66%': { color: 'green' },
  '100%': { color: props.initialColor }
})

renderer.renderKeyframe(keyframe, { initialColor: 'blue' }) // => k1
renderer.renderKeyframe(keyframe, { initialColor: 'black' }) // => k2

Tips & Tricks

  • Be sure to only use animateable properties. Other properties will be ignored.
  • Keyframe objects must at least have the steps 0% and 100% or rather from and to. Otherwise it might not be used at all.

renderFont

Renders a @font-face rule using the family as reference.

Arguments

Argument Type Description
family string A font family reference which is later required to use this font face.
files string[] An array of valid source paths. It may either be relative (within your project) or absolute (hosted on an external server).
It must have one of the following file extensions: .woff, .woff2, .eof, .ttf or .svg
properties Object? Additional font properties which are fontVariant, fontWeight, fontStretch, fontStyle and unicodeRange.

Returns

(string) The font family reference that was passed in.

Example

import { createRenderer } from 'fela'

const renderer = createRenderer(mountNode)

const files = [
  '../fonts/Lato.ttf',
  '../fonts/Lato.woff',
  '../fonts/Lato.eof'
]

renderer.renderFont('Lato', files, { fontWeight: 300 })

Caveats

  • If you are using relative paths such as ../fonts/Lato.ttf, keep in mind that it is relative to your index.html.

renderStatic

Renders static styles.

Arguments

Argument Type Description
style string
Object
Either a pure CSS string or an object of style declarations.
selector string If style is passed as an object you must specify a selector selector.

Example

import { createRenderer } from 'fela'

const renderer = createRenderer(mountNode)

// string type style
renderer.renderStatic('html,body{box-sizing:border-box;margin:0}').

// object type style
renderer.renderStatic({
  boxSizing: 'border-box',
  margin: 0
}, 'html,body')

Tips & Tricks

  • Only use static styles for global CSS rules such as resets.
  • Use string styles to include legacy and third-party CSS.
Pro Tip

You can even reuse existing formatted CSS using ECMAScript 2015 template strings.

import { createRenderer } from 'fela'

const renderer = createRenderer(mountNode)

renderer.renderStatic(`
html, body {
  box-sizing: border-box;
  margin: 0
}

div {
  display: -webkit-flex;
  display: -moz-flex;
  display: flex
}
`)

subscribe

Adds a change listener to get notified when changes happen.

Arguments

Argument Type Description
listener Function A callback function that is called on every change. It passes a change object containing information on what actually got rendered or changed. Every change object at least has a unique type and optionally some meta data. In addition it passes the renderer that triggered the change.

Returns

(Object): An object containing the corresponding unsubscribe-method.

Example

import { createRenderer } from 'fela'

const renderer = createRenderer(mountNode)

const rule = props => ({
  fontSize: props.fontSize,
  color: 'blue'
})

const subscription = renderer.subscribe(console.log)
renderer.renderRule(rule, { fontSize: '12px '})
// { type: 'rule', style: 'font-size:12px', selector: 'a', media: '' }
// { type: 'rule', style: 'color:blue', selector: 'b', media: '' }

// Unsubscribing removes the event listener
subscription.unsubscribe()

clear

Clears the whole cache and updates the DOM node to remove all CSS rules.

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