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

ArgumentTypeDefaultDescription
ruleFunctionA 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()
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(new tab) 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

ArgumentTypeDefaultDescription
keyframeFunctionA 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()
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(new tab). 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

ArgumentTypeDescription
familystringA font family reference which is later required to use this font face.
filesstring[]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
propertiesObject?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()
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

ArgumentTypeDescription
stylestring
Object
Either a pure CSS string or an object of style declarations.
selectorstringIf
style
is passed as an object you must specify a
selector
selector.

Example

import { createRenderer } from 'fela'
const renderer = createRenderer()
// 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(new tab) template strings(new tab).

import { createRenderer } from 'fela'
const renderer = createRenderer()
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

ArgumentTypeDescription
listenerFunctionA 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()
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.