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 style 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

clear

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. It must only use React Native supported properties.
props	Object?	`{}`	An object containing properties to resolve dynamic rule values.

Returns

(string): The style object provided by

StyleSheet.create

Example

import { createRenderer } from 'fela'

const renderer = createRenderer()

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

renderer.renderRule(rule, { size: 12 })
// => { backgroundColor: 'red', fontSize: 12, color: 'blue' }
renderer.renderRule(rule)
// => { backgroundColor: 'red', color: 'blue' }

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 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, { justifyContent: 'center' })
// => { justifyContent: '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' }

Adds a change

listener

to get notified when changes happen.

Argument Type Description

listener

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.

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.

(Object): An object containing the corresponding

unsubscribe

-method.

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: 12, color: 'blue' })
// { type: 'rule', style: { fontSize: 12, color: 'blue' } }

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

clear

Clears the whole cache.