wc-context
A comprehensive context implementation for web components
Features
✓ Small and fast. No Internet Explorer support
✓ Flexible ways to define context providers and consumers
✓ Ability to provide or consume one or more contexts per element
✓ Context can be provided or consumed by any HTML element
✓ Context can be identified by string or unique identifier
✓ Works with shadow dom and slotted content (handles timing issues)
✓ Easy to implement unit tests. Most of the time, same as components without context
✓ Builtin integration with LitElement
✓ Builtin ContextProvider (Reactive Controller) with primitives for lazy loading
✓ Builtin context-provider and context-consumer elements
Live examples
Usage
Context can be identified by string or an unique identifier returned by a call to createContext
import { createContext } from 'wc-context'
const themeContext = createContext('theme') // or just a string like 'theme'
To define how a context is provided or consumed, use one of the methods described below. It is possible to mix different methods. For example, a context provided using Lit integration can be consumed by a dedicated custom element and vice versa.
Lit integration
The easiest way to use wc-context
is with the Lit integration exported in the lit namespace (wc-context/lit
). It provides a withContext
class mixin that hooks into the property reactivity system allowing to define context using the property declaration. The context is automatically propagated when the property is updated.
Providing a context
To provide a context add providedContext
to the property declaration
import { withContext } from 'wc-context/lit'
import { LitElement } from 'lit'
class Provider extends withContext(LitElement) {
static properties = {
value: { type: String, providedContext: 'theme' },
activeTitle: { type: String, providedContext: 'title' },
}
toggleTheme() {
this.value = 'newtheme'
}
toggleTitle() {
this.activeTitle = 'New title'
}
}
Consuming a context
To consume a context add context
to the property declaration
import { withContext } from 'wc-context/lit'
import { LitElement, html } from 'lit'
class Consumer extends withContext(LitElement) {
static properties = {
theme: { type: String, context: 'theme' },
titleProp: { type: String, context: 'title' },
}
render() {
return html`<div>Theme is ${this.theme}, title is ${this.titleProp}</div>`
}
}
Custom elements integration
The withContext
class mixin exported in the root namespace, implements an API similar to DOM observedAttributes
/attributeChangedCallback
.
Contexts are defined in an custom element through static providedContexts
field where the key is the context name and value holds a configuration object. The configuration can have a value
property defining the default context value or a property
one defining from what component property the context will retrieve its value.
This mixin can be used in any web component including the created with Lit or other libraries
Providing a context
import { withContext } from 'wc-context'
class Provider extends withContext(HTMLElement) {
static providedContexts = {
theme: { value: 'blue' },
title: 'activeTitle', // shorthand for { property: 'activeTitle' }
}
get activeTitle() {
return this._activeTitle
}
set activeTitle(value) {
this._activeTitle = value
this.updateContext('title')
}
toggleTheme() {
this.updateContext('theme', 'newtheme')
}
toggleTitle() {
this.activeTitle = 'New title'
}
}
Consuming a context
import { withContext } from 'wc-context'
class Consumer extends withContext(HTMLElement) {
static observedContexts = ['theme', ['title', 'titleProp']]
contextChangedCallback(name, oldValue, value) {
console.log(`theme changed from "${oldValue}" to "${value}"`)
// updates el accordingly
}
connectedCallback() {
super.connectedCallback()
this.innerHTML = `<div>Theme is ${this.theme}, title is ${this.titleProp}</div>`
}
}
Dedicated custom elements
The context-provider
and context-consumer
custom elements allows to provide and consume contexts declaratively.
In both elements, the context
attribute / property defines the context. Setting the value
property of context-provider
will change the context value propagating to context-consumer
value
property (or any other context consumer)
An context-update
event is triggered on context-consumer
when context value changes
import 'wc-context/context-provider.js'
import 'wc-context/context-consumer.js'
document.body.innerHTML = `
<context-provider context="theme" value="light">
<div>
<context-consumer context="theme"></context-consumer>
</div>
</context-provider>`
const provider = document.querySelector('context-provider')
const consumer = document.querySelector('context-consumer')
consumer.addEventListener('context-update', ({ context, value }) => {
console.log(`Context ${context}:${value}`)
})
provider.value = 'dark'
Low level API
The low level functions are exported in wc-context/core
and can be used to handle specific cases or create a new interface / integration.
For example, is possible to provide a context in body element to be consumed anywhere in the page.
import { registerContext, updateContext } from 'wc-context/core'
registerContext(document.body, 'theme', 'light')
document.querySelector('#theme-toggle-button').addEventListener('click', () => {
updateContext(document.body, 'theme', 'dark')
})
License
MIT Copyright © 2022 Luiz Américo Pereira Câmara