cache-element

Cache a bel element. Makes rendering elements very fast™. Analogous to React's .shouldComponentUpdate() method, but only using native DOM methods.

Features

• makes rendering elements very fast™
• widget API helps with creating stateful wrappers around 3rd party libs
• implemented in only a couple of lines
• only uses native DOM methods

Usage

Caching

Here we take a regular element, and cache it so re-renders are fast. We compare the arguments on each run passing it to the compare function:

const cache = require('cache-element')
const html = require('bel')

const renderEl = cache(createEl, function compare (curr, prev) {
  if (curr && !prev) return true
  return (prev === curr)
})

let el = renderEl('Tubi') // creates new element
let el = renderEl('Tubi', 'Tubi') // returns cached element (proxy)
let el = renderEl('Babs', 'Tubi') // creates new element

function createEl (name, age) {
  return html`
    <p>The person's name is ${name}</p>
  `
}

Widget

Here we take a widget (e.g. d3, gmaps) and wrap it so it return a DOM node once, and then only has to worry about managing its lifecycle:

const widget = require('cache-element/widget')
const html = require('bel')

const renderEl = widget(function createEl (update) {
  let name = null
  let age = null

  update(onupdate)

  return html`
    <p onload=${onload} onunload=${onunload}>
      Name is ${name} and age is ${age}
    </p>
  `

  function onupdate (newName, newAge) {
    name = newName
    age = newAge
  }

  function onload () {
    console.log('added to DOM')
  }

  function onunload () {
    name = null
    age = null
    console.log('removed from DOM')
  }
})

let el = renderEl('Tubi', 12) // creates new element
let el = renderEl('Tubi', 12) // returns cached element (proxy)
let el = renderEl('Babs', 25) // returns cached element (proxy)

API

createEl = cache(render(...args), compare?)

Cache an element. The compare function is optional, and by default compares the first and second argument with ===:

function compare (curr, prev) {
  if (curr && !prev) return true
  return (prev === curr)
}

createEl = widget(render(update(onupdate((...args))))

Render a widget. Takes a render function which exposes an update function which takes an onupdate function which is passed arguments whenever arguments are passed into the tree. Unlike cache, widget takes no compare function as it will always return a proxy element. If you want to prevent any updates from happening, run a comparison inside onupdate.

el = createEl(...args)

Render an element, passing it arbitrary arguments. The arguments are passed to the element's compare function (onupdate for widget).

FAQ

Where does this run?

Make sure you're running a diffing engine that checks for .isSameNode(), if it doesn't you'll end up with super weird results because proxy nodes will probably be rendered which is not what should happen. Probably make sure you're using morphdom. Seriously.

What's a proxy node?

It's a node that overloads Node.isSameNode() to compare it to another node. This is needed because a given DOM node can only exist in one DOM tree at the time, so we need a way to reference mounted nodes in the tree without actually using them. Hence the proxy pattern, and the recently added support for it in certain diffing engines:

const html = require('bel')

const el1 = html`<div>pink is the best</div>`
const el2 = html`<div>blue is the best</div>`

// let's proxy el1
const proxy = html`<div></div>`
proxy.isSameNode = function (targetNode) {
  return (targetNode === el1)
}

el1.isSameNode(el1)   // true
el1.isSameNode(el2)   // false
proxy.isSameNode(el1) // true
proxy.isSameNode(el2) // false

How does it work?

Morphdom is a diffing engine that diffs real DOM trees. It runs a series of checks between nodes to see if they should either be replaced, removed, updated or reordered. This is done using a series of property checks on the nodes.

Since v2.1.0 morphdom also runs Node.isSameNode(otherNode). This allows us to override the function and replace it with a custom function that proxies an existing node. Check out the code to see how it works. The result is that if every element in our tree uses cache-element, only elements that have changed will be recomputed and rerendered making things very fast.

What's the exact difference between cache and widget?

• cache return a proxy node if the arguments were the same. If arguments change, it'll rerender and return a new node.
• widget will always return a proxy node. It also listens for the node to be unmounted from the DOM so it can clean up internal references, making it more expensive to use.

Installation

$ npm install cache-element

License

MIT