layzr.js
v2.2.2
A modern lazy loading library for images.
0/weekUpdated 3 years agoMIT
Published by Michael Cavalea
npm install layzr.jsLayzr.js
A modern lazy loading library for images.
Getting Started
Follow these steps:
1. Install
2. Setup Images
3. Instantiate
4. Review Options
5. Review Events
6. Review API
7. Review Example Code
* Examples progress like a coffee addiction: small -> medium -> large
Install
Layzr was developed with a modern JavaScript workflow in mind. Though not required, it's convenient to have a build system in place that can transpile ES6, and bundle modules. For a minimal boilerplate that does so, try outset.
Choose an installation option based on your workflow:
* npm
* CDN
* Download
* Framework Bridge
Refer to the releases page for version specific information.
$3
Install Layzr, and add it to your package.json dependencies.
``
$ npm install layzr.js --save
Then import it into the file where you'll use it.
`es6`
import Layzr from 'layzr.js'
$3
Copy and paste one of the following
`
#### cdnjs
`html`
$3
Download the latest version, and load the script in the dist folder.
`html`
$3
Thank you to the community members who created these framework bridges!
* Ruby on Rails: layzr-rails by Mohit Jain
Setup Images
Layzr intelligently chooses the best image source available based on an image's data attributes and browser feature detection.
* In browsers that support srcset, if available, it will be used to determine the source.
* In browsers that don't, the normal or retina source will be chosen based on the devicePixelRatio and availability.
Note that all attribute names are configurable via the options passed to Layzr. To indicate potential sources, add the following attributes to your images:
| Name | Required | Optional |
| :----------------------------- | :------: | :------: |
| data-normal | ✓ | |
| data-retina | | ✓ |
| data-srcset | | ✓ |
$3
Put the _normal resolution_ source in the data-normal attribute.
`html`
Note that Layzr selects elements using this attribute. Elements without it won't be tracked, and will never load.
$3
Add the _retina/high resolution_ source in the data-retina attribute.
`html`
$3
Add the _source set_ in the data-srcset attribute. For information on the proper syntax, read the official specification.
`html`
Instantiate
Create an instance, optionally passing in your options.
Be sure to assign your Layzr instance to a variable. Using your instance, you can:
* start and stop the event listeners
* add and remove event handlers
* accommodate dynamically added elements
`es6
// using the default options
const instance = Layzr()
// using custom options
const instance = Layzr({
// ...
})
`
Options are explained in the following section.
Options
Default options are shown below, and an explanation of each follows:
`es6`
const instance = Layzr({
normal: 'data-normal',
retina: 'data-retina',
srcset: 'data-srcset',
threshold: 0
})
$3
Customize the attribute the normal resolution source is taken from.
`es6`
const instance = Layzr({
normal: 'data-normal'
})
$3
Customize the attribute the retina/high resolution source is taken from.
`es6`
const instance = Layzr({
retina: 'data-retina'
})
$3
Customize the attribute the source set is taken from.
`es6`
const instance = Layzr({
srcset: 'data-srcset'
})
$3
Adjust when images load, relative to the viewport. _Positive values make images load sooner, negative values make images load later_.
Threshold is a percentage of the viewport height, identical to the CSS vh unit.
`es6`
const instance = Layzr({
threshold: 0
})
Events
Layzr instances are extended with Knot.js, a browser-based event emitter. Use the event emitter syntax to add and remove handlers. Review the emitter syntax here.
Layzr emits the following events:
$3
This event is emitted immediately _before an image source is set_. The image node is passed to the event handler.
`es6`
instance.on('src:before', (element) => {
// 'this' is your Layzr instance
// 'element' is the image node
// ...
})
Load event handlers should be attached using this event. See the example, and note the caveats associated with image load events before proceeding.
$3
This event is emitted immediately _after an image source is set_. The image node is passed to the event handler.
`es6`
instance.on('src:after', (element) => {
// 'this' is your Layzr instance
// 'element' is the image node
// ...
})
Note that the image is not necessarily done loading when this event fires.
API
All API methods are chainable, including those from the emitter.
$3
Add or remove the scroll and resize event handlers.
`es6`
instance
.handlers(true) // 'true' adds them
.handlers(false) // 'false' removes them
$3
Manually check if elements are in the viewport.
This method is called while the window is scrolled or resized.
`es6`
instance.check()
$3
Update the elements Layzr is checking.
`es6``
instance.update()
Dynamically added elements should be handled using this method. See the example.
Browser Support
Layzr depends on the following browser APIs:
* classList
* ES5 array methods: forEach, slice
* requestAnimationFrame
It supports the following natively:
* Chrome 24+
* Firefox 23+
* Safari 6.1+
* Opera 15+
* Edge 12+
* IE 10+
* iOS Safari 7.1+
* Android Browser 4.4+
To support older browsers, consider including polyfills/shims for the APIs listed above. There are no plans to include any in the library, in the interest of file size.
Colophon
* Site Design: Chris Allen
* Stock Photos: Unsplash
License
MIT. © 2016 Michael Cavalea
