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useIntersection

Adds 2 new behaviors to your Stimulus controller : appear and disappear.

This behavior is built on top of the Intersection Observer API

Reference

useIntersection(controller, options = {})

controller : a Stimulus Controller (usually 'this')

options :

Option Description       Default value      
dispatchEvent Whether to dispatch appear, disappear events or not. true
element The root element listening to intersection events. The controller element
eventPrefix Whether to prefix or not the emitted event. Can be a boolean or a string.
- true prefix the event with the controller identifier card:appear
- someString prefix the event with the given string someString:appear
- false to remove prefix		 true
root The element that is used as the viewport for checking visibility of the target. Must be the ancestor of the target. Defaults to the browser viewport if not specified or if null.
rootMargin Margin around the root. Can have values similar to the CSS margin property, e.g. "10px 20px 30px 40px" (top, right, bottom, left). The values can be percentages. This set of values serves to grow or shrink each side of the root element's bounding box before computing intersections. "0px 0px 0px 0px"
threshold Either a single number or an array of numbers which indicate at what percentage of the target's visibility the observer's callback should be executed. If you only want to detect when visibility passes the 50% mark, you can use a value of 0.5. If you want the callback to run every time visibility passes another 25%, you would specify the array [0, 0.25, 0.5, 0.75, 1]. The default is 0 (meaning as soon as even one pixel is visible, the callback will be run). A value of 1.0 means that the threshold isn't considered passed until every pixel is visible. 0

Usage

Composing

import { Controller } from 'stimulus'
import { useIntersection } from 'stimulus-use'

export default class extends Controller {
  options = {
    threshold: 0, // default
  }

  connect() {
    useIntersection(this, this.options)
  }

  appear(entry) {
    // callback automatically triggered when the element
    // intersects with the viewport (or root Element specified in the options)
  }

  disappear(entry) {
    // callback automatically triggered when the element
    // leaves the viewport (or root Element specified in the options)
  }
}

Extending a controller

import { IntersectionController } from 'stimulus-use'

export default class extends IntersectionController {
  options = {
    threshold: 0, // default
  }

  appear(entry) {
    // ...
  }

  disappear(entry) {
    // ...
  }
}

Events

This module adds two new events appear and disapear event that you may use to triggers stimulus actions

For example, to count all visible elements on a page we could listen to individual appear/disappear events to update a counter

<div class="modal" data-controller="counter" data-action="appear@window->counter#increase disappear@window->counter#decrease" >
</div>

Get the emitting controller and entry object for an appear event

count(event) {
  const { controller, entry } = event.detail
}

Example

Rails Infinite scroll : https://github.com/adrienpoly/infinite-scroll-stimulus-js

Polyfill

Availability:

https://caniuse.com/#feat=intersectionobserver

Exisiting polyfill:

https://github.com/w3c/IntersectionObserver/tree/master/polyfill