Observe changes in the intersection of a target element.

React component to observe changes in the intersection of a target element or within the document's viewport by using Intersection Observer API.

Intersection information for a react component is needed for many reasons and can be used as:

• Infinite scrolling of web sites, where more and more content is loaded and rendered as scrolled,
• Lazy loading of images or other content as the page is scrolled,
• Controlling elements in-viewport, in case of their loss of visibility,
• Performing tasks or animations based on whether or not the user will see the result,
• Reporting advertisements' visibility in order to calculate ad revenues.

Installation

npm install react-dev-comps.intersection-observer --save

Usage

• Using as infinite-scroller:

Place it as the latest item in list and execute fetch handler on your onEnter callback.

import InfiniteScroller from 'react-dev-comps.intersection-observer';

<div className="list-container">
    <div className="each-todo-item" />
    <div className="each-todo-item" />
    <div className="each-todo-item" />

    { /*... any other items ...*/ }

    <InfiniteScroller onEnter={this.fetchNextList}/>
</div>

• Using as lazy-loader:

Place it as the latest item in list and execute image handler on your onEnter callback.

import LazyLoad from 'react-dev-comps.intersection-observer';

// Since we have 3 images on each row, not to broke style, we used targetStyle prop.

<div className="image-container">
    <img className="each-image" src="" />
    <img className="each-image" src="" />
    <img className="each-image" src="" />

    { /*... any other images ...*/ }

    <LazyLoad
        targetStyle={{ width: "100%", height: "0" }}
        onEnter={this.getNextImages}
    />
</div>

• Using as inview-controller:

Lets say we have an image and zoom-control buttons to zoom in/out. When zoomed in/out, image may be lost from viewport. (as an implementation side-effect)

<div className="app">
    <ImageContainer />
    <ZoomController />
</div>

To overcome this issue, wrap the image with InViewController component and reset the image position on your onLeave callback.

import InViewController from 'react-dev-comps.intersection-observer';

<div className="app">
    <InViewController onLeave={this.resetImagePosition}>
        <ImageContainer />
    </InViewController>
    <ZoomController />
</div>

• Using as task-performer:

Wrap the video container with InViewController component and perform your tasks/animations on your onEnter and onLeave callbacks.

import InViewController from 'react-dev-comps.intersection-observer';

<div className="app">
    <InViewController
        onEnter={this.playVideo}
        onLeave={this.pauseVideo}
    >
        <VideoContainer />
    </InViewController>
</div>

• Using as ads-visibility-timer:

Wrap the AdvertisementPanel with InViewController component and start/stop your timers on your callbacks.

import InViewController from 'react-dev-comps.intersection-observer';

<div className="app">
    <InViewController
        onEnter={this.startTimer}
        onLeave={this.stopTimer}
    >
        <AdvertisementPanel />
    </InViewController>
</div>

API

react-dev-comps.intersection-observer exposes a React Component which takes the following props:

• onIntersect: A cb, invoked when target element intersects with the root. (either intersecting in or out)
• onEnter: A cb, invoked when target element is visible within the root element. (or viewport)
• onLeave: A cb, invoked when target element has lost visibility within the root element. (or viewport)
• onUpdate: An array of variables which must be described when any of your callbacks are using any kind of data. Variables must be the datas you are using on your callbacks. Example
• options: An object passed to IntersectionObserver() constructor to let you control the circumstances for invocation of observer's callbacks. Defaults to {root: null, rootMargin: '0', threshold: 1}
• targetClass: A string class name for target element. Defaults to "intersection-target"
• targetStyle: An object for inner styling target element. Defaults to empty object
• children: A react children component which will be the child of the intersection target element. Defaults to null

Callbacks will be invoked with 2 arguments.

• IntersectionObserverEntry
• IntersectionObserver

Extra Use Cases

1. Let's say, you're depending on some data (variables, props, state-variables etc.) on any of your callbacks. For that situation, you must provide those variables as onUpdate elements to use those variables' latest/updated versions on your callbacks. If you don't use onUpdate prop as required, your callbacks will always be executed with the initial values of those variables.

   import InfiniteScroller from 'react-dev-comps.intersection-observer';
   
   <div className="list-container">
       <div className="each-todo-item" />
       <div className="each-todo-item" />
       <div className="each-todo-item" />
   
       { /*... any other items ...*/ }
   
       // We need fresh versions of *anyPropVariable* and *anyStateVariable* datas.
       <InfiniteScroller
           onEnter={() => {
               if (this.props.anyPropVariable) {
                   // do someting...
               }
   
               if (this.state.anyStateVariable) {
                   // do someting...
               }
           }}
   
           // We **must** provide those variables on our onUpdate array prop.
           onUpdate={[ this.props.anyPropVariable, this.state.anyStateVariable ]}
       />
   </div>

2. Let's say, you have to execute some essential tasks (fetching new items and append to the list) and some background tasks which are non-essential to be completed priorly. For that situation, react-dev-comps.intersection-observer also exposes background-tasks-api with the method name scheduleTasksOnBackground. Although it is a standalone package on npm, you can import and use it as:

   import InfiniteScroller, { scheduleTasksOnBackground } from 'react-dev-comps.intersection-observer';
   
   var nonEssentialWork1 = () => { /* do something */ };
   var nonEssentialWork2 = () => { /* do something */ };
   var tasks = [ nonEssentialWork1, nonEssentialWork2 ];
   
   <div className="list-container">
       <div className="each-todo-item" />
       <div className="each-todo-item" />
       <div className="each-todo-item" />
   
       { /*... any other items ...*/ }
   
       <InfiniteScroller
           onEnter={() => {
               scheduleTasksOnBackground(tasks); // non-essential tasks will be executed on idle callbacks.
               this.fetchNextList(); // free to execute prior tasks on main thread.
           }}
       />
   </div>

License

Licensed under the MIT License, Copyright © 2019-present.

See LICENSE for more information.