React Observed

React component using the browser's Intersection Observer API to watch for when an element is within (or intersecting with) the viewport.

Example

Storybook Demos

Install

npm install react-observed --save

or with Yarn

yarn add react-observed

Note: For complete browser support you must also provide an Intersection Observer polyfill.

npm install intersection-observer --save

or

yarn add intersection-observer

Usage

<Observed> takes a function as a child which gives you access to the isInView and mapRef properties.

mapRef is the ref function callback that must be declared on the observed target element like <div ref={mapRef} />. Then isInView will give you the current state of the observed element.

Here's an example:

<Observed>
    {({ isInView, mapRef }) => (
        <div ref={mapRef}>
            {isInView ? (
                <span>I'm in view!</span>
            ) : (
                <span>I'm not in view</span>
            )}
        </div>
    )}
</Observed>

<Observed> Props

<Observed> takes the props as shown by the following example:

<Observed
    initialViewState // the initial view state state; defaults to `false`
    intersectionRatio={0.5} // target's visibility must pass the 50% threshold to be considered visible
    once // discontinue observing the target once it's become visible
    onChange={isInView => {}} // handler called with the current `isInView` state whenever it changes
    onEnter={() => {}} // handler called when the observed element enters
    onExit={() => {}} // handler called when the observed element exits
    onIntersect={entry => {}} // handler called when each threshold is met with the entry data
    options={{
        // IntersectionObserver constuctor options
        root: null,
        rootMargin: "0px",
        threshold: 0.5
    }}
/>

Name	Type	Default	Description
initialViewState	Boolean	false	Optionally sets the initial isInView state. By default this is false until the observer updates the state.
intersectionRatio	Number	0	The intersection ratio (a value between 0–1) that when >= to the target's intersect ratio and not equal to 0 will be considered in view.
once	Boolean	false	If once is true the observer will disconnect after the target element has entered the view. Useful for triggering something when in view for a single time.
onChange	Function		Handler to be called with the current isInView state whenever it changes.
onEnter	Function		Handler to be called when the isInView state changes to true.
onExit	Function		Handler to be called when the isInView state changes to false.
onIntersect	Function		Handler to be called when each threshold is met with the current entry data.
options	Object	*see below	Options passed to the IntersectionObserver constructor.

*IntersectionObserver Options

These are the options passed to the IntersectionObserver constructor. The default options to the <Observed> component are:

options: {
    root: null,
    rootMargin: '0px',
    threshold: [0],
}

The following is as described on MDN:

Name	Type	Default	Description
root	Element	null	The DOM element that is used as the viewport for checking visiblity of the target. Must be the ancestor of the target. Defaults to the browser viewport if not specified or if null.
rootMargin	String	0px	Margin around the root. Can have values similar to the CSS margin property, e.g. 10px 20px 30px 40px (top, right, bottom, left). If the root element is specified, 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. Defaults to all zeros.
threshold	Number or Array	[0]	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 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.

Render Callback

The child function takes one object parameter like such { isInView, mapRef }. It's required to map a ref function to a DOM element otherwise <Observed> will throw an error.

Example:

<Observed>{({ mapRef }) => <div ref={mapRef} />}</Observed>

The keys of which are:

Name	Type	Description
isInView	Boolean	This is true when the observed element's intersection ratio is >= to the intersectionRatio prop.
mapRef	Function	This a function that is declared as the ref of the DOM element to be observed. Note that this is required for the observer to work.

Browser Support

Intersection Observer is supported by the latest major browsers. For older browsers a good polyfill is necessary.