Evaluate IntersectionObserver support

[joshwnj]

Suggested by @Andarist in #54 (comment)

What would you say about implementing IntersectionObserver support? It could even be the default with scroll and interval being used only as a fallback settings. Its already implemented in 2 browsers.

https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API

It would be cool to try this, and compare to the current approach. We can try the polyfill for browsers that don't support it yet:

https://github.com/WICG/IntersectionObserver/tree/gh-pages/polyfill

http://caniuse.com/#search=Intersection

[Andarist]

I might try to implement this in following week or so, no promises yet though.

NOTE (for myself): https://github.com/WICG/EventListenerOptions/blob/gh-pages/explainer.md - could be added too to the basic scroll listener
https://gist.github.com/threepointone/0564095704d76757c631b891f634899a - IntersectionObserver's example usage in React

[Andarist]

Didnt have time to tackle this yet, if somebody wants to pick it up faster, I do not mind - if not, I will still try to do this when I have some spare time

[mshustov]

Actually I used in my project both react-visibility-sensor and IntersectionObserver (IO). So I have some notices. even if main functionality could be replaced, some API are incompatible.

1. checking on startWatching ensures that API consumer gets isVisible status on componentDidMount, for example. but IntersectionObserver calls callback only when real status has changed (elements became visible/invisible)
2. with IO containment must be ancestor of target element
   An IntersectionObserver with a root Element can observe any target Element that is a descendant of the root in the containing block chain.
3. current implementation RVS works for hidden elements (for example, wrapped in element with display: none). IO doesn't work for hidden elements
4. imho implementing things like partialVisibility='side' could be hard without changing rendering API. for getting more info check how algorithm works
   solution is possible with adding some intermediate threshold points, but seems to be performance overhead

Probably, I missed something else.
Any thoughts about feasibility such changes?