Adding 'once' option for IntersectionObserverInit (feature request)

[ziyunfei]

I have a real use case here: getting all IMG elements which at least 50% are displayed in the viewport at 2 seconds after the page is loaded.

setTimeout(() => {
  let observer = new IntersectionObserver(entries => {
    entries.forEach(entry => {
      console.log(entry.target)
    })
  }, {
    threshold: 0.5,
    // once: true
  })
  Array.from(document.querySelectorAll("img")).forEach(item => observer.observe(item))
  setTimeout(() => observer.disconnect(), 20) // a workaround.
}, 2000)

[szager-chromium]

I think you can accomplish this by other means:

setTimeout(() => {
  letObserver = ...
  Array.from(document.querySelectorAll("img").forEach(item => observer.observe(item))
  requestAnimationFrame(() => { setTimeout(() => { observer.disconnect() })})
})

By putting setTimeout inside requestAimationFrame, you are guaranteed that the IntersectionObserver has calculated intersections at least once.

[ojanvafai]

FWIW, it looks like we're adding "once" to addEventListener. If we're adding it there, it seems reasonable to me to add it here in v2.

[mackelito]

would also be nice to have some kind of throttle :)