如何判断屏幕上是否呈现HTML元素？

Question

I'd like an element to do a CSS3 animation once the page is scrolled down enough for it to be visible, and I'm wondering if there's any way to accomplish this. Anything involving JavaScript or CSS would work. I've done many Google searches and Stackoverflow searches and can't find exactly what I need.

Answer 1

It depends on what you want to do specifically. I would look at these resources:

http://daneden.github.io/animate.css/

http://www.w3schools.com/css/css3_animations.asp

http://css-tricks.com/almanac/properties/a/animation/

Answer 2

Depending on the complexity of your layout, it could be as simple as finding the scroll position, the height of the window, and where the element is on the page.

function scrollEvent() {
  var el = document.getElementsByTagName('a')[0];
  var body = document.getElementsByTagName('body')[0];
  var posY = (window.innerHeight + body.scrollTop) - el.offsetTop;
  var onScreen = (posY > 0 && posY < window.innerHeight) ? true : false;
}

window.onscroll = scrollEvent;

Use the same technique if you're worried about horizontal positioning, as well.

Answer 3

Put your CSS3 animation style in a class, but don't assign it to your element until it has been scrolled completely into view.

Assuming your element has an id of sprite , this should get you going:

<style>
  .animate {
    //CSS3 animation style
  }
</style>

window.onscroll= function() {
  var sprite = document.getElementById('sprite');
  if(sprite.getBoundingClientRect().top>=0 && sprite.getBoundingClientRect().bottom<=window.innerHeight) {
    sprite.className= 'animate';
  }
}