Make “scrollLeft” / “scrollTop” changes not trigger scroll event listener

Question

Currently my program is in a spot where it both listens for the user to scroll a certain element, but also, at times, automatically scrolls this element by itself. (Not a gr

Answer 1

How about a setter?

var _preventEvent = false; // set global flag 

function setScrollTop(amount) {
  _preventEvent = true; // set flag
  document.documentElement.scrollTop = amount * Math.random();
}

function setScrollLeft(amount) {
  _preventEvent = true; // set flag
  document.documentElement.scrollLeft = amount * Math.random();
}

// scroll event listener
window.onscroll = function() {
  console.clear();
  
  if (_preventEvent) {
    _preventEvent = false; // reset flag
    return;
  }
  
  console.log('normal scroll');
}

html{ height:500%; width:500%; } 
button{ position:fixed; }
button + button{ top:50px; }  

<button onclick=setScrollTop(1000)>Random scrollTop</button>
<button onclick=setScrollLeft(1000)>Random scrollLeft</button>

Answer 2

You could constantly check if the scroll position changes...

Answer 3

You could try storing the offset top of your element and matching it against the scrollTop when you step inside onScroll:

function onScroll(){
    var scrollTop = (document.documentElement.scrollTop || document.body.scrollTop),
    offsetTop = $('selector').offset().top;
    if(offsetTop > scrollTop){
      //user has not scrolled to element so do auto scrolling
    }
}

Answer 4

All right, my final solution, building off of Shog9 (not my real code, but my approach):

var oldScrollLeft = element.scrollLeft;
element.scrollLeft = x;
if(element.scrollLeft != oldScrollLeft) {
  ignoreScrollEvents = true;
}

function onScroll() {
  if(!ignoreScrollEvents) performGreatActions();
  ignoreScrollEvents = false;
}

The if statement only sets the ignore flag when the scrollLeft value actually ends up changing, so cases like setting from 0 to 0 don't set the flag incorrectly.

Thanks, all!

Answer 5

Easiest generic method? Just reset the flag in the event handler. You'll want to check first if you're actually changing the value, as otherwise an event won't be fired - as Rodrigo notes, a good practice here is to factor this out into so-called "setter" functions:

function setScrollLeft(x)
{
  if ( element.scrollLeft != x )
  {
    ignoreScrollEvents = true;
    element.scrollLeft = x;
  }
} 

...

function onScroll() 
{
  var ignore = ignoreScrollEvents;
  ignoreScrollEvents = false;

  if (ignore) return false;

  ...
}

But, depending on your needs, you may already be storing the scroll position somewhere; if so, just update and check that as your flag. Something like:

element.scrollLeft = currentScrollLeft = x;

...

function onScroll() 
{
  // retrieve element, etc... 

  if (element.scrollLeft == currentScrollLeft) return false;

  ...
}