CODEWITHSUNDEEP
javascriptjqueryhtml
Matchu
Matchu

Reputation: 85794

Make "scrollLeft" / "scrollTop" changes not trigger scroll event listener

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 gradual, pretty scroll, but an instant jump. It makes sense in context, I swear.)

Is there a way to make the scroll event not trigger if the scrolling was done by setting scrollLeft or scrollTop? My first thought was a basic switch, like:

ignoreScrollEvents = true;
element.scrollLeft = x;
ignoreScrollEvents = false;

function onScroll() {
  if(ignoreScrollEvents) return false;
}

but since events don't trigger immediately (oops, duhh), that's not a workable solution. What other sort of answers could I try? I'm also using jQuery, if that helps anything.

Upvotes: 35

Views: 41861

Answers (4)

Shog9
Shog9

Reputation: 159618

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() {
  const 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 = x;
currentScrollLeft = x;

...

function onScroll() {
  // retrieve element, etc... 
  if (element.scrollLeft == currentScrollLeft) {
    return false;
  }

  ...
}

Upvotes: 31

Rodrigo
Rodrigo

Reputation: 4395

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>

Upvotes: 10

Matchu
Matchu

Reputation: 85794

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!

Upvotes: 0

Colin
Colin

Reputation: 2502

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
    }
}

Upvotes: 0

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