Position observer

Runs a callback function when position or size of an observed element changes as a result of scroll or resize of a parent container or window, zooming in and out the page, or setting left and top coordinates of an observed element. Uses IntersectionObserver to detect position change. When position change detected, stops observing the element and starts a requestAnimationFrame loop in which the callback will be invoked. When position stops changing, it stops the requestAnimationFrame loop and returns to observing the element.

• It doesn't run anything in the background when the observed element's bounding box doesn't change.
• It doesn't listen to scroll or resize events.

See Position observer demo page.

Also, for more details see Detecting size and position change of a DOM element as a result of scroll, resize or zoom with IntersectionObserver..

Installation

npm install @itihon/position-observer

Usage

  import PositionObserver from '@itihon/position-observer';

  const ctx = {
    // a context to be passed in the callback
  };

  function callback(target, targetRect, ctx) {
    // do something when position change of the target is detected
  }

  const positionObserver = new PositionObserver(callback, ctx);

  // ... 
  // ... 

  // start observing target's position change
  positionObserver.observe(target);

  // ... 
  // ... 
  
  // stop observing target's position change
  positionObserver.unobserve(target);

  // ... 
  // ... 
  
  // stop observing all targets' position change
  positionObserver.disconnect();

  // ... 
  // ... 
  
  // get an iterator of currently observed target elements
  positionObserver.getTargets();

Debugging

If you encountered a situation in which for some reason it doesn't behave as you expected, you can change import of PositionObserver to debug subfolder. It will be displaying actual observed areas of each side observer as it is demonstrated on this page How it works?. In order to see them, you may need to temporarily make all document's body children half-transparent opacity: .5;.

import PositionObserver from '@itihon/position-observer/debug';