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A Computed Style Observer

This prototype ComputedStyleObserver provides the ability to watch for changes being made to the style properties of DOM elements.

Example

<style>
  #test {
    padding: 1em;
    background-color: yellow;
  }
  #test:hover {
    background-color: red;
  }
</style>

<div id="test">
  Hover over me and watch the console
</div>

<!-- load the script -->
<script src="dist/computedStyleObserver.min.js"></script>

<script>
  // Setup a callback
  const callback = entries => {
    entries.forEach(entry => {
      console.log(`Property '${entry.property}' changed from '${entry.previousValue}' to '${entry.value}'`);
    });
  }

  // create the observer
  let computedStyleObserver = new ComputedStyleObserver(callback, ['background-color']);

  // observe an element
  computedStyleObserver.observe(document.getElementById('test'));
</script>

⚠️ A note on performance

Internally, ComputedStyleObserver calls getComputedStyle for each observed element, every animation frame. Since getComputedStyle will trigger a style recalc (and sometimes reflows), performance will suffer as the number of observed elements increases.

ComputedStyleObserver

Constructor

ComputedStyleObserver(callback, properties)

Creates and returns a new ComputedStyleObserver which will invoke a specified callback function when style changes for the given properties occur.

The callback function will receive an array of ComputedStyleObserverEntry objects, one for each property change that occured.

properties is an array of strings specifying which CSS property names should be observed. (i.e. "background-color", "border-radius" etc.)

Methods

disconnect()

Prevents the ComputedStyleObserver instance from receiving further notifications until observe() is called again.

observe(targetElement)

Configures the ComputedStyleObserver instance to begin receiving notifications for the specificed targetElement through its callback function when style changes occur.

unobserve(targetElement)

Prevents the ComputedStyleObserver instance from receiving further notifications for the specificed targetElement.

ComputedStyleObserverEntry

A ComputedStyleObserverEntry represents an individual style mutation. It is the object that is passed to ComputedStyleObserver's callback.

Properties

target

The DOM node the mutation affected

property

The style property that mutated

value

The current value of the property

previousValue

The previous value of the property

Contributing

Requirements

  • Node / NPM

Setup

  1. Clone this repo.
  2. Install dependencies: npm install
  3. Build the project with the watch task: npm run dev
  4. Start editing...

Other build options

  • npm run dist - builds the both the unminified and minified distribution files to the /dist/ folder.