cssVarListener

doesn't rely on getComputedStyle() or other functions that cause reflow

install from NPM:

npm install css-var-listener

Usage

import cssVarListener from "css-var-listener"

cssVarListener.add("--yourvar", callback, options)

Where:

• "--yourvar" is any custom property you want to add functionality to
• callback is a function that takes 1 argument, wich is an object containing:
  • target - the element your CSS is applying "--yourvar" to
  • value - the value of "--yourvar" as determined by the document's stylesheets (see Value determination and specificity at the bottom of this readme)
  • oldValue - the previous value of "--yourvar" if a change in the DOM, :hover states, or stylesheet adding/removal caused it to change
  • compiledValue - the value of "--yourvar" with any var(--dep) replaced with its value as determined by the document's stylesheets
  • oldCompiledValue - the previous compiled value
  • prop - the css variable name "--yourvar"
• options is an object with optional properties:
  • ignoreAttr: "data-optional-ignore-attr" causes DOM changes within to be ignored (see Ignoring Observer Changes below)

Lifecycle

• when cssVarListener is called with a prop, it searches the style tags and link'd stylesheets for that prop and caches selectors that set it (plus the specificity and the var value of that rule).
• The callback is called for every element it applies to initially - just once with the final value.
• Rules in the CSS with :hover selectors (that set your var) will result in mouseenter and mouseleave events being attached so the var value will be recalculated as needed (and callbacks will be called if/when the var's value changes for an element)
  • CSS selectors with unnecessary/extra :hover at multiple levels are not currently supported
  • for example: div:hover > a:hover is not supported
  • but div:hover > a, div > a:hover is fine
• an internal observer watches for elements or attributes to be added/changed/removed from the DOM. On any change, it runs the selector cache to determine any callbacks that need to happen
• if an element is removed or otherwise has your var no longer applied, the callback will be called with the element and undefined for the newValue
• When a style tag or link'd stylesheet is added or removed, the selector cache is rebuilt and callbacks are called only for elements with a changed value
• ::pseudo-elements aren't captured by document.querySelectorAll so no callbacks happen for these rules

Ignoring Observer Changes

You can use the built in data-css-var-listener-ignore to ignore all element changes within for all vars enhanced with cssVarListener. This prevents the cascade from checking if the cached selectors apply (or stop applying) to new/different elements.

The ignore attribute is especially important to add to javascript animation containers since every DOM change is observed.

Adding ignoreAttr to cssVarListener's options param (ignoreAttr: "data-optional-ignore-attr") allows specific callbacks (or depentant packages) to individually opt-out of cascade checks from dom changes if needed.

Note: Initial callbacks and initially applied :hover checks for your CSS var still happen if they apply to elements within. Only further changes are ignored. Stylesheets added within are still tracked.

Ignoring styleSheets

Putting data-css-var-listener-ignore-stylesheets on a stylesheet's element or ancestor will cause the stylesheet to be ignored. This prevents newly added or removed stylesheets from rebuilding the selector cache (and subsequent cascade check).

Value determination and specificity

• If your style has a space ( between: these; ), the value will start with the space. All values are strings (or undefined)
• !important flag is ignored in the specificity calculation
• Inline styles (style attribute on the element) are not considered (your callback could override the var inline without blocking potential future callbacks)
• Any other deviation from CSS in specificity is unintentional, please file an issue if you encounter one