Platform

Floating UI’s core is essentially a bunch of mathematical calculations performed on rectangles. These calculations are pure and agnostic, allowing Floating UI to work on any platform that can execute JavaScript.

To make it work with a given platform, methods are used to allow it to hook into measurement APIs, for instance, to measure the bounding box of a given element.

Possible platforms other than the DOM include React Native, Canvas/WebGL, etc.

This is Floating UI running in a pure <canvas /> element!

Check it out on CodeSandbox.

Custom platform object

If you’re building a platform from scratch, e.g. your own tiny custom DOM platform, you’ll be using the @floating-ui/core package — see Methods.

If you’re extending or customizing the existing DOM methods, and are using @floating-ui/dom, this is accessible via the platform import:

import {platform} from '@floating-ui/dom';
 
computePosition(referenceEl, floatingEl, {
  platform: {
    ...platform,
    // Overwrite the methods above with your own.
  },
});

Shadow DOM fix

The spec says to hide the actual CSS offsetParent inside shadow DOM hierarchies, notable when using web components. This causes the position to be wrong when a positioned parent has a shadowed child floating element.

Since Chrome 109, the offsetParent property now matches the spec, as well as Safari and Firefox for quite some time, which causes the positioning issue.

In many cases, using the 'fixed' strategy instead of 'absolute' will fix this positioning issue, but in case it does not, you can add the following “ponyfill”:

npm i composed-offset-position

import {platform} from '@floating-ui/dom';
import {offsetParent} from 'composed-offset-position';
 
computePosition(referenceEl, floatingEl, {
  platform: {
    ...platform,
    getOffsetParent: (element) =>
      platform.getOffsetParent(element, offsetParent),
  },
});

Concepts

The library works largely with a Rect:

interface Rect {
  width: number;
  height: number;
  x: number;
  y: number;
}

This data can come from anywhere, and the library will perform the right computations. x and y represent the coordinates of the element relative to another one.

import {computePosition} from '@floating-ui/core';
 
computePosition(referenceElement, floatingElement, {
  platform: {
    // ...
  },
});

Methods

A platform is a plain object consisting of 3 required and 7 optional methods. These methods allow the platform to interface with Floating UI’s logic.

Each of these methods can be either async or sync. This enables support of platforms whose measurement APIs are async, like React Native.

Required methods

getElementRects

Takes in the elements and the positioning strategy and returns the element Rect objects.

function getElementRects({reference, floating, strategy}) {
  return {
    reference: {width: 0, height: 0, x: 0, y: 0},
    floating: {width: 0, height: 0, x: 0, y: 0},
  };
}

reference

The x and y values of a reference Rect should be its coordinates relative to the floating element’s offsetParent element if required rather than the viewport.

floating

Both x and y are not relevant initially, so you can set these both of these to 0.

getDimensions

Returns the dimensions of an element.

function getDimensions(element) {
  return {width: 0, height: 0};
}

getClippingRect

Returns the Rect (relative to the viewport) whose outside bounds will clip the given element. For instance, the viewport itself.

function getClippingRect({element, boundary, rootBoundary}) {
  return {
    width: 0,
    height: 0,
    x: 0,
    y: 0,
  };
}

Optional methods

Depending on the platform you’re working with, these may or may not be necessary.

convertOffsetParentRelativeRectToViewportRelativeRect

This function will take a Rect that is relative to a given offsetParent element and convert its x and y values such that it is instead relative to the viewport.

function convertOffsetParentRelativeRectToViewportRelativeRect({
  rect,
  offsetParent,
  strategy,
}) {
  return rect;
}

getOffsetParent

Returns the offsetParent of a given element. The following four properties are what is accessed on an offsetParent.

function getOffsetParent(element, polyfill) {
  return {
    clientWidth: 0,
    clientHeight: 0,
    clientLeft: 0,
    clientTop: 0,
  };
}

The polyfill parameter exists only for @floating-ui/dom and is optional to fix the Shadow DOM bug.

getDocumentElement

Returns the document element.

function getDocumentElement(element) {
  return {};
}

getClientRects

Returns an array of ClientRects.

function getClientRects(element) {
  return [];
}

isElement

Determines if the current value is an element.

function isElement(value) {
  return true;
}

isRTL

Determines if an element is in RTL layout.

function isRTL(element) {
  return false;
}

getScale

Determines the scale of an element.

function getScale(element) {
  return {x: 1, y: 1};
}

Usage

All these methods are passed to platform:

import {computePosition} from '@floating-ui/core';
 
computePosition(referenceEl, floatingEl, {
  platform: {
    // Required
    getElementRects,
    getDimensions,
    getClippingRect,
 
    // Optional
    convertOffsetParentRelativeRectToViewportRelativeRect,
    getOffsetParent,
    getDocumentElement,
    getClientRects,
    isElement,
    isRTL,
    getScale,
  },
});