Facebook attempt at normalizing wheel event

facebook-normalize-wheel.js

/**
 * Copyright (c) 2015, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule normalizeWheel
 * @typechecks
 */

'use strict';

var UserAgent_DEPRECATED = require('UserAgent_DEPRECATED');

var isEventSupported = require('isEventSupported');


// Reasonable defaults
var PIXEL_STEP  = 10;
var LINE_HEIGHT = 40;
var PAGE_HEIGHT = 800;

/**
 * Mouse wheel (and 2-finger trackpad) support on the web sucks.  It is
 * complicated, thus this doc is long and (hopefully) detailed enough to answer
 * your questions.
 *
 * If you need to react to the mouse wheel in a predictable way, this code is
 * like your bestest friend. * hugs *
 *
 * As of today, there are 4 DOM event types you can listen to:
 *
 *   'wheel'                -- Chrome(31+), FF(17+), IE(9+)
 *   'mousewheel'           -- Chrome, IE(6+), Opera, Safari
 *   'MozMousePixelScroll'  -- FF(3.5 only!) (2010-2013) -- don't bother!
 *   'DOMMouseScroll'       -- FF(0.9.7+) since 2003
 *
 * So what to do?  The is the best:
 *
 *   normalizeWheel.getEventType();
 *
 * In your event callback, use this code to get sane interpretation of the
 * deltas.  This code will return an object with properties:
 *
 *   spinX   -- normalized spin speed (use for zoom) - x plane
 *   spinY   -- " - y plane
 *   pixelX  -- normalized distance (to pixels) - x plane
 *   pixelY  -- " - y plane
 *
 * Wheel values are provided by the browser assuming you are using the wheel to
 * scroll a web page by a number of lines or pixels (or pages).  Values can vary
 * significantly on different platforms and browsers, forgetting that you can
 * scroll at different speeds.  Some devices (like trackpads) emit more events
 * at smaller increments with fine granularity, and some emit massive jumps with
 * linear speed or acceleration.
 *
 * This code does its best to normalize the deltas for you:
 *
 *   - spin is trying to normalize how far the wheel was spun (or trackpad
 *     dragged).  This is super useful for zoom support where you want to
 *     throw away the chunky scroll steps on the PC and make those equal to
 *     the slow and smooth tiny steps on the Mac. Key data: This code tries to
 *     resolve a single slow step on a wheel to 1.
 *
 *   - pixel is normalizing the desired scroll delta in pixel units.  You'll
 *     get the crazy differences between browsers, but at least it'll be in
 *     pixels!
 *
 *   - positive value indicates scrolling DOWN/RIGHT, negative UP/LEFT.  This
 *     should translate to positive value zooming IN, negative zooming OUT.
 *     This matches the newer 'wheel' event.
 *
 * Why are there spinX, spinY (or pixels)?
 *
 *   - spinX is a 2-finger side drag on the trackpad, and a shift + wheel turn
 *     with a mouse.  It results in side-scrolling in the browser by default.
 *
 *   - spinY is what you expect -- it's the classic axis of a mouse wheel.
 *
 *   - I dropped spinZ/pixelZ.  It is supported by the DOM 3 'wheel' event and
 *     probably is by browsers in conjunction with fancy 3D controllers .. but
 *     you know.
 *
 * Implementation info:
 *
 * Examples of 'wheel' event if you scroll slowly (down) by one step with an
 * average mouse:
 *
 *   OS X + Chrome  (mouse)     -    4   pixel delta  (wheelDelta -120)
 *   OS X + Safari  (mouse)     -  N/A   pixel delta  (wheelDelta  -12)
 *   OS X + Firefox (mouse)     -    0.1 line  delta  (wheelDelta  N/A)
 *   Win8 + Chrome  (mouse)     -  100   pixel delta  (wheelDelta -120)
 *   Win8 + Firefox (mouse)     -    3   line  delta  (wheelDelta -120)
 *
 * On the trackpad:
 *
 *   OS X + Chrome  (trackpad)  -    2   pixel delta  (wheelDelta   -6)
 *   OS X + Firefox (trackpad)  -    1   pixel delta  (wheelDelta  N/A)
 *
 * On other/older browsers.. it's more complicated as there can be multiple and
 * also missing delta values.
 *
 * The 'wheel' event is more standard:
 *
 * http://www.w3.org/TR/DOM-Level-3-Events/#events-wheelevents
 *
 * The basics is that it includes a unit, deltaMode (pixels, lines, pages), and
 * deltaX, deltaY and deltaZ.  Some browsers provide other values to maintain
 * backward compatibility with older events.  Those other values help us
 * better normalize spin speed.  Example of what the browsers provide:
 *
 *                          | event.wheelDelta | event.detail
 *        ------------------+------------------+--------------
 *          Safari v5/OS X  |       -120       |       0
 *          Safari v5/Win7  |       -120       |       0
 *         Chrome v17/OS X  |       -120       |       0
 *         Chrome v17/Win7  |       -120       |       0
 *                IE9/Win7  |       -120       |   undefined
 *         Firefox v4/OS X  |     undefined    |       1
 *         Firefox v4/Win7  |     undefined    |       3
 *
 */
function normalizeWheel(/*object*/ event) /*object*/ {
  var sX = 0, sY = 0,       // spinX, spinY
      pX = 0, pY = 0;       // pixelX, pixelY

  // Legacy
  if ('detail'      in event) { sY = event.detail; }
  if ('wheelDelta'  in event) { sY = -event.wheelDelta / 120; }
  if ('wheelDeltaY' in event) { sY = -event.wheelDeltaY / 120; }
  if ('wheelDeltaX' in event) { sX = -event.wheelDeltaX / 120; }

  // side scrolling on FF with DOMMouseScroll
  if ( 'axis' in event && event.axis === event.HORIZONTAL_AXIS ) {
    sX = sY;
    sY = 0;
  }

  pX = sX * PIXEL_STEP;
  pY = sY * PIXEL_STEP;

  if ('deltaY' in event) { pY = event.deltaY; }
  if ('deltaX' in event) { pX = event.deltaX; }

  if ((pX || pY) && event.deltaMode) {
    if (event.deltaMode == 1) {          // delta in LINE units
      pX *= LINE_HEIGHT;
      pY *= LINE_HEIGHT;
    } else {                             // delta in PAGE units
      pX *= PAGE_HEIGHT;
      pY *= PAGE_HEIGHT;
    }
  }

  // Fall-back if spin cannot be determined
  if (pX && !sX) { sX = (pX < 1) ? -1 : 1; }
  if (pY && !sY) { sY = (pY < 1) ? -1 : 1; }

  return { spinX  : sX,
           spinY  : sY,
           pixelX : pX,
           pixelY : pY };
}


/**
 * The best combination if you prefer spinX + spinY normalization.  It favors
 * the older DOMMouseScroll for Firefox, as FF does not include wheelDelta with
 * 'wheel' event, making spin speed determination impossible.
 */
normalizeWheel.getEventType = function() /*string*/ {
  return (UserAgent_DEPRECATED.firefox())
           ? 'DOMMouseScroll'
           : (isEventSupported('wheel'))
               ? 'wheel'
               : 'mousewheel';
};

module.exports = normalizeWheel;