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Filename : scrollUtils.js
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/*
 * Copyright (C) 2017 - present Instructure, Inc.
 *
 * This file is part of Canvas.
 *
 * Canvas is free software: you can redistribute it and/or modify it under
 * the terms of the GNU Affero General Public License as published by the Free
 * Software Foundation, version 3 of the License.
 *
 * Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
 * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
 * A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
 * details.
 *
 * You should have received a copy of the GNU Affero General Public License along
 * with this program. If not, see <http://www.gnu.org/licenses/>.
 */

function isScrollPositionAtTop(wind) {
  return wind.pageYOffset === 0
}

function isScrollPositionAtBottom(wind) {
  const doc = wind.document.documentElement
  const docBottom = doc.getBoundingClientRect().bottom
  const clientHeight = doc.clientHeight
  // clientHeight is rounded to an integer, while the rect is a more precise
  // float. This means sometimes bottom is greater than the clientHeight
  // because clientHeight has been rounded off. Also, there appears to be
  // a bug with chrome when zoomed out and this will be off by more than
  // half a pixel. So to fix this, we'll pad the clientHeight just a little
  // so things will work when we're near the bottom.

  return docBottom <= clientHeight + 2
}

function isWheelUpEvent(e) {
  return e.deltaY < 0
}

function isWheelDownEvent(e) {
  return e.deltaY > 0
}

function handleScrollAttempt(cb, _e) {
  cb()
}

function handleWindowWheel(pastCb, futureCb, wind, e) {
  if (isScrollPositionAtTop(wind) && isWheelUpEvent(e)) {
    handleScrollAttempt(pastCb, e)
  } else if (isScrollPositionAtBottom(wind) && isWheelDownEvent(e)) {
    handleScrollAttempt(futureCb, e)
  }
}

// User drags a finger down the screen to scroll up.
// When she gets to the top, and keeps on pulling down, call the callback
// and vice versa
let ongoingTouch = null
function handleTouchStart(e) {
  if (ongoingTouch === null) {
    ongoingTouch = e.changedTouches[0]
  }
}

function handleWindowTouchMove(pastCb, futureCb, wind, e) {
  if (!ongoingTouch) return
  const thisTouch = e.changedTouches[ongoingTouch.identifier]
  if (!thisTouch) return
  if (isScrollPositionAtTop(wind) && thisTouch.screenY - ongoingTouch.screenY > 3) {
    pastCb()
  } else if (isScrollPositionAtBottom(wind) && thisTouch.screenY - ongoingTouch.screenY < -3) {
    futureCb()
  }
}
function handleTouchEnd(_e) {
  ongoingTouch = null
}

class ScrollHandler {
  mostRecentScrollPosition = 0

  callbackThrottle = false

  constructor(scrollCb, wind) {
    this.scrollCb = scrollCb
    this.wind = wind
    wind.addEventListener('scroll', this.handleScrollEvent)
  }

  throttledScrollEvent = () => {
    try {
      this.scrollCb(this.mostRecentScrollPosition)
    } finally {
      this.callbackThrottle = false
    }
  }

  handleScrollEvent = () => {
    this.mostRecentScrollPosition = this.wind.pageYOffset
    if (!this.callbackThrottle) {
      this.callbackThrottle = true
      this.wind.setTimeout(this.throttledScrollEvent, 0)
    }
  }
}

export function registerScrollEvents({
  scrollIntoPast: scrollIntoPastCb,
  scrollIntoFuture: scrollIntoFutureCb,
  scrollPositionChange: scrollCb,
  window: wind,
}) {
  wind = wind || window
  const boundWindowWheel = handleWindowWheel.bind(
    undefined,
    scrollIntoPastCb,
    scrollIntoFutureCb,
    wind
  )
  wind.addEventListener('wheel', boundWindowWheel)

  wind.addEventListener('touchstart', handleTouchStart)
  wind.addEventListener('touchend', handleTouchEnd)
  const boundTouchMove = handleWindowTouchMove.bind(
    undefined,
    scrollIntoPastCb,
    scrollIntoFutureCb,
    wind
  )
  wind.addEventListener('touchmove', boundTouchMove)

  new ScrollHandler(scrollCb, wind)
}