index.js

/* global MutationObserver */
const document = require('global/document')
const window = require('global/window')
const watch = Object.create(null)
const KEY_ID = 'onloadid' + Math.random().toString(36).slice(2)
const KEY_ATTR = 'data-' + KEY_ID
let INDEX = 0

if (window && window.MutationObserver) {
  const observer = new MutationObserver(function (mutations) {
    if (!isHashValid(watch)) return
    let i = mutations.length
    while (i--) {
      if (mutations[i].attributeName === KEY_ATTR) {
        eachAttr(mutations[i], turnon, turnoff)
        continue
      }
      eachMutation(mutations[i].removedNodes, function (index, el) {
        if (!document.documentElement.contains(el)) turnoff(index, el)
      })
      eachMutation(mutations[i].addedNodes, function (index, el) {
        if (document.documentElement.contains(el)) turnon(index, el)
      })
    }
  })

  observer.observe(document.documentElement, {
    childList: true,
    subtree: true,
    attributes: true,
    attributeOldValue: true,
    attributeFilter: [KEY_ATTR]
  })
}

module.exports = function onload (el, on, off, caller) {
  on = on || function () {}
  off = off || function () {}
  el.setAttribute(KEY_ATTR, 'o' + INDEX)
  watch['o' + INDEX] = [on, off, 0, caller || onload.caller]
  INDEX += 1
  return el
}

module.exports.KEY_ATTR = KEY_ATTR
module.exports.KEY_ID = KEY_ID

function turnon (index, el) {
  if (watch[index][0] && watch[index][2] === 0) {
    watch[index][0](el)
    watch[index][2] = 1
  }
}

function turnoff (index, el) {
  if (watch[index][1] && watch[index][2] === 1) {
    watch[index][1](el)
    watch[index][2] = 0
  }
}

function eachAttr (mutation, on, off) {
  const newValue = mutation.target.getAttribute(KEY_ATTR)
  if (sameOrigin(mutation.oldValue, newValue)) {
    watch[newValue][2] = watch[mutation.oldValue][2]
    return
  }
  if (watch[mutation.oldValue]) {
    off(mutation.oldValue, mutation.target)
  }
  if (watch[newValue]) {
    on(newValue, mutation.target)
  }
}

function sameOrigin (oldValue, newValue) {
  if (!oldValue || !newValue) return false
  return watch[oldValue][3] === watch[newValue][3]
}

function eachMutation (nodes, fn) {
  const keys = Object.keys(watch)
  for (let i = 0; i < nodes.length; i++) {
    if (nodes[i] && nodes[i].getAttribute && nodes[i].getAttribute(KEY_ATTR)) {
      const onloadid = nodes[i].getAttribute(KEY_ATTR)
      keys.forEach(function (k) {
        if (onloadid === k) {
          fn(k, nodes[i])
        }
      })
    }
    if (nodes[i] && nodes[i].childNodes.length > 0) {
      eachMutation(nodes[i].childNodes, fn)
    }
  }
}

function isHashValid (hash) {
  /* eslint-disable no-unreachable-loop */
  for (const k in hash) {
    return k
  }
}