git ssb

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// forked because of weird non-filling and over-resuming problems
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// should really PR this, but Dominic might not accept it :D
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var pull = require('pull-stream')
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var Pause = require('pull-pause')
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var Obv = require('obv')
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var next = 'undefined' === typeof setImmediate ? setTimeout : setImmediate
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var buffer = Math.max(window.innerHeight * 2, 1000)
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var u = require('pull-scroll/utils'),
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  assertScrollable = u.assertScrollable,
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  isEnd = u.isEnd,
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  isVisible = u.isVisible
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module.exports = Scroller
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function Scroller(scroller, content, render, isPrepend, isSticky, cb) {
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  assertScrollable(scroller)
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  var obv = Obv()
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  //if second argument is a function,
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  //it means the scroller and content elements are the same.
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  if('function' === typeof content) {
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    cb = isSticky
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    isPrepend = render
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    render = content
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    content = scroller
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  }
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  if(!cb) cb = function (err) { if(err) throw err }
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  scroller.addEventListener('scroll', scroll)
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  var pause = Pause(function () {})
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  var queue = []
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  //apply some changes to the dom, but ensure that
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  //`element` is at the same place on screen afterwards.
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  function add () {
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    if(queue.length) {
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      var m = queue.shift()
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      var r = render(m)
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      append(scroller, content, r, isPrepend, isSticky)
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      obv.set(queue.length)
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    }
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  }
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  function scroll (ev) {
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    if(isEnd(scroller, buffer, isPrepend)) {
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      pause.resume()
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    }
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  }
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  pause.pause()
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  //wait until the scroller has been added to the document
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  next(function next () {
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    if(scroller.parentElement) pause.resume()
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    else                       setTimeout(next, 100)
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  })
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  var stream = pull(
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    pause,
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    pull.drain(function (e) {
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      queue.push(e)
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      obv.set(queue.length)
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      if(content.clientHeight < window.innerHeight)
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        add()
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      if (isVisible(content)) {
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        if (isEnd(scroller, buffer, isPrepend))
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          add()
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      }
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      if(queue.length > 5)
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        pause.pause()
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    }, function (err) {
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      if(err) console.error(err)
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      cb ? cb(err) : console.error(err)
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    })
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  )
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  stream.visible = add
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  stream.observ = obv
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  return stream
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}
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function append(scroller, list, el, isPrepend, isSticky) {
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  if(!el) return
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  var s = scroller.scrollHeight
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  var st = scroller.scrollTop
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  if(isPrepend && list.firstChild)
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    list.insertBefore(el, list.firstChild)
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  else
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    list.appendChild(el)
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  //scroll down by the height of the thing added.
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  //if it added to the top (in non-sticky mode)
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  //or added it to the bottom (in sticky mode)
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  if(isPrepend !== isSticky) {
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    var d = (scroller.scrollHeight - s)
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    var before = scroller.scrollTop
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    //check whether the browser has moved the scrollTop for us.
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    //if you add an element that is not scrolled into view
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    //it no longer bumps the view down! but this check is still needed
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    //for firefox.
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    //this seems to be the behavior in recent chrome (also electron)
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    if(st === scroller.scrollTop) {
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      scroller.scrollTop = scroller.scrollTop + d
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    }
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  }
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}
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