git ssb

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var pull = require('pull-stream')
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var Pause = require('pull-pause')
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var Value = require('mutant/value')
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var onceIdle = require('mutant/once-idle')
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var computed = require('mutant/computed')
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module.exports = Scroller
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function Scroller (scroller, content, render, opts) {
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  if (typeof opts === 'function') {
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    opts = {onDone: opts}
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  } else if (!opts) {
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    opts = {}
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  }
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  var toRenderCount = Value(0)
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  var toAppendCount = Value(0)
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  var pendingVisible = new Set()
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  var queueLength = computed([toRenderCount, toAppendCount], (a, b) => a + b)
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  var pause = Pause(function () {})
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  var running = true
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  var appendQueue = []
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  function appendLoop () {
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    var distanceFromBottom = scroller.scrollHeight - (scroller.scrollTop + scroller.clientHeight)
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    if (distanceFromBottom < scroller.clientHeight) {
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      while (appendQueue.length) {
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        var element = appendQueue.shift()
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        content.appendChild(element)
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        pendingVisible.add(element)
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      }
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    }
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    toAppendCount.set(appendQueue.length)
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    if (queueLength() < 5) {
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      // queue running low, resume stream
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      pause.resume()
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    }
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    if (running || queueLength()) {
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      window.requestAnimationFrame(appendLoop)
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    }
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  }
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  var stream = pull(
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    pause,
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    pull.drain(function (msg) {
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      toRenderCount.set(toRenderCount() + 1)
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      onceIdle(() => {
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        var element = render(msg)
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        appendQueue.push(element)
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        toRenderCount.set(toRenderCount() - 1)
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      })
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      if (queueLength() > 5) {
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        pause.pause()
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      }
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    }, function (err) {
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      running = false
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      clearInterval(visibleInterval)
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      opts.onDone ? opts.onDone(err) : console.error(err)
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    })
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  )
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  var visibleInterval = setInterval(() => {
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    // check for visible items every 2 seconds
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    Array.from(pendingVisible).forEach(checkVisible)
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  }, 2000)
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  stream.queue = queueLength
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  appendLoop()
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  return stream
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  function checkVisible (element) {
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    var height = scroller.clientHeight
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    var rect = element.getBoundingClientRect()
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    if (height > 50 && rect.bottom < height) {
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      pendingVisible.delete(element)
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      if (opts.onItemVisible) {
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        onceIdle(() => opts.onItemVisible(element))
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      }
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    }
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  }
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}
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