git ssb

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/* A lazy binding take on computed */
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// doesn't start watching observables until itself is watched, and then releases if unwatched
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// avoids memory/watcher leakage
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var resolve = require('./resolve')
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module.exports = computed
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function computed (observables, lambda) {
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  var values = []
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  var releases = []
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  var computedValue = null
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  var live = false
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  var lazy = false
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  var listeners = []
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  var result = function (listener) {
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    if (!listener) {
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      return getValue()
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    }
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    if (typeof listener !== 'function') {
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      throw new Error('Listeners must be functions.')
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    }
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    listeners.push(listener)
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    listen()
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    return function remove () {
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      for (var i = 0, len = listeners.length; i < len; i++) {
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        if (listeners[i] === listener) {
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          listeners.splice(i, 1)
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          break
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        }
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      }
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      if (!listeners.length) {
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        unlisten()
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      }
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    }
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  }
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  return result
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  // scoped
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  function listen () {
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    if (!live) {
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      for (var i = 0, len = observables.length; i < len; i++) {
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        if (typeof observables[i] === 'function') {
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          releases.push(observables[i](onUpdate))
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        }
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      }
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      live = true
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      lazy = true
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    }
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  }
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  function unlisten () {
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    if (live) {
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      live = false
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      while (releases.length) {
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        releases.pop()()
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      }
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    }
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  }
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  function update () {
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    var changed = false
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    for (var i = 0, len = observables.length; i < len; i++) {
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      var newValue = resolve(observables[i])
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      if (newValue !== values[i] || typeof newValue === 'object') {
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        changed = true
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        values[i] = newValue
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      }
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    }
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    if (changed) {
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      var newComputedValue = lambda.apply(null, values)
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      if (newComputedValue !== computedValue || typeof newComputedValue === 'object') {
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        computedValue = newComputedValue
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        return true
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      }
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    }
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    return false
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  }
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  function onUpdate () {
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    if (update()) {
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      for (var i = 0, len = listeners.length; i < len; i++) {
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        listeners[i](computedValue)
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      }
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    }
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  }
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  function getValue () {
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    if (!live || lazy) {
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      lazy = false
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      update()
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    }
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    return computedValue
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  }
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}
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