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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// - attaches to inner observables if these are returned from value
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// - doesn't broadcast if value is same as last value (and is `value type` or observable - can't make assuptions about reference types)
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// - doesn't broadcast if value is computed.NO_CHANGE
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var resolve = require('./resolve')
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var isObservable = require('./is-observable')
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module.exports = computed
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computed.NO_CHANGE = {}
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function computed (observables, lambda) {
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  if (!Array.isArray(observables)) {
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    observables = [observables]
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  }
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  var values = []
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  var releases = []
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  var computedValue = null
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  var inner = null
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  var releaseInner = null
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  var live = false
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  var lazy = false
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  var initialized = 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 (isObservable(observables[i])) {
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          releases.push(observables[i](onUpdate))
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        }
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      }
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      if (inner) {
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        releaseInner = inner(onInnerUpdate)
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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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      if (releaseInner) {
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        releaseInner()
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        releaseInner = null
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      }
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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] || isReferenceType(newValue)) {
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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 || !initialized) {
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      initialized = true
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      var newComputedValue = lambda.apply(null, values)
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      if (newComputedValue === computed.NO_CHANGE) {
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        return false
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      }
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      if (newComputedValue !== computedValue || (isReferenceType(newComputedValue) && !isObservable(newComputedValue))) {
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        if (releaseInner) {
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          releaseInner()
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          inner = releaseInner = null
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        }
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        if (isObservable(newComputedValue)) {
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          // handle returning observable from computed
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          computedValue = newComputedValue()
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          inner = newComputedValue
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          if (live) {
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            releaseInner = inner(onInnerUpdate)
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          }
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        } else {
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          computedValue = newComputedValue
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        }
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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 onInnerUpdate (value) {
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    if (value !== computedValue || isReferenceType(computedValue)) {
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      computedValue = value
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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 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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function isReferenceType (value) {
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  return typeof value === 'object' && value !== null
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}
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