git ssb

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module.exports = function (update, onBind, onUnbind) {
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  var lazy = false
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  var listeners = []
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  var obj = {
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    live: false,
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    broadcast: broadcast,
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    update: update,
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    value: null,
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    onUpdate: function () {
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      if (update()) {
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        broadcast()
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      }
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    },
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    getValue: function () {
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      if (!obj.live || lazy) {
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        lazy = false
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        update()
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      }
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      return obj.value
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    },
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    addListener: function (listener) {
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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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      if (!obj.live) {
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        obj.live = true
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        lazy = true
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        onBind()
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      }
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      return function release () {
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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 && obj.live) {
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          obj.live = false
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          onUnbind()
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        }
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      }
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    }
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  }
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  return obj
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  // scoped
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  function broadcast () {
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    var cachedListeners = listeners.slice(0)
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    for (var i = 0, len = cachedListeners.length; i < len; i++) {
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      cachedListeners[i](obj.value)
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    }
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  }
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}
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