git ssb

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module.exports = function (update, onBind, onUnbind) {
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  var lazy = false
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  var context = this
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  var updating = false
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  var obj = {
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    live: false,
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    nextTick: false,
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    suspended: false,
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    update: update,
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    value: null,
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    listeners: [],
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    broadcast: function () {
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      if (obj.nextTick) {
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        if (!updating) {
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          updating = true
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          setImmediate(broadcast)
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        }
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      } else {
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        broadcast()
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      }
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    },
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    transaction: function (value, cb) {
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      var originalValue = obj.suspended
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      obj.suspended = true
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      cb(value)
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      obj.suspended = originalValue
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      obj.broadcast()
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    },
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    onUpdate: function () {
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      if (obj.nextTick) {
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        if (!updating) {
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          updating = true
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          setImmediate(obj.updateAndBroadcast)
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        }
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      } else {
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        obj.updateAndBroadcast()
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      }
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    },
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    updateAndBroadcast: function () {
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      updating = false
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      if (update.call(context)) {
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        broadcast()
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      }
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    },
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    checkUpdated: function () {
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      if (!obj.live || lazy || updating) {
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        lazy = false
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        if (obj.nextTick && obj.live && lazy) {
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          obj.onUpdate() // use cached value to make more responsive
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        } else {
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          update.apply(context)
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        }
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      }
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    },
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    getValue: function () {
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      obj.checkUpdated.apply(context)
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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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      obj.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.apply(context)
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      }
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      return function release () {
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        for (var i = 0, len = obj.listeners.length; i < len; i++) {
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          if (obj.listeners[i] === listener) {
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            obj.listeners.splice(i, 1)
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            break
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          }
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        }
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        if (!obj.listeners.length && obj.live) {
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          obj.live = false
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          onUnbind.apply(context)
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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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    if (!obj.suspended) {
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      var cachedListeners = obj.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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}
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