traced-dispatch.js

define(function (require) {
  var _ = require('underscore')
    , dispatch = require('d3-dispatch').dispatch

  return function createDispatcher(/* types */) {
    var dispatcher = dispatch.apply(
          null
        , [ 'firstsubscribed', 'lastunsubscribed' ].concat(_.toArray(arguments))
        )
      , baseOn = dispatcher.on
      , listenersMap = {}

    dispatcher.hasListeners = hasListeners
    dispatcher.on = on

    return dispatcher

    function hasListeners(type) {
      return (type || '').indexOf('.') >= 0
        ? !!listenersMap[type]
        : Object.keys(listenersMap).some(matchesType)

      function matchesType(key) {
        return key == type || (key.indexOf(type + '.') == 0)
      }
    }

    function on(type, listener) {
      var baseType = type.split('.', 1)[0]
        , hadBaseListeners = hasListeners(baseType)

      if (arguments.length > 1) {
        if (_.isNull(listener)) {
          delete listenersMap[type]
          if (hadBaseListeners && !hasListeners(baseType)) {
            dispatcher.lastunsubscribed(baseType)
          }
        } else {
          listenersMap[type] = true
          if (!hadBaseListeners && hasListeners(baseType)) {
            dispatcher.firstsubscribed(baseType)
          }
        }
      }

      return baseOn.apply(dispatcher, arguments)
    }
  }
})