packages/@weex/plugins/debug-server/runtime/EventEmitter.js

function EventEmitter() {
  this._handlers = {};
}
EventEmitter.prototype = {
  constructor: EventEmitter,
  off: function (method, handler) {
    if (handler) {
      for (var i = 0; i < this._handlers[method].length; i++) {
        if (this._handlers[method][i] === handler) {
          this._handlers[method].splice(i, 1);
          i--;
        }
      }
    } else {
      this._handlers[method] = [];
    }
  },
  once: function (method, handler) {
    var self = this;
    var fired = false;

    function g() {
      self.off(method, g);
      if (!fired) {
        fired = true;
        handler.apply(self, Array.prototype.slice.call(arguments));
      }
    }

    this.on(method, g);
  },
  on: function (method, handler) {
    if (this._handlers[method]) {
      this._handlers[method].push(handler);
    } else {
      this._handlers[method] = [handler];
    }
  },

  _emit: function (method, args, context) {
    var handlers = this._handlers[method];
    if (handlers && handlers.length > 0) {
      handlers.forEach(function (handler) {
        handler.apply(context, args)
      });
      return true;
    } else {
      return false;
    }
  },

  emit: function (method) {
    var context = {};
    var args = Array.prototype.slice.call(arguments, 1);
    if (!this._emit(method, args, context)) {
      this._emit('*', args, context)
    }
    this._emit('$finally', args, context);
    return context;
  }
};