Cordova Windows plugin: Winmd + event subscription in JS

BatteryProxy.js

cordova.define("org.apache.cordova.battery-status.Battery", function(require, exports, module) { /*
 *
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 *
 */

var eventHandlerWrapper;

function handleResponse(successCb, errorCb, jsonResponse) {
    var info = JSON.parse(jsonResponse);

    if (info.hasOwnProperty("exceptionMessage")) {
        errorCb(info.exceptionMessage);
        return;
    }

    successCb(info, { keepCallback: true });
}


var Battery = {
    start: function (win, fail, args, env) {
        stopped = false;
        try {
            if (WinJS.Utilities.isPhone === true) {
                function getBatteryStatus(success, error) {
                    handleResponse(success, error, BatteryStatus.BatteryStatus.getBatteryStatus());
                }
               
                getBatteryStatus(win, fail);
                eventHandlerWrapper = function(json) {
                     handleResponse(win, fail, json);
                };

                // Issue: This event callback is not being called
                BatteryStatus.BatteryStatus.addEventListener("remainingchargepercentchanged", eventHandlerWrapper);
            } else {
                fail("The operation is unsupported by this platform.");
            }
        } catch(e) {
            fail(e);
        }        
    },

    stop: function () {
        stopped = true;
        BatteryStatus.BatteryStatus.removeEventListener("remainingchargepercentchanged", eventHandlerWrapper);
    }
};

require("cordova/exec/proxy").add("Battery", Battery);

});

BatteryStatus.cs

using System;
using System.IO;
using System.Runtime.Serialization;
using System.Runtime.Serialization.Json;
using Windows.Phone.Devices.Power;

namespace BatteryStatus
{
    public sealed class BatteryStatus
    {
        private static Battery battery = Battery.GetDefault();

        static BatteryStatus()
        {
            battery.RemainingChargePercentChanged += BatteryOnRemainingChargePercentChanged;
        }

        public static event EventHandler<string> RemainingChargePercentChanged;
        public static string getBatteryStatus()
        {
            try
            {
                return Serialize(typeof(BatteryInfo), new BatteryInfo
                {
                    Level = battery.RemainingChargePercent
                });                
            }
            catch (Exception ex)
            {
                return Serialize(typeof(ExceptionInfo), new ExceptionInfo { Message = ex.Message });
            }
        }

        private static void BatteryOnRemainingChargePercentChanged(object sender, object o)
        {
            if(RemainingChargePercentChanged != null)
                RemainingChargePercentChanged(null, getBatteryStatus());
        }

        private static string Serialize(Type type, object obj)
        {
            using (var stream = new MemoryStream())
            {
                var jsonSer = new DataContractJsonSerializer(type);
                jsonSer.WriteObject(stream, obj);
                stream.Position = 0;
                return new StreamReader(stream).ReadToEnd();
            }
        }

        [DataContract]
        private class BatteryInfo
        {
            [DataMember(Name = "level")]
            public int Level;

            // Not supported by native API
            [DataMember(Name = "isPlugged")]
            public string IsPlugged;
        };

        [DataContract]
        private class ExceptionInfo
        {
            [DataMember(Name = "exceptionMessage")]
            public string Message = string.Empty;
        };
    }
}