Add a test case to test the ID observer

OneSignalExample/Assets/OneSignal/Example/OneSignalExampleBehaviour.cs

@@ -33,6 +33,7 @@
 using OneSignalSDK;
 using OneSignalSDK.Notifications;
 using OneSignalSDK.InAppMessages;
+using OneSignalSDK.User;
 using OneSignalSDK.User.Models;
 using OneSignalSDK.Debug.Models;
 using OneSignalSDK.Debug.Utilities;
@@ -159,6 +160,7 @@ private void Start() {
 
         // Setup the below to listen for and respond to state changes
         OneSignal.User.PushSubscription.Changed += _pushSubscriptionChanged;
+        OneSignal.User.Changed += _userStateChanged;
     }
 
     /*
@@ -211,6 +213,11 @@ private void _pushSubscriptionChanged(object sender, PushSubscriptionChangedEven
         _log($"Push subscription changed to current: {JsonUtility.ToJson(e.State.Current)}");
     }
 
+    private void _userStateChanged(object sender, UserStateChangedEventArgs e) {
+        _log($"onesignalId changed : {JsonUtility.ToJson(e.State.onesignalId)}");
+        _log($"externalId changed : {JsonUtility.ToJson(e.State.externalId)}");
+    }
+
     /*
      * SDK setup
      */

com.onesignal.unity.android/Runtime/AndroidUserManager.cs

@@ -37,6 +37,7 @@ namespace OneSignalSDK.Android.User {
     internal sealed class AndroidUserManager : IUserManager {
         private readonly AndroidJavaObject _user;
         private AndroidPushSubscription _pushSubscription;
+        public event EventHandler<UserStateChangedEventArgs> Changed;
 
         public AndroidUserManager(AndroidJavaClass sdkClass) {
             _user = sdkClass.CallStatic<AndroidJavaObject>("getUser");
@@ -102,6 +103,7 @@ public void RemoveSms(string sms)
 
         public void Initialize() {
             _pushSubscription.Initialize();
+            _user.Call("addObserver", new InternalUserChangedHandler(this));
         }
 
         private sealed class InternalUserChangedHandler : OneSignalAndroidJavaProxy {

com.onesignal.unity.core/Editor/Platform/UserManager.cs

@@ -25,6 +25,7 @@
  * THE SOFTWARE.
  */
 
+using System;
 using System.Collections.Generic;
 using OneSignalSDK.User.Models;
 
@@ -38,6 +39,8 @@ public string externalId {
             get => "";
         }
 
+        public event EventHandler<UserStateChangedEventArgs> Changed;
+
         private PushSubscription _subscription = new PushSubscription();
 
         public IPushSubscription PushSubscription {

com.onesignal.unity.core/Runtime/User/IUserManager.cs

@@ -25,7 +25,10 @@
  * THE SOFTWARE.
  */
 
+using System;
 using System.Collections.Generic;
+using System.Threading.Tasks;
+using UnityEngine;
 using OneSignalSDK.User.Models;
 
 namespace OneSignalSDK.User {
@@ -59,6 +62,11 @@ public interface IUserManager {
         /// </summary>
         string externalId { get; }
 
+        /// <summary>
+        /// When onesignalId or externalId has changed
+        /// </summary>
+        event EventHandler<UserStateChangedEventArgs> Changed;
+
         /// <summary>
         /// The 2-character language either as a detected language or explicitly set for this user. See
         /// https://documentation.onesignal.com/docs/language-localization#what-languages-are-supported

com.onesignal.unity.core/Runtime/User/Models/IUserState.cs

@@ -27,7 +27,19 @@
 
 using System;
 
-namespace OneSignalSDK.User.Models {
+namespace OneSignalSDK.User {
+    /// <summary>
+    /// Several states associated with the SDK can be changed in and outside of the application.
+    /// </summary>
+    public class UserStateChangedEventArgs : EventArgs
+    {
+        public IUserState State { get; }
+
+        public UserStateChangedEventArgs(IUserState state) {
+            State = state;
+        }
+    }
+
     public interface IUserState {
         /// <summary>
         /// The unique identifier for your OneSignal account. This will be an empty string until the