I have upadetd the sdk with getLoggedInUser API in App42API class which you can call as follows:
If you are using createUser API of User Service then on successfull user creation, you can use the above API every time you open your app to check if user is loggedIn.
You can aslo set loggeIn user manually as follows:
[App42API setLoggedInUser:@"Your user name"];
You can get the user later on as stated above.
Please download the latest SDK from here.
Let me know if you face any problem.