Introduction
When writing a mobile application you mostly always need a way to store the information outside of the application itself so the data is accessible to not only the application itself on a specific device, but on every device the application is installed on and perhaps even web applications. This means the application will need some sort of a backend service/API to communicate with. This usually also means the application will use some sort of account/user management.

There are several so-called mobile backend as a service platform which facilitate this. The word mobile is a bit misplaced I think because the clients of those platforms are not always mobile devices.  I therefor call those platforms Backend As a Service (BAAS). Parse.com is one of those platforms.

This post talks about the way you can manage users with Parse.com and specifically how to use anonymous users and convert between an anonymous and registered user.

Anonymous user
A lot of apps require the user to register (create a user account) or login with a Facebook or similar account. If this is a required process, chances are that a certain group of users will not use your app because of the required login. If your app functionality allows, a possible work-around for this is to provide the app with anonymous user login. By using an anonymous user, the user of your app can experience all or most of the functionality of the app without requiring a user account. If the user likes your app, he/she can then sign-up for a registered account. Ideally, all of the data gathered during anonymous access, should be transferred to the registered account. Fortunately, the above functionality is fairly easy to implement using the Parse.com platform.

Enabling anonymous access
To enable your Parse.com app for using anonymous access, you have to do the following:

1. Enable anonymous access in the Parse.com console. Go to Settings -> Authentication and enable "Allow anonymous access".

2. Add the following code in the onCreate method of your Android Application class:

@Override
    public void onCreate() {
        super.onCreate();
        Parse.initialize(this, "APPLICATION_ID", "CLIENT_KEY");
        ParseUser.enableAutomaticUser();
}

By enabling automatic user, the call to ParseUser.getCurrentUser() always returns a user and thus is never null. You can check if the user is an anonymous user or a registered one by using the following code:

ParseAnonymousUtils.isLinked(ParseUser.getCurrentUser());

This can be useful to check if the sign-up button should be displayed or disabling some functionality which is only accessible to registered users.

Converting an anonymous user into a registered one
An anonymous user can be converted to a registered one. The data belonging to the anonymous user is also present on the registered one.

Before converting an anonymous user, there are some things to consider:

1. The username can not be left blank. You must explicitly specify a username and password on the user which is to be converted into a registered one.
2. It is adviced to save the anonymous user to the backend as soon as it is created. If this is not done and a call to saveInBackground is called on the registered user (after converting it from an anonymous one) a stack overflow is generated from the Android parse SDK. See also the following question on Stack-overflow (created by me): http://stackoverflow.com/questions/27595057/converting-an-anonymous-user-to-a-regular-user-and-saving

@Override
    public void onCreate() {
        super.onCreate();
        Parse.initialize(this, "APPLICATION_ID", "CLIENT_KEY");
        ParseUser.enableAutomaticUser();
        ParseUser.getCurrentUser.saveInBackground();
}

findViewById(R.id.createUser).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(final View v) {
                final String accountUsername = username.getText().toString();
                final String accountPassword = password.getText().toString();
                final ParseUser user = ParseUser.getCurrentUser();
                user.setUsername(accountUsername);
                user.setPassword(accountPassword);
                user.signUpInBackground(new SignUpCallback() {
                    @Override
                    public void done(final ParseException e) {
                        if (e != null) {
                            Toast.makeText(MainActivity.this, "Signup Fail", Toast.LENGTH_SHORT).show();
                            Log.e(TAG, "Signup fail", e);
                        } else {
                            Toast.makeText(MainActivity.this, "Signup success", Toast.LENGTH_SHORT).show();
                            final ParseUser user = ParseUser.getCurrentUser();
                            user.put("phone_no", "31612345678");
                            user.saveInBackground(new SaveCallback() {
                                @Override
                                public void done(final ParseException e) {
                                    if (e != null) {
                                        Toast.makeText(MainActivity.this, "Save data Fail", Toast.LENGTH_SHORT).show();
                                        Log.e(TAG, "Signup fail", e);
                                    } else {
                                        Toast.makeText(MainActivity.this, "Save data success", Toast.LENGTH_SHORT).show();
                                    }
                                }
                            });
                        }
                    }
                });
            }
        })