If you retrieve your current session via
id session = [[[Twitter sharedInstance] sessionStore] session];
You get an interface which is under the hood of the type TWTRSession.
TWTRSession contains the property userName which is handy to show the user with which account he/she is logged in.
You can of course cast this object and return the username but that (with protocol oriented programming) should not be needed and might even be considered a bad practice.
Could you please add the userName property to the interface?
Small side note: shouldn’t it be username instead of userName
The username is the (usually unique) thing you type in with your password, for example: bobsmith66.
The user name is the name of the user, the user’s real life name, for example: Bob Smith. User name is sometimes used for username, but occasionally it makes a difference, so be clear and avoid the ambiguity. (Better still, use full name when you want them to enter Bob Smith.)
User-name is a variant of username, but this is rarely used if ever.