文本字段的最小字符?(迅速)

I´m currently working on a login and registration(Xcode 7, Swift2). If a user registers and types his desired username in the text field,i would like him to type at least 5 characters. So if he leaves the text field and haven´t typed in at least 5 characters, a message get´s displayed that tells him to type in at least 5 characters. I only found how to determine the maximum amount of characters, but was not able to adjust it to my needs. This is my current code:I´m currently working on a login and registrati