大寫和小寫字串

要使字串中的所有字元都為大寫或小寫：

Version = 2.2

let text = "AaBbCc"
let uppercase = text.uppercaseString // "AABBCC"
let lowercase = text.lowercaseString // "aabbcc"

Version = 3.0

let text = "AaBbCc"
let uppercase = text.uppercased() // "AABBCC"
let lowercase = text.lowercased() // "aabbcc"