extension String {
    var length: Int { return countElements(self) }  // Swift 1.1
}

extension String {
    var length: Int { return count(self)         }  // Swift 1.2
}

let globe = "" // U+1F30D EARTH GLOBE EUROPE-AFRICA
count(globe)     // -> 1

// The equivalent of NSString.length is counting the elements in the string's UTF-16 representation
count(globe.utf16) // -> 2

Ref：

http://stackoverflow.com/questions/24037711/get-the-length-of-a-string

http://oleb.net/blog/2014/07/swift-strings/