A little truncate function extension for the default String type

string-truncate.swift

extension String {
    /// Truncates the string to length number of characters and 
    /// appends optional trailing string if longer
    func truncate(length: Int, trailing: String? = nil) -> String {
        if countElements(self) > length {
            return self.substringToIndex(advance(self.startIndex, length)) + (trailing ?? "")
        } else {
            return self
        }
    }
}

// Example
let str = "This is a long string".truncate(10, trailing: "...") // "This is a ..."

Thanks! I created a fork with this function updated for Swift 2.0: https://gist.github.com/jesskturner/8def9c0cf756b3bbde79

Swift 4 version: https://gist.github.com/budidino/8585eecd55fd4284afaaef762450f98e