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 self.characters.count > length {
            return self.substringToIndex(self.startIndex.advancedBy(length)) + (trailing ?? "")
        } else {
            return self
        }
    }
}

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

Thanks for the 2.0 version.
Made very small changes https://gist.github.com/carlhunterroach/445e47525779ec634e9e

Great job! Figured I'd update it for Swift 3.0 and add proper documentation. https://gist.github.com/ViccAlexander/0224ab078f76a3af6d79986369d5240b

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