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 ..."