A truncate function extension for the default String type that honors words.

string-truncate.swift

extension String {
	/// Truncates the string to length number of characters and
	/// doesn't truncate within a word.
	/// appends optional trailing string if longer
		func truncate(_ length: Int, wordSeparator: String = " ", trailing: String = "…") -> String {
		// only truncate if needed
		if self.characters.count > length {
			// count the trailing characters first
			var cumulativeCharacters = trailing.characters.count
			let words = self.components(separatedBy: wordSeparator)
			var wordsToInclude:[String] = []
			for word in words {
				cumulativeCharacters += word.characters.count + 1
				print("\(word) is \(word.characters.count) characters long, cumulative: \(cumulativeCharacters)")
				if cumulativeCharacters <= length {
					wordsToInclude.append(word)
				} else {
					return wordsToInclude.joined(separator: wordSeparator) + trailing
				}
				
			}
			return self.substring(to: self.characters.index(self.startIndex, offsetBy: length)) + trailing
		} else {
			return self
		}
	}
}

// Examples:
"This is a long string".truncate(15) // "This is a…"
"This-is-a-long-string".truncate(15, wordSeparator: "-") // "This-is-a…"
"This is a long string".truncate(15, wordSeparator: " ", trailing: " more…") // "This is more…"

Updated for swift 3 and variable length "trailing" values.