iOS - Swift - UITextView scroll to bottom extension

UITextView+scrolls.swift

import UIKit

extension UITextView {
    
    func scrollToBotom() {
        let range = NSMakeRange(text.characters.count - 1, 1);
        scrollRangeToVisible(range);
    }
    
}

This is both slow with a lot of text (the .count property accessor is O(n) for collections which do not conform to SequenceType, and you will see very noticeable slowdowns once you get to 100,000+ characters), and doesn't work properly with things like emoji.

Here's why: scrollRangeToVisible expects an NSRange over the content of the NSTextStorage object, which is an NSAttributedString and stores its actual text content as an NSString, which is indexed by UTF-16 code units. A Swift String's .characters member, on the other hand, presents the String's content as a collection of Unicode extended grapheme clusters (which correspond to visible characters a.k.a. glyphs).

In cases where there is not a one-to-one mapping between UTF-16 code units and extended grapheme clusters (such as with emoji, which generally require two UTF-16 code units to represent — see https://www.objc.io/issues/9-strings/unicode/ for a good discussion), .characters.count of String will report a very different value than .length of NSString.

Fortunately, there is a simple modification which solves both problems (speed and full Unicode compatibility). Just replace this line:

let range = NSMakeRange(text.characters.count - 1, 1);

with this line:

let range = NSMakeRange((text as NSString).length - 1, 1);

That's it.

Works perfectly! Thank you!