Looping Through NSAttributedString Attributes to Increase Font SIze

Something like this should work:

NSMutableAttributedString *res = [self.richTextEditor.attributedText mutableCopy];[res beginEditing];__block BOOL found = NO;[res enumerateAttribute:NSFontAttributeName inRange:NSMakeRange(0, res.length) options:0 usingBlock:^(id value, NSRange range, BOOL *stop) {    if (value) {        UIFont *oldFont = (UIFont *)value;        UIFont *newFont = [oldFont fontWithSize:oldFont.pointSize * 2];        [res removeAttribute:NSFontAttributeName range:range];        [res addAttribute:NSFontAttributeName value:newFont range:range];        found = YES;    }}];if (!found) {    // No font was found - do something else?}[res endEditing];self.richTextEditor.attributedText = res;

At this point res has a new attributed string with all fonts being twice their original size.

Create an NSMutableAttributedString from your original attributed string before you start. On each iteration of the loop, call addAttribute:value:range: on the mutable attributed string (this will replace the old attributes in that range).

Here is a Swift port of maddy's answer (which works really well for me!). It's wrapped in a little extension.

import UIKitextension NSAttributedString {    func changeFontSize(factor: CGFloat) -> NSAttributedString {        guard let output = self.mutableCopy() as? NSMutableAttributedString else {            return self        }        output.beginEditing()        output.enumerateAttribute(NSAttributedString.Key.font,                                  in: NSRange(location: 0, length: self.length),                                  options: []) { (value, range, stop) -> Void in            guard let oldFont = value as? UIFont else {                return            }            let newFont = oldFont.withSize(oldFont.pointSize * factor)            output.removeAttribute(NSAttributedString.Key.font, range: range)            output.addAttribute(NSAttributedString.Key.font, value: newFont, range: range)        }        output.endEditing()        return output    }}