Truncate a AttributedString with something other than just "..."

nsattributedstring-ext.swift

import Foundation
import UIKit

extension NSAttributedString {
    // Get the number of lines an attributed string takes
    func lineCount(atSize size: CGSize) -> Int {
        let attrs = self.attributes(at: 0, effectiveRange: nil)
        guard let font = (attrs[NSFontAttributeName] as? UIFont) else {
            return 0
        }
        let paragraph = attrs[NSParagraphStyleAttributeName] as? NSParagraphStyle
        let fontMultiplyer = paragraph?.lineHeightMultiple ?? 1.0
        let lineSpacing = paragraph?.lineSpacing ?? 0
        
        // Take the font's lineHeight, multiply it by the lineHeightMultiplyer and add lineSpacing 
        // (Linespacing is maybe wrong here?) to get height of a single line
        let singleLineHeight = ceil(font.lineHeight * fontMultiplyer + lineSpacing)
        
        // Get our own text height
        let textHeight = self.boundingRect(with: size, options: .usesLineFragmentOrigin, context: nil).height
        return Int(ceil(textHeight / singleLineHeight))
    }
    
    func truncated(withAttrString truncation: NSAttributedString, atLine numLines: Int, width: CGFloat) -> NSAttributedString {
        if lineCount(atSize: CGSize(width: width, height: CGFloat.infinity)) <= numLines {
            // Smaller than number of lines we care about
            return self
        }
        
        var subAttr = NSMutableAttributedString()
        var lastEnd: Int = 0
        
        // 1. Enumerate through words
        (self.string as NSString).enumerateSubstrings(in: NSMakeRange(0, self.string.characters.count), options: .byWords) { (substr, strRange, strRangeWithEnd, stop) in
            // 2. Add the word (without the whitespace) and the truncation string (ie. "... more") to the str
            subAttr.append(self.attributedSubstring(from: strRange))
            subAttr.append(truncation)
            
            // 3. See if that attributed string is too many lines
            if subAttr.lineCount(atSize: CGSize(width: width, height: CGFloat.infinity)) > numLines {
                // 3a. If it is then delete the last word (and any whitespace before it) but leave the truncation string
                
                // We need to delete to the end of the previous range (and then add the difference to the length of the range) because there can be multiple whitespace characters together
                subAttr.deleteCharacters(in: NSMakeRange(lastEnd, strRange.length + (strRange.location - lastEnd)))
                stop.pointee = true
            } else {
                // 3b. If it is too short track the end of this (for sake of deleting whitespace on next pass),
                //     delete the word from the end and then add the word + whitespace and go back to #2
                lastEnd = strRange.location + strRange.length
                let deleteRange = NSMakeRange(strRange.location, strRange.length + truncation.length)
                subAttr.deleteCharacters(in: deleteRange)
                subAttr.append(self.attributedSubstring(from: strRangeWithEnd))
            }
        }
        return subAttr
    }
}

This will be crash if NSAttributedString.string has emoji.

I think this can be changed as bellow

(self.string as NSString).enumerateSubstrings(in: NSMakeRange(0, self.string.characters.count), options: .byWords) { (substr, strRange, strRangeWithEnd, stop)
-->
(self.string as NSString).enumerateSubstrings(in: NSMakeRange(0, self.string.characters.count), options: .byComposedCharacterSequences) { (substr, strRange, strRangeWithEnd, stop)