Bold words that start with letter - swift

Question

I want to set a label's text with a string that's partly bold. The words I want to make bold all begin with the same letter, say "~".
For example I could have the string, "This ~word is bold, and so is ~this"
Then the label's text would contain the string "This word is bold, and so is this".
Does anybody know if it's possible to make a function like this? I tried the following:
func makeStringBoldForLabel(str: String) {
var finalStr = ""
let words = str.components(separatedBy: " ")
for var word in words {
if word.characters.first == "~" {
var att = [NSFontAttributeName : boldFont]
let realWord = word.substring(from: word.startIndex)
finalStr = finalStr + NSMutableAttributedString(string:realWord, attributes:att)
} else {
finalStr = finalStr + word
}
}
}
but get the error:
Binary operator '+' cannot be applied to operands of type 'String' and 'NSMutableAttributedString'

Answer 1

Easy to solve problem.
use:
func makeStringBoldForLabel(str: String) {
let finalStr = NSMutableAttributedString(string: "")
let words = str.components(separatedBy: " ")
for var word in words {
if word.characters.first == "~" {
var att = [NSFontAttributeName : boldFont]
let realWord = word.substring(from: word.startIndex)
finalStr.append(NSMutableAttributedString(string:realWord, attributes:att))
} else {
finalStr.append(NSMutableAttributedString(string: word))
}
}
}

Answer 2

The error message is clear, you cannot concatenate String and NSAttributedString with the + operator.
You are looking for the API enumerateSubstrings:options. It enumerates strings word by word passing the .byWords option. Unfortunately the tilde (~) is not recognized as a word separator, so we have to check if a word has a preceding tilde. Then change the font attributes at the specific range.
let string = "This ~word is bold, and so is ~this"
let attributedString = NSMutableAttributedString(string: string, attributes:[NSFontAttributeName : NSFont.systemFont(ofSize: 14.0)])
let boldAttribute = NSFont.boldSystemFont(ofSize: 14.0)
string.enumerateSubstrings(in: string.startIndex..<string.endIndex, options: .byWords) { (substring, substringRange, enclosingRange, stop) -> () in
if substring == nil { return }
if substringRange.lowerBound != string.startIndex {
let tildeIndex = string.index(before: substringRange.lowerBound)
if string[tildeIndex..<substringRange.lowerBound] == "~" {
let location = string.distance(from: string.startIndex, to: tildeIndex)
let length = string.distance(from: tildeIndex, to: substringRange.upperBound)
attributedString.addAttribute(NSFontAttributeName, value: boldAttribute, range: NSMakeRange(location, length))
}
}
}