replace entire text string in NSAttributedString without modifying other attributes

You can use NSMutableAttributedString and just update the string, the attributes won't change. Example:

NSMutableAttributedString *mutableAttributedString = [[NSMutableAttributedString alloc] initWithString:@"my string" attributes:@{NSForegroundColorAttributeName: [UIColor blueColor], NSFontAttributeName: [UIFont systemFontOfSize:20]}];

//update the string
[mutableAttributedString.mutableString setString:@"my new string"];

Swift

Change the text while keeping the attributes:

let myString = "my string"
let myAttributes = [NSAttributedString.Key.foregroundColor: UIColor.blue, NSAttributedString.Key.font: UIFont.systemFont(ofSize: 40)]
let mutableAttributedString = NSMutableAttributedString(string: myString, attributes: myAttributes)

let myNewString = "my new string"
mutableAttributedString.mutableString.setString(myNewString)

The results for mutableAttributedString are

• 
• 

Notes

Any sub-ranges of attributes beyond index 0 are discarded. For example, if I add another attribute to the last word of the original string, it is lost after I change the string:

// additional attribute added before changing the text
let myRange = NSRange(location: 3, length: 6)
let anotherAttribute = [ NSAttributedString.Key.backgroundColor: UIColor.yellow ]
mutableAttributedString.addAttributes(anotherAttribute, range: myRange)

Results:

• 
• 

From this we can see that the new string gets whatever the attributes are at index 0 of the original string. Indeed, if we adjust the range to be

let myRange = NSRange(location: 0, length: 1)

we get

• 
• 

See also

• my main answer about Swift attributed strings

I made a little extension to make this really easy:

import UIKit

extension UILabel {
    func setTextWhileKeepingAttributes(string: String) {
        if let newAttributedText = self.attributedText {
            let mutableAttributedText = newAttributedText.mutableCopy()

            mutableAttributedText.mutableString.setString(string)

            self.attributedText = mutableAttributedText as? NSAttributedString
        }
    }
}

https://gist.github.com/wvdk/e8992e82b04e626a862dbb991e4cbe9c