How to replace a specific character in a string inside a UITextField?

Question

I have a custom textField's input view - it is a Numpad style keyboard. Numpad is using to add numbers and math symbols to a textField.

I can't figure out how can I change a math symbol in a string if user already add one and wants to change it on another straight away. Here is an example of what I need:

Here is the code I use:

 func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {

//number formatter
let formatter = NumberFormatter()
formatter.numberStyle = .decimal
formatter.maximumFractionDigits = 2
formatter.locale = .current
formatter.roundingMode = .down

//all possible math operation symbols user can add
let symbolsSet = Set(["+","-","x","/"])
var amountOfSymbols = 0

let numberString = textField.text ?? ""
guard let range = Range(range, in: numberString) else { return false }
let updatedString = numberString.replacingCharacters(in: range, with: string)
let correctDecimalString = updatedString.replacingOccurrences(of: formatter.groupingSeparator, with: "")
let completeString = correctDecimalString.replacingOccurrences(of: formatter.decimalSeparator, with: ".")

//current math symbol user add
let symbol = symbolsSet.filter(completeString.contains).last ?? ""
//if user add math symbol to an empty string - do not insert
if string == symbol, numberString.count == 0 { return false }

//count how much math symbols string has. If more that one - do not insert, string can have only one
completeString.forEach { character in
    if symbolsSet.contains(String(character)) {
        amountOfSymbols += 1
    }
}
if amountOfSymbols > 1 { return false }

//count how much decimals string has. If more that one - do not insert because it can have only one per number
let numbersArray = completeString.components(separatedBy: symbol)
for number in numbersArray {
    let amountOfDecimalSigns = number.filter({$0 == "."}).count
    if amountOfDecimalSigns > 1 { return false }
}

//create numbers from a string
guard let firstNumber = Double(String(numbersArray.first ?? "0")) else { return true }
guard let secondNumber = Double(String(numbersArray.last ?? "0")) else { return true }

//format numbers and turn them back to string
let firstFormattedNumber = formatter.string(for: firstNumber) ?? ""
let secondFormattedNumber = formatter.string(for: secondNumber) ?? ""

//assign formatted numbers to a textField
textField.text = completeString.contains(symbol) ? "\(firstFormattedNumber)\(symbol)\(secondFormattedNumber)" : "\(firstFormattedNumber)"

return string == formatter.decimalSeparator
}

The logic for me was to use textField.deleteBackwards() method to delete an old one and add a new math symbol after, but with above code it doesn't work: it deletes symbol, but a new one doesn't appear - I should press again so new symbol can appear.

What should I do to change a math symbol in a string?

Test project on GitHub

Answer 1

Inside the shouldChangeCharactersIn delegate use a logic as follows.

     func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        
        var shouldChange : Bool = true
        
        //current text visible in the text field
        let numberString = (textField.text ?? "").replacingOccurrences(of: ",", with: "")
        
        //last charachter in the text field
        let lastCharachter = numberString.last
        
        //get all the numbers displayed in the textfield by sperating them from symbols
        let numbersArray = numberString.components(separatedBy: CharacterSet(charactersIn: "+-x/"))
        
        
        if string == ""{
            
        }
        
        //stop entering a symbol in the first place
        else if numberString == ""{
            if !Character(string).isNumber{
                shouldChange = false
            }
        }
        
        //if a number is entered check for the decimal points
        else if Character(string).isNumber{
            if numbersArray.last!.contains(".") && numbersArray.last?.components(separatedBy: ".").last?.count == 2 && Character(string).isNumber{
                shouldChange =  false
            }else{
                var symbol : String?
                for n in numberString{
                    if "+-/x".contains(n){
                        symbol = String(n)
                        break
                    }
                }
                
                if (symbol != nil){
                    textField.text = "\(numbersArray.first!.withCommas())\(symbol!)\((numbersArray.last! + string).withCommas())"
                }else{
                    textField.text = "\((numbersArray.first! + string).withCommas())"
                }
                shouldChange = false
            }
        // if symbol is entered
        }else{
            if string == "."{
                if !lastCharachter!.isNumber{
                    shouldChange = false
                }
            }

            //if there are more than 1 numbers numbersArray, calculate the value
            else if lastCharachter!.isNumber{
                if numbersArray.count > 1{
                    let expression = NSExpression(format: numberString)
                    let answer =  expression.expressionValue(with:nil, context: nil) as! Double
                    textField.text = "\(forTrailingZero(temp: answer).withCommas())\(string)"
                    shouldChange = false
                }
            }else{
                //change the symbol
                textField.text = "\(textField.text!.dropLast())\(string)"
                shouldChange =  false
            }
        }
        
        return shouldChange
    }

    
    //use to remove trailing zeros
    func forTrailingZero(temp: Double) -> String {
        let tempVar = String(format: "%g", temp)
        return tempVar
    }

extension String {
    func withCommas() -> String {
        let numberFormatter = NumberFormatter()
        numberFormatter.numberStyle = .decimal
        numberFormatter.locale = .current
        return numberFormatter.string(from: NSNumber(value: Double(self)!))!
    }
}

There may be different ways of achieving the expected output.

---

This is another way you can achieve the above result.

    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        
        let formatter = NumberFormatter()
        formatter.numberStyle = .decimal
        formatter.maximumFractionDigits = 2
        formatter.minimumFractionDigits = 1
        formatter.locale = .current
        formatter.roundingMode = .down
        
        let numberString = "\(textField.text ?? "")".replacingOccurrences(of: ",", with: "")
        let lastCharachter = numberString.last
        
        let numbersArray = numberString.components(separatedBy: CharacterSet(charactersIn: "+-x/"))
        
        let amountOfDecimalSigns = "\(numbersArray.last!)\(string)".filter({$0 == "."}).count
        
        if numberString.last == "." && string == "0"{
            formatter.minimumFractionDigits = 1
        }else{
            formatter.minimumFractionDigits = 0
        }
        
        if string == ""{
            return true
        }

        
        if numberString == ""{
            if Character(string).isNumber{
                textField.text = string
            }else{
                return false
            }
        }
        else if amountOfDecimalSigns > 1{
            return false
        }
        
        else if numbersArray.count > 1 {
            
            var symbol = ""
            for str in numberString{
                if "+-/x".contains(str){
                    symbol = String(str)
                    break
                }
            }
            
            if Character(string).isNumber{
                textField.text = "\(formatter.string(for: Float("\(numbersArray.first!)")! as NSNumber)!)\(symbol)\(formatter.string(for: Float("\(numbersArray.last!)\(string)")! as NSNumber)!)"
            }else if string == "."{
                textField.text = "\(textField.text!)\(string)"
            }
            else{
                if lastCharachter!.isNumber{
                    let expression = NSExpression(format: numberString)
                    let answer =  expression.expressionValue(with:nil, context: nil)
                    textField.text = "\(formatter.string(from: answer as! NSNumber)!)\(string)"
                }else if lastCharachter! == "."{
                    let expression = NSExpression(format: String(numberString.dropLast()))
                    let answer =  expression.expressionValue(with:nil, context: nil)
                    textField.text = "\(formatter.string(from: answer as! NSNumber)!)\(string)"
                }
                else{
                    textField.text = "\(numberString.dropLast())\(string)"
                }

            }
        }else{
            if Character(string).isNumber{
                textField.text = "\(formatter.string(for: Float("\(numbersArray.first!)\(string)")! as NSNumber)!)"
            }else{
                if lastCharachter!.isNumber{
                    textField.text = "\(textField.text!)\(string)"
                }else{
                    textField.text = "\(numberString.dropLast())\(string)"
                }
                    
            }
        }
        
        return false
    }

Check the logic in both answers.

Answer 2

You should add a listener for your UITextView text change.

NotificationCenter.default.addObserver(forName: UITextField.textDidChangeNotification, object: myTextField, queue: OperationQueue.main) { [weak self] (notification) in
    guard let self = self else { return }
            
    guard let textField = notification.object as? UITextField else { return }
            
    //here write a logic which verifies if the last character in the text is one of the mathematical symbols, and the one previous to that is also a math symbol, then you would replace the 'lastIndex - 1' with the last character
    guard let text = textField.text, text.count > 2 else { return }
    guard let lastChar = text.last else { return }
    let previousChar: Character = Array(text)[text.count - 2]
    //compare these two characters and make sure the 'previousChar' is a math symbol but not a number, if so - replace it with 'lastChar'
}