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

UPDATE:

The closest working solution I could get is:

     var completeString = correctDecimalString.replacingOccurrences(of: formatter.decimalSeparator, with: ".")

        //count how much math symbols string has. If more than one - do not insert, string can have only one
    completeString.forEach { character in
        if symbolsSet.contains(String(character)) {
            amountOfSymbols  = 1
        }
    }
    
    if amountOfSymbols > 1 {
        let newSymbol = completeString.last?.description ?? ""
        completeString.removeAll { symbolsSet.contains(String($0))}
        textField.text = completeString newSymbol
        return false
    }

But then I have a problem when adding a second number and pressing a math symbol again:

Maybe my approach is wrong? How to do it correct?

CodePudding user response：

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'
}

CodePudding user response：

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 ?? ""
        
        //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
            }
        // 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))\(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
    }

There may be different ways of achieving the expected output.