How can I achieve this functionality to enter DOB in UITextField - ios

Question

I used a variable to store the text being entered and modifying it by appending the remaining suffix of "mm/dd/yyyy". I am getting the functionality, but if I try to update the cursor to the right position, its creating a problem.
I used the textfield.selectedTextRange to move the cursor from EOF to position I need. But, it is replacing the text entered with last "y" from "mm/dd/yyyy". So If I enter "12" the text changes from mm/dd/yyyy| to "yy|/mm/yyyy" instead of "12|/dd/yyy"
Am I doing it the wrong way?
let dobPlaceholderText = "mm/dd/yyyy"
var enteredDOBText = ""
#objc func textFieldDidChange(_ textField: UITextField){
enteredDOBText.append(textField.text.last ?? Character(""))
let modifiedText = enteredDOBText + dobPlaceholderText.suffix(dobPlaceholderText.count - enteredDOBText.count)
textField.text = modifiedText
setCursorPosition(input: dob.textField, position: enteredDOBText.count)
}
private func setCursorPosition(input: UITextField, position: Int){
let position = input.position(from: input.beginningOfDocument, offset: position)!
input.selectedTextRange = input.textRange(from: position, to: position)
}

Answer 1

The suggestion of matt and aheze in the comments to use textField(_:shouldChangeCharactersIn:replacementString:) is very promising in such use cases.
The procedure could look something like this:
determine the resulting text as it would look like after the user input and filter out all non-numeric characters
apply the pattern as defined in your dobPlaceholderText
set the result in the UIText field
determine and set new cursor position
In source code it could look like this:
let dobPlaceholderText = "mm/dd/yyyy"
let delims = "/"
let validInput = "0123456789"
func textField(_ textField: UITextField,
shouldChangeCharactersIn range: NSRange,
replacementString string: String) -> Bool {
let filteredText = filtered(textField.text, range: range, replacement: string)
let newText = applyPattern(filteredText, pattern: Array(dobPlaceholderText))
textField.text = newText
let newPosition = newCursorPosition(range: range, replacement: string, newText: newText)
setCursorPosition(input: textField, position: newPosition)
return false
}
The actual implementation of the three methods filtered, applyPattern and newCursorPosition depends on the exact detail behavior you want to achieve. This is just a sample implementation, use something that reflects your requirements.
private func filtered(_ text: String?,range:NSRange, replacement: String) -> Array<Character> {
let textFieldText: NSString = (text ?? "") as NSString
let textAfterUpdate = textFieldText.replacingCharacters(in: range, with: replacement)
var filtered = Array(textAfterUpdate.filter(validInput.contains))
if filtered.count >= dobPlaceholderText.count {
filtered = Array(filtered[0..<dobPlaceholderText.count])
}
return filtered
}
private func applyPattern(_ filtered: Array<Character>, pattern: Array<Character>) -> String {
var result = pattern
var iter = filtered.makeIterator()
for i in 0..<pattern.count {
if delims.contains(pattern[i]) {
result[i] = pattern[i]
} else if let ch = iter.next() {
result[i] = ch
} else {
result[i] = pattern[i]
}
}
return String(result)
}
private func newCursorPosition(range: NSRange, replacement: String, newText: String) -> Int {
var newPos = 0
if replacement.isEmpty {
newPos = range.location
}
else {
newPos = min(range.location + range.length + 1, dobPlaceholderText.count)
if newPos < dobPlaceholderText.count && delims.contains(Array(newText)[newPos]) {
newPos += 1
}
}
return newPos
}
Demo