Cannot delete all characters from a UITextField - ios

Question

In my application, there is a possibility to add transactions. The transaction has an attribute called amount and this attribute is a Double. I have implemented the function to add a negative and a positive amount. I do this with a UISegmentedControll. If the user makes the amount negative, the amountTextField.textgets to `"-" + amountTextField.text. That the user can input just Doubles I added this function :
func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
guard let oldText = textField.text, let r = Range(range, in: oldText) else{
return true
}
let newText = oldText.replacingCharacters(in: r, with: string)
let isNumeric = newText.isEmpty || (Double(newText) != nil)
let numberOfDots = newText.components(separatedBy: ".").count - 1
let numberOfDecimalDigits: Int
if let dotIndex = newText.index(of: "."){
numberOfDecimalDigits = newText.distance(from: dotIndex, to: newText.endIndex) - 1
} else {
numberOfDecimalDigits = 0
}
return isNumeric && numberOfDots <= 1 && numberOfDecimalDigits <= 2
}
When there is a minus in front of the positive Double, it is impossible to delete the first number of the string. For example if the amountTextField.text is -399.99, and the user presses the delete button as often as he wants, the textField will show -3. In my debugging work I found out that the function I added to the code is the reason for this.
How can I fix this issue?

Answer 1

Here is one approach:
func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
guard let oldText = textField.text, let r = Range(range, in: oldText) else {
return true
}
let newText = oldText.replacingCharacters(in: r, with: string)
if newText == "-" {
// result will be "-" so just
return true
}
let isNumeric = newText.isEmpty || (Double(newText) != nil)
let numberOfDots = newText.components(separatedBy: ".").count - 1
let numberOfDecimalDigits: Int
if let dotIndex = newText.index(of: "."){
numberOfDecimalDigits = newText.distance(from: dotIndex, to: newText.endIndex) - 1
} else {
numberOfDecimalDigits = 0
}
if isNumeric && numberOfDots <= 1 && numberOfDecimalDigits <= 2 {
// value passes those tests, so make sure the leading "-" is still there
// if not, prepend it, set the text and return false
if newText.first != "-" {
textField.text = "-" + newText
return false
}
}
return isNumeric && numberOfDots <= 1 && numberOfDecimalDigits <= 2
}
We have a couple of additional if blocks to handle:
user moves the insertion point to delete the "-"
user does a "select all" and taps a new digit or delete, or pastes a value
I expect you already know you'll also want a bool check to handle this differently if the segmented control is not in the negative position.