How does replacingCharacters work in a delegate? - ios

Question

I'm new to Swift and I'm learning about delegates:
class ZipCodeTextFieldDelegate: NSObject, UITextFieldDelegate {
func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
var newText = textField.text! as NSString
newText = newText.replacingCharacters(in: range, with: string) as NSString // this line
return newText.length <= 5
}
func textFieldShouldReturn(_ textField: UITextField) -> Bool {
textField.resignFirstResponder()
return true;
}
}
This is basically a delegate to limit the character count to within 5. But, I'm not quite sure what role this method replacingCharacters is playing in this function. Apple documentation shows that it
Returns a new string in which the characters in a specified range of
the receiver are replaced by a given string.
which seems pretty straight forward. It replaces some range of characters with a replacement of your choice. I can see that textField has a parameter called range and replacementString, but what range is being provided and what are they being replaced with?
The main ViewController that the delegate is being used in doesn't provide either of them. It simply instantiate the delegate and applies it to the relevant textField
let zipCodeDelegate = ZipCodeTextFieldDelegate()
// MARK: Outlets
#IBOutlet weak var textField: UITextField!
// MARK: Life Cycle
override func viewDidLoad() {
super.viewDidLoad()
self.textField.delegate = self.zipCodeDelegate
}

Answer 1

First, you should understand the purpose of shouldChangeCharactersIn. Whenever the user tries to change the text of a text field in some way, the text field asks its delegates whether it "should change the characters", i.e. whether it should allow the user to do the change that they are trying to do.
Note that the user can change the textfield's text in a lot of ways, not just by "entering a single character". Here's a list of possible ways (you might be able to think of others):
entering a single character, or pasting some text
deleting a single character
selecting a range of characters, and deleting it
selecting a range of characters, and entering a single character, or pasting some text. This will replace the selected range with the single character, or pasted text
Notice that all those ways are just special cases of Way 4. In all of these ways, you are just replacing a range in the textfield's text with some new text. Let me reword the first 3 ways:
replacing a zero-length range at some position with a single character, or some pasted text
replacing a one-length range of the textfield with an empty string
replacing some range of the textfield with an empty string
When the user tries to change the textfield's text in any way, the textfield will make a call like similar to this:
delegate?.textField(self, shouldChangeCharactersIn: someRange, replacementString: someString)
You should now understand what the parameters someRange and someString are for. No matter how the user changes the text, it can always be modelled as "replacing a range of text (someRange) with some other text (someString)".
So what does
newText.replacingCharacters(in: range, with: string)
do?
It simply does the replacement mentioned before. Recall that this delegate method is the textfield asking you, "should this change (modelled as a range and a replacement string) be allowed?" And to determine that, you try to do the change first. If after the change, the text field's text has a length of at most 5, then it is allowed.
The line tries to do the change.
Of course, you are not changing the textfield's text when doing this change, which is why you assign it to a variable called newText.