Format UITextField text without having cursor move to the end

To complement the other correct answers in this thread here are some code snippets for Swift 4.2 and for both UITextField and UITextView

UITextField

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    // Update Cursor
    let positionOriginal = textField.beginningOfDocument
    let cursorLocation = textField.position(from: positionOriginal, offset: (range.location + string.count))
    if let cursorLocation = cursorLocation {
        textField.selectedTextRange = textField.textRange(from: cursorLocation, to: cursorLocation)
    }
    return false
}

UITextView

func textView(_ textView: UITextView, shouldChangeTextIn range: NSRange, replacementText text: String) -> Bool {
    // Update Cursor
    let positionOriginal = textView.beginningOfDocument
    let cursorLocation = textView.position(from: positionOriginal, offset: (range.location + text.count))
    if let cursorLocation = cursorLocation {
        textView.selectedTextRange = textView.textRange(from: cursorLocation, to: cursorLocation)
    }
    return false
}

The way to make this work is to grab the location of the cursor, update the field contents, and then replace the cursor to its original position. I'm not sure of the exact equivalent in Swift, but the following is how I would do it in Obj-C.

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    UITextPosition *beginning = textField.beginningOfDocument;
    UITextPosition *cursorLocation = [textField positionFromPosition:beginning offset:(range.location + string.length)];

    textField.text = [textField.text stringByReplacingCharactersInRange:range withString:string];

    /* MAKE YOUR CHANGES TO THE FIELD CONTENTS AS NEEDED HERE */

    // cursorLocation will be (null) if you're inputting text at the end of the string
    // if already at the end, no need to change location as it will default to end anyway
    if(cursorLocation)
    {
        // set start/end location to same spot so that nothing is highlighted
        [textField setSelectedTextRange:[textField textRangeFromPosition:cursorLocation toPosition:cursorLocation]];
    }

    return NO;
}

This is an old question, but I had a similar issue and resolved it with the following Swift:

// store the current cursor position as a range
var preAttributedRange: NSRange = textField.selectedRange 
// apply attributed string
var attributedString:NSMutableAttributedString = NSMutableAttributedString(string: fullString)
attributedString = format(textField.text) as NSMutableAttributedString
textField.attributedText = attributedString
// reapply the range
textField.selectedRange = preAttributedRange

It works in the context of my app, hopefully it's useful for someone!

Thanks @Stonz2 by the code in Objective-C. It works like a charm! I used it in my Swift project. The same code in Swift:

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

    let positionOriginal = textField.beginningOfDocument
    let cursorLocation = textField.position(from: positionOriginal, offset: (range.location + NSString(string: string).length))

    /* MAKE YOUR CHANGES TO THE FIELD CONTENTS AS NEEDED HERE */

    if let cursorLoc = cursorLocation {
        textField.selectedTextRange = textField.textRange(from: cursorLoc, to: cursorLoc)
    }
    return false 
}