限制UITextField 输入字数

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
// Prevent crashing undo bug – see note below.
if(range.length + range.location > textField.text.length)
{
return NO;
}

NSUInteger newLength = [textField.text length] + [string length] - range.length;
return newLength <= 25;
}

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

let currentCharacterCount = textField.text?.characters.count ?? 0
if (range.length + range.location > currentCharacterCount){
return false
}
let newLength = currentCharacterCount + string.characters.count - range.length
return newLength <= 25
}