iOS UITextField限制输入字数

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    if (!string.length)
    {
        return YES;
    }
   
    if (textField.keyboardType == UIKeyboardTypeNumberPad)
    {
        if ([string rangeOfCharacterFromSet:[[NSCharacterSet decimalDigitCharacterSet] invertedSet]].location != NSNotFound)
        {
            return NO;
        }
    }
    
    NSString *updatedText = [textField.text stringByReplacingCharactersInRange:range withString:string];
    
    if (updatedText.length > 11) // 4 was chosen for SSN verification
    {
        if (string.length > 1)
        {
            // BasicAlert(@"", @"This field accepts a maximum of 4 characters.");
        }
        
        return NO;
    }    
    return YES;
}