iOS UITextField限制输入字数

关于iOS的文本框有时需要限制字数，如手机号，在UITextField的代理单纯写一个判断，在字数超过限制时，这时再想删除就删除不掉，可以在代理这样写，就解决

- (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 > ) // 4 was chosen for SSN verification
    {
        if (string.length > )
        {
            // BasicAlert(@"", @"This field accepts a maximum of 4 characters.");
        }

        return NO;
    }
    return YES;
}