limit textField textView input char length

this is a category for NSString to calculate the lengh of a string

+ (NSUInteger)countLengthOfString:(NSString *)textString {

    NSUInteger strlength = 0;
    
    char *p = (char *)[textString cStringUsingEncoding:NSUnicodeStringEncoding];
    
    for (int i = 0; i < [textString lengthOfBytesUsingEncoding:NSUnicodeStringEncoding]; i++) {
        if (*p) {
            p++;
            strlength++;
        } else {
            p++;
        }
    }
    
    return strlength;
}

this is the limit method

 // limit textField's char length
    UITextRange *rangeOfMarked = [textField markedTextRange];
    if (!rangeOfMarked) {
        if ([NSString countLengthOfString:textField.text] > LIMIT_CHARACTOR) {
            NSString* tmp;
            for (NSInteger i = textField.text.length; i > 0; i--) {
                tmp = [textField.text substringToIndex:i];
                if ([NSString countLengthOfString:tmp] <= LIMIT_CHARACTOR) {
                    textField.text = tmp;
                    break;
                }
            }
        }        
    }