Retrieve specific query string parameter from NSURL

querystring.swift

func getQueryStringParameter(url: String, param: String) -> String? {
        
    let url = NSURLComponents(string: url)!
      
    return
      (url.queryItems? as [NSURLQueryItem])
      .filter({ (item) in item.name == param }).first?
      .value()
  }

why guard?

extension URL {
    func getQueryString(parameter: String) -> String? {
        return URLComponents(url: self, resolvingAgainstBaseURL: false)?
            .queryItems?
            .first { $0.name == parameter }?
            .value
    }
}

I'm using Objective - C and Categories

Header file

#import <Foundation/Foundation.h>

@interface NSString (getQueryStringParameter)

/**
 * @method getQueryStringParameter:url:key
 * @abstract Method get specific value from NSURL
 *
 * I wrote this method follow these guides:
 * - https://gist.github.com/gillesdemey/509bb8a1a8c576ea215a
 * - https://stackoverflow.com/questions/8756683/best-way-to-parse-url-string-to-get-values-for-keys/26406426#26406426
 *
 */
+ (NSString *)getQueryStringParameter:(NSURL *)url :(NSString *)key;

@end

Implementation

#import <Foundation/Foundation.h>

#import "NSString+getQueryStringParameter.h"

@implementation NSString (Validation)

+ (NSString *)getQueryStringParameter:(NSURL *)url :(NSString *)key {
    NSURLComponents *urlComponents = [NSURLComponents componentsWithURL:url
                                                resolvingAgainstBaseURL:NO];
    NSArray *queryItems = urlComponents.queryItems;
    
    if ([queryItems count] == 0) return @"";
    
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name=%@", key];
    NSURLQueryItem *queryItem = [[queryItems
                                  filteredArrayUsingPredicate:predicate]
                                 firstObject];
    
    return queryItem.value;
}