Question

I am getting response back in JSON format and have never parsed it in the past, so could somebody please help me understand this.
Here’s the Json response i am getting:

Twitter response:

(
    {
        contributors = "<null>";
        coordinates = "<null>";
        "created_at" = "Tue Nov 29 15:48:35 +0000 2011";
        entities =         {
            hashtags =             (
            );
            media =             (
                                {
                    "display_url" = "pic.twitter.com/650E1WRY";
                    "expanded_url" = "http://twitter.com/ashu1702/status/141544088850796545/photo/1";
                    id = 141544088854990848;
                    "id_str" = 141544088854990848;
                    indices =                     (
                        22,
                        42
                    );
                    "media_url" = "http://p.twimg.com/AfbdmVBCEAAPJvT.jpg";
                    "media_url_https" = "https://p.twimg.com/AfbdmVBCEAAPJvT.jpg";
                    sizes =                     {
                        large =                         {
                            h = 279;
                            resize = fit;
                            w = 215;
                        };
                        medium =                         {
                            h = 279;
                            resize = fit;
                            w = 215;
                        };
                        small =                         {
                            h = 279;
                            resize = fit;
                            w = 215;
                        };
                        thumb =                         {
                            h = 150;
                            resize = crop;
                            w = 150;
                        };
                    };
                    type = photo;
                    url = "http://t.co/650E1WRY";
                }
            );
            urls =             (
            );
            "user_mentions" =             (
            );
        };
        favorited = 0;
        geo = "<null>";
        id = 141544088850796545;
        "id_str" = 141544088850796545;
        "in_reply_to_screen_name" = "<null>";
        "in_reply_to_status_id" = "<null>";
        "in_reply_to_status_id_str" = "<null>";
        "in_reply_to_user_id" = "<null>";
        "in_reply_to_user_id_str" = "<null>";
        place = "<null>";
        "possibly_sensitive" = 0;
        "retweet_count" = 0;
        retweeted = 0;
        source = "<a href=\"http://www.apple.com\" rel=\"nofollow\">iOS</a>";
        text = "I am in Syria @(null) http://t.co/650E1WRY";
        truncated = 0;
        user =         {
            "contributors_enabled" = 0;
            "created_at" = "Sun May 01 15:20:52 +0000 2011";
            "default_profile" = 1;
            "default_profile_image" = 1;
            description = "<null>";
            "favourites_count" = 0;
            "follow_request_sent" = "<null>";
            "followers_count" = 0;
            following = "<null>";
            "friends_count" = 5;
            "geo_enabled" = 0;
            id = 291164338;
            "id_str" = 291164338;
            "is_translator" = 0;
            lang = en;
            "listed_count" = 0;
            location = "<null>";
            name = "Ashutosh Tiwari";
            notifications = "<null>";
            "profile_background_color" = C0DEED;
            "profile_background_image_url" = "http://a0.twimg.com/images/themes/theme1/bg.png";
            "profile_background_image_url_https" = "https://si0.twimg.com/images/themes/theme1/bg.png";
            "profile_background_tile" = 0;
            "profile_image_url" = "http://a2.twimg.com/sticky/default_profile_images/default_profile_3_normal.png";
            "profile_image_url_https" = "https://si0.twimg.com/sticky/default_profile_images/default_profile_3_normal.png";
            "profile_link_color" = 0084B4;
            "profile_sidebar_border_color" = C0DEED;
            "profile_sidebar_fill_color" = DDEEF6;
            "profile_text_color" = 333333;
            "profile_use_background_image" = 1;
            protected = 0;
            "screen_name" = ashu1702;
            "show_all_inline_media" = 0;
            "statuses_count" = 62;
            "time_zone" = "<null>";
            url = "<null>";
            "utc_offset" = "<null>";
            verified = 0;
        };
    }
)

Specifically i just need the “expanded_url” and i am doing this in my code to get it:

NSDictionary *dict = [NSJSONSerialization JSONObjectWithData:responseData options:0 error:&error];// where response data is coming from the call made to twitter timeline_api

                          NSString * callBackURL = [dict objectForKey:@"expanded_url"];

On doing this i am getting this error:

2011-11-29 09:50:31.214 L'Occitane[1119:19d07] -[__NSCFArray objectForKey:]: unrecognized selector sent to instance 0x85a50d0
2011-11-29 09:50:31.215 L'Occitane[1119:19d07] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFArray objectForKey:]: unrecognized selector sent to instance 0x85a50d0'

PLease help!!!

Thanks,

Answer 1

When converted to Cocoa Foundation objects, JSON decomposes into a bunch of nested NSArrays and NSDictionarys ultimately leading to NSStrings, NSNumbers and NSNulls. Ideally you’d work forwards from the web service’s documentation but you can work backwards from the description that the root object prints through NSLog.

From your log I can see that expanded_url is a member of an unnamed object which is the first entry in the media array that is a member of the entities object. The unnamed object containing entities is part of an array, which is what the JSON deserialiser will return. So you want to do something like (if written out in long form for exposition, and without any validation):

NSArray *result = [NSJSONSerialization ...

NSDictionary *firstObject = [result objectAtIndex:0];
NSDictionary *entities = [firstObject objectForKey:@"entities"];
NSArray *media = [entities objectForKey:@"media"];
NSDictionary *relevantObject = [media objectAtIndex:0];
NSString *expandedUrl = [relevantObject objectForKey:@"expanded_url"];

The main risk in there is that the web service may return an empty array for media, causing objectAtIndex:0 to raise an exception, or that it will return something other than an array as the root object or in some other way adjust a type somewhere along the line. Objective-C’s usual rules about messaging nil should save you from most problems with the dictionaries. Quite how much validation to apply is a judgment call.