How to cache JSON web response locally on iOS - objective c

Question

I am building a mobile iOS app for a web backend. I retrieve the JSON response using the following code:

NSError *error;
NSString *url_string = [NSString stringWithFormat: @"https://myURL"];
NSData *data = [NSData dataWithContentsOfURL: [NSURL URLWithString:url_string]];
NSDictionary *jsonDict = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableContainers error:&error];

What would be the best/simplest way to store this data locally so that I can use a local database when internet connectivity is not available? The intention is to check web connectivity at launch of the app. If connection is available, get JSON response, parse and update local copy, if connection is not available parse the data from local storage.

Thanks in advance.

Answer 1

If you wish to cache the JSON data so you can still use it offline, I would write the JSON dictionary (or the data) to a file in your app's sandbox. A subfolder of the "Application Support" folder would be a good place. You don't want to use the Caches folder in this case because the files could be purged by iOS when you need them offline.

The trick is to map a given URL to a filename. You need this mapping to both save a file for a given URL and to later load the file if offline. You should be able convert a URL to a useful filename simply by converting all / characters to something else such as an underscore.

You probably don't want these files backed up when a user backups their iOS device so be sure you mark the files with the "do not backup" attribute. There are many existing question covering that topic.

Answer 2

The best way is CoreData and the simplest way is NSUserDefaults

NSUserDefaults Class Reference

[[NSUserDefaults standardUserDefaults] setObject: jsonDict forKey:@"dictionaryKey"];
 //...
 NSDictionary * myDictionary = [[NSUserDefaults standardUserDefaults] dictionaryForKey:@"dictionaryKey"];