Caching data with NSCoding?

Question

In my app, I retrieve url data from the server. As an optimization, I normalize the data by storing it into a custom class so that the data can be quickly accessed later on. However, I need to store an instance of this class locally and offline so that I can avoid retrieving data from the server if I've already done so. Also, the website changes daily, so the cached data is only useful if it's from the same day.

This scenario applies to 2 separate classes (4 objects in total), and I've tried saving this data with NSCoding like so. (I've only tried implementing 1 object so far, but I'm planning on creating a separate docPath for each object)

- (void) saveStateToDocumentNamed:(NSString*)docName
{
    NSError       *error;
    NSFileManager *fileMan = [NSFileManager defaultManager];
    NSArray       *paths   = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString      *docPath = [paths[0] stringByAppendingPathComponent:docName];

    if ([fileMan fileExistsAtPath:docPath])
        [fileMan removeItemAtPath:docPath error:&error];

    // Store the hours data
    Hours *h = [self parseHoursFromServer];
    NSDictionary *state = [NSDictionary dictionaryWithObjectsAndKeys:h, @"Hours", nil];

    // There are many ways to write the state to a file. This is the simplest
    // but lacks error checking and recovery options.
    [NSKeyedArchiver archiveRootObject:state toFile:docPath];
    NSLog(@"end");
}

- (NSDictionary*) stateFromDocumentNamed:(NSString*)docName
{
    NSError       *error;
    NSFileManager *fileMan = [NSFileManager defaultManager];
    NSArray       *paths   = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString      *docPath = [paths[0] stringByAppendingPathComponent:docName];

    if ([fileMan fileExistsAtPath:docPath])
        return [NSKeyedUnarchiver unarchiveObjectWithFile:docPath];

    return nil;
}

However, when I tried running this code, I received a [Hours encodeWithCoder:]: unrecognized selector sent to instance 0 because my class currently does not support NSCoding. After reading about how I need to manually encode all the properties for my custom class, I want to make sure that NSCoding is an ideal solution to my data caching problem.

EDIT:

Here is how I currently create the url

NSURL *url = [NSURL URLWithString:urlString];
NSData *pageData = [NSData dataWithContentsOfURL:url];

Answer 1

TL;DL; The system is already caching for you!

When a request is made, the URL loading system will check the shared URL cache to see if there is a valid matching response in the cache. If there is, it will use that rather than making a network connection. This is transparent to your networking code, it works the same wether it's being given data from the cache or the network. You don't have to implement anything other than tell NSURLCache to use the disk:

NSURLCache *URLCache = [[NSURLCache alloc] initWithMemoryCapacity:(4 * 1024 * 1024) diskCapacity:(20 * 1024 * 1024) diskPath:nil];
[NSURLCache setSharedURLCache:URLCache];

That would allow it to use 4MB RAM, 20MB on disk, and the default cache location on disk. This should be done once, when your application starts up. From that point on, NSURLCache will write any cacheable network responses to memory as well as disk. As they expire or the cache runs out of space they will be removed. How the network connection uses the cache is determined by the NSURLRequest's cachePolicy. The default of NSURLRequestUseProtocolCachePolicy uses the cache in accordance with the rules of the protocol (HTTP). HTTP responses include information about how long a response should be considered valid for, and this policy obeys those rules. For the most part, this works well - unless the server you are communicating with implements HTTP response caching information incorrectly. If you want to alter the cache policy to NOT allow the URL loading system to attempt to load anything from the network, ONLY do so if the device's radios are offline. If the device is in airplane mode you would construct your NSURLRequest with a different cache policy (assuming you are using Reachability to determine network status):

if (networkStatus == NotReachable){
    request = [[NSURLRequest alloc] initWithURL:someURL cachePolicy: NSURLRequestReturnCacheDataDontLoad timeoutInterval:timeout];
} else {
    request = [[NSURLRequest alloc] initWithURL:someURL cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:timeout];
}

This would use the policy NSURLRequestReturnCacheDataDontLoad if the device was in airplane mode or neither cellular or wifi had a signal. NSURLRequestReturnCacheDataDontLoad will load responses from the cache (even if expired), and not attempt to access the network for items that are not in the cache.

You can read more about this in the URL Loading System Programming Guide: Understanding Cache Access