Question

I have a singleton class that will initialize it’s data from a web service then save itself to a file (using NSCoding and NSKeyedUnarchiver) so I can just initialize this file instead of pulling all the data from the web again. Because it’s a singleton instance that is being saved, I want to be able to call the singleton getter like you normally would, check to see if there’s an archived copy and if not, pull the data down and archive it. Archiving is working fine, but when I try to call [[NSKeyedUnarchiver unarchiveObjectWithFile: filePath] retain] it calls the sharedInstance getter before sharedInstance is initialized. This causes init to be called and the app then downloads all the data again, just to be subsequently overwritten by the unarchiver.

Am I doing something wrong with my setup, or is there another way of Serializing this data?

Here’s some (simplified) code:

@implementation Helmets
@synthesize helmetList, helmetListVersion;
//Class Fields
static Helmets *sharedInstance = nil;

// Get the shared instance and create it if necessary.
+ (Helmets *)sharedInstance {
    //if(sharedInstance == nil){
        //[Helmets unarchive];             //This does not work! Calls sharedInstance() again (recursion)
        if(sharedInstance == nil){
            sharedInstance = [[super allocWithZone:NULL] init];    //Pull from web service
            [Helmets archive];    //Save instance
        //}
    //}
    return sharedInstance;
}

- (id)init {
    self = [super init];
    if (self) {
            helmetList = [[NSMutableArray alloc]init];

            //Get our data from the web service and save it (excluded)
            }
    }
    return self;
}

//This works!
+(void)archive{
    if([NSKeyedArchiver archiveRootObject:sharedInstance toFile:[Helmets getFilePath]]){
        NSLog(@"Archiving Successful");
    }
    else{
        NSLog(@"Archiving Failed");
    }
}

//This works, but calls getInstance causing data to be downloaded anyways!
+(void)unarchive{
    // Check if the file already exists
    NSFileManager *filemgr = [NSFileManager defaultManager];
    NSString *filePath = [Helmets getFilePath];
    if ([filemgr fileExistsAtPath: filePath])
    {
       sharedInstance = [[NSKeyedUnarchiver unarchiveObjectWithFile: filePath] retain];
    }
    [filemgr release];
}

Instance is initialized like:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{
    ...
    [Helmets unarchive];        //This calls sharedInstance() too soon!
    [Helmets sharedInstance];
}

The class implements NSCoding in the .h and overrides initWithCoder and encodeWithCoder (Archiving is working).

Thanks in advance!

Answer 1

In the end you need a private method to set your shared instance in addition to the one you have, and you need a different init, again private to the implementation.

- (id)initAndFetch:(BOOL)fetch
{
    if((self = [super init])) {
        ...

        if(fetch) { do the web fetch };
             ...   
    }
}

In the +sharedInstance method, you will pass YES.

Then your decode will look like:

- (id)initWithCoder:(NSCoder *)decoder
{
    if((self = [self initAndFetch:NO])) {
        title = [decoder decodeObjectForKey:@"title"];
        ...
        sharedInstance = self;
    }
    return self;
}