Check AppStore if an update for app is available.

appStoreUpdate.m

-(void) checkForAppUpdate{
    BOOL updateAvailable = NO;
    NSDictionary* infoDictionary = [[NSBundle mainBundle] infoDictionary];
    NSString* appID = infoDictionary[@"CFBundleIdentifier"];
    NSURL* url = [NSURL URLWithString:[NSString stringWithFormat:@"http://itunes.apple.com/lookup?bundleId=%@", appID]];
    NSData* data = [NSData dataWithContentsOfURL:url];
    NSDictionary* lookup = [NSJSONSerialization JSONObjectWithData:data options:0 error:nil];
    
    if ([lookup[@"resultCount"] integerValue] == 1){
        NSString* appStoreVersion = lookup[@"results"][0][@"version"];
        NSString* currentVersion = infoDictionary[@"CFBundleShortVersionString"];
        if (![appStoreVersion isEqualToString:currentVersion]){
            //NSLog(@"Need to update [%@ != %@]", appStoreVersion, currentVersion);
            updateAvailable = YES;
            NSString * appITunesItemIdentifier =  lookup[@"results"][0][@"trackId"]; 
            [self openStoreProductViewControllerWithITunesItemIdentifier:[appITunesItemIdentifier intValue]];        
        }
    }
    updateAvailable = NO;

}
- (void)openStoreProductViewControllerWithITunesItemIdentifier:(NSInteger)iTunesItemIdentifier {
    SKStoreProductViewController *storeViewController = [[SKStoreProductViewController alloc] init];
    
    storeViewController.delegate = self;
    
    NSNumber *identifier = [NSNumber numberWithInteger:iTunesItemIdentifier];
    
    NSDictionary *parameters = @{ SKStoreProductParameterITunesItemIdentifier:identifier };
    UIViewController *viewController = self.window.rootViewController;
    [storeViewController loadProductWithParameters:parameters
                                   completionBlock:^(BOOL result, NSError *error) {
                                       if (result)
                                           [viewController presentViewController:storeViewController
                                                                        animated:YES
                                                                      completion:nil];
                                       else NSLog(@"SKStoreProductViewController: %@", error);
                                   }];
    
}
#pragma mark - SKStoreProductViewControllerDelegate

- (void)productViewControllerDidFinish:(SKStoreProductViewController *)viewController {
    [viewController dismissViewControllerAnimated:YES completion:nil];
}