Force app to wait for method completion (data download) - ios

Question

I'm working with an app that requests data from an OAuth2.0 protected server. When I use the GTM OAuth Library to retrieve data, the program continues to run while the data is being downloaded in the background. I need some sort of mechanism to either force my application to wait until the didFinishWithData selector is called,or I need a way to notify my ViewController of the download's completion, so I can then utilize the data immediately.
I've tried conditional blocks, but those aren't doing it for me. I've also tried polling the object whose data I'm interested in, but if I do that, the data never seems to download. I've heard I can somehow utilize the Notification Center to accomplish this task, so I'll look more into that while I'm waiting for replies here.
Here is basically what is going on:
-(void) getAlert{
// Define the URL of the API module we'd like to utilize.
NSURL *url = [[NSURL alloc] initWithString:#"https://access.active911.com/interface/open_api/api/alerts"];
// Constructs a an HTTP request object to send to the server in order to obtain data.
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url];
[request setValue:#"1" forHTTPHeaderField:#"alert_days"];
// This fetcher sends the request along with the authentication header in a recognizable manner.
GTMHTTPFetcher *fetcher = [[GTMHTTPFetcher alloc] initWithRequest:request];
// Attach the OAuth credentials for the fetcher's use.
[fetcher setAuthorizer:auth];
// Execute the operation.
[fetcher waitForCompletionWithTimeout:10];
NSLog(#"About to get alert");
[fetcher beginFetchWithDelegate:self didFinishSelector:#selector(responseHandler:finishedWithData:finishedWithError:)];
NSLog(#"got alert");
}
-(void)responseHandler:(id)valueNotUsed finishedWithData:(NSData *)data finishedWithError:(NSError *)error{
// Retrieve the server data in a usable object
// All that's being done here is conversion to an NSDictionary
// followed by the creation of subdictionaries from that dictionary
// until our final value can be picked directly out of the resulting dict
NSData *jsonData = [[NSData alloc] initWithData:data];
NSError *dictError;
NSDictionary* json = [NSJSONSerialization
JSONObjectWithData:jsonData //1
options:kNilOptions
error:&dictError];
NSDictionary *token = [json objectForKeyedSubscript:#"message"];
NSArray *alerts = [token objectForKeyedSubscript:#"alerts"];
NSDictionary *alertData = alerts[0];
mapCode = [alertData objectForKeyedSubscript:#"map_code"];
NSString *city = [alertData objectForKeyedSubscript:#"city"];
NSLog(#"Map code: '%#' with city '%#' and access token %#", mapCode, city, accessToken);
}
And I need to pass the mapCode to my view controller.
Thanks for the help!

Answer 1

First off, please rethink about having the UI halt while you fetch results from the server. This can create an extremely bad UX for the app and only should be done if absolutely necessary.
Second, does your responseHandler method work? And do you only need mapCode in the VC that responseHandler is in?
If so, you don't even need to use Notifications. Simply do:
-(void)responseHandler:(id)valueNotUsed finishedWithData:(NSData *)data finishedWithError:(NSError *)error{
...
...
mapCode = [alertData objectForKeyedSubscript:#"map_code"];
[self updateVCWithMapCode:mapCode];
}
That will call the method after the response has been received. Passing it explicitly too so you don't need to have mapCode be a property as well.