Best practice for making async calls sync in iOS?? - iphone

Question

I have a singleton class, APIClient, which needs to have userId and authToken set up before it can make calls to my backend.
We are currently storing userId and authToken in NSUserDefaults. For fresh installs, these values do not exist and we query the server for them.
Currently, we have code in our ViewControllers' viewDidLoad methods that manually query the server if these values do not exist.
I am interested to make this class "just work". By this, I mean have the client check if it has been initialized, if not fire a call to the server and set the appropriate userId and authToken - all without manual interference.
This has proven to be a rather tricky due to:
I can't make asyncObtainCredentials synchronous because I was told by folks at #iphonedev that the OS will kill my app if I have to freeze the main thread for a network operation
For what we have right now, the first call will always fail because of the asynchronous nature of asyncObtainCredential. Nil will be returned and first calls will always fail.
Does anyone know of a good work around for this problem?
`
#interface APIClient ()
#property (atomic) BOOL initialized;
#property (atomic) NSLock *lock;
#end
#implementation APIClient
#pragma mark - Methods
- (void)setUserId:(NSNumber *)userId andAuthToken:(NSString *)authToken;
{
self.initialized = YES;
[self clearAuthorizationHeader];
[self setAuthorizationHeaderWithUsername:[userId stringValue] password:authToken];
}
#pragma mark - Singleton Methods
+ (APIClient *)sharedManager {
static dispatch_once_t pred;
static APIClient *_s = nil;
dispatch_once(&pred, ^{
_s = [[self alloc] initWithBaseURL:[NSURL URLWithString:SERVER_ADDR]];
_s.lock =[NSLock new] ;
});
[_s.lock lock];
if (!(_s.initialized)) {
NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];
NSNumber *userId = #([prefs integerForKey:KEY_USER_ID]);
NSString *authToken = [prefs stringForKey:KEY_AUTH_TOKEN];
// If still doesn't exist, we need to fetch
if (userId && authToken) {
[_s setUserId:userId andAuthToken:authToken];
} else {
/*
* We can't have obtainCredentials to be a sync operation the OS will kill the thread
* Hence we will have to return nil right now.
* This means that subsequent calls after asyncObtainCredentials has finished
* will have the right credentials.
*/
[_s asyncObtainCredentials:^(NSNumber *userId, NSString *authToken){
[_s setUserId:userId andAuthToken:authToken];
}];
[_s.lock unlock];
return nil;
}
}
[_s.lock unlock];
return _s;
}
- (void)asyncObtainCredentials:(void (^)(NSNumber *, NSString *))successBlock {
AFHTTPClient *client = [[AFHTTPClient alloc] initWithBaseURL:[NSURL URLWithString:SERVER_ADDR]];
NSDictionary *params = [NSDictionary dictionaryWithObjectsAndKeys:[OpenUDID value], #"open_udid", nil];
NSMutableURLRequest *request = [client requestWithMethod:#"GET" path:#"/get_user" parameters:params];
AFJSONRequestOperation *operation = \
[AFJSONRequestOperation JSONRequestOperationWithRequest:request success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) {
...
// Do not use sharedManager here cause you can end up in a deadlock
successBlock(userId, authToken);
} failure:^(NSURLRequest *request , NSURLResponse *response , NSError *error , id JSON) {
NSLog(#"obtain Credential failed. error:%# response:%# JSON:%#",
[error localizedDescription], response, JSON);
}];
[operation start];
[operation waitUntilFinished];
}

Answer 1

You should check if those values are there in NSUserDefaults during the application launch. If they are not there, make a call to fetch it from server and show a loading overlay on the screen. Once you have fetched it, you can proceed with the next step.
If you dont want to use loading overlay, you can set some isLoading flag in APIClient class and check that to know if the asyn is still fetching. So whenever you are making a service call and you need these values, you know that how to handle it based this flag. Once you have got the required values and stored in NSUserDefaults, you can proceed with the next step. You can use Notifications/Blocks/KVO to notify your viewcontrollers to let them know that you have fetched these values.