Waiting for network call to finish - ios

Question

What I'm trying to achieve is to make a network request and wait for it to finish, so that I can make a decission what should be apps next step.
Normally I would avoid such solution, but this is a rare case in which codebase has a lot of legacy and we don't have enough time to apply necessary changes in order to make things right.
I'm trying to write a simple input-output method with following definition:
- (nullable id<UserPaymentCard>)validCardForLocationWithId:(ObjectId)locationId;
The thing is that in order to perform some validation inside this method I need to make a network request just to receive neccessary information, so I'd like to wait for this request to finish.
First thing that popped in my mind was using dispatch_semaphore_t, so I ended up with something like this:
- (nullable id<UserPaymentCard>)validCardForLocationWithId:(ObjectId)locationId {
id<LocationsReader> locationsReader = [self locationsReader];
__block LocationStatus *status = nil;
dispatch_semaphore_t sema = dispatch_semaphore_create(0);
[locationsReader fetchLocationProviderStatusFor:locationId completion:^(LocationStatus * _Nonnull locationStatus) {
status = locationStatus;
dispatch_semaphore_signal(sema);
} failure:nil];
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
return [self.paymentCards firstCardForStatus:status];
}
Everything compiles and runs, but my UI freezes and I actually never receive sempahore's signal.
So, I started playing with dispatch_group_t with exactly the same results.
Look like I might have some problems with where code gets executed, but I don't know how to approach this and get the expected results. When I try wrapping everything in dispatch_async I actually stop blocking main queue, but dispatch_async return immediatelly, so I return from this method before the network request finishes.
What am I missing? Can this actually be acheived without some while hacks? Or am I trying to fight windmills?
I was able to achieve what I want with the following solution, but it really feels like a hacky way and not something I'd love to ship in my codebase.
- (nullable id<UserPaymentCard>)validCardForLocationWithId:(ObjectId)locationId {
id<LocationsReader> locationsReader = [self locationsReader];
__block LocationStatus *status = nil;
__block BOOL flag = NO;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[locationsReader fetchLocationProviderStatusFor:locationId completion:^(LocationStatus * _Nonnull locationStatus) {
status = locationStatus;
flag = YES;
} failure:nil];
});
while (CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0, true) && !flag){};
return [self.paymentCards firstCardForStatus:status];
}

Answer 1

I guess fetchLocationProviderStatusFor:completion:failure: calls those callbacks in main queue. That's why you get deadlock. It's impossible. We can't time travel yet.

Answer 2

The deprecated NSURLConnection.sendSynchronousRequest API is useful for those instances when you really can't (or just can't be bothered to) do things properly, like this example:
private func pageExists(at url: URL) -> Bool {
var request = URLRequest(url: url)
request.httpMethod = "HEAD"
request.timeoutInterval = 10
var response: URLResponse?
try! NSURLConnection.sendSynchronousRequest(request,
returning: &response)
let httpResponse = response as! HTTPURLResponse
if httpResponse.statusCode != 200 { return false }
if httpResponse.url != url { return false }
return true
}

Answer 3

Currently, your method causes work to be done on the main thread, which freezes the UI. Your solution works, but it would be best to change the method to include a completion block. Then, you could call the completion block at the end of the async block. Here's the example code for that:
- (void)validCardForLocationWithId:(ObjectId)locationId completion:(nullable id<UserPaymentCard> (^)(void))completion {
id<LocationsReader> locationsReader = [self locationsReader];
__block LocationStatus *status = nil;
[locationsReader fetchLocationProviderStatusFor:locationId completion:^(LocationStatus * _Nonnull locationStatus) {
status = locationStatus;
completion([self.paymentCards firstCardForStatus:status]);
} failure:nil];
}