react-native iOS native module: calling method that activates network indicator asynchronously

Question

I am writing a native module that calls functions from a closed source framework binary that I don't have access to. It makes blocking network calls and activates the network indicator. I want to be able to run it in the background asyncronously, It kind of works out of the box, but when I do that I get this warning/error and the call stalls a bit:

Main Thread Checker: UI API called on a background thread: -[UIApplication setNetworkActivityIndicatorVisible:]

If I put it on the main queue using the following code in my In my RN Module, the calls block everything until they complete.

- (dispatch_queue_t)methodQueue
{
  return dispatch_get_main_queue();
}

Here is an example of a method exposed to JS:

RCT_REMAP_METHOD(createSession,
                 userIDtoken:(NSString *)userIDtoken
                 createSessionWithResolver:(RCTPromiseResolveBlock)resolve
                 rejecter:(RCTPromiseRejectBlock)reject)
{
    Boolean connected = [_networkController startSession:userIDtoken isTest:true];
    NSString *success = @"session created";
    if (connected){
      resolve(success);
    } else {
      NSError *error = [NSError errorWithDomain:@"ConnectionError" code:1 userInfo:nil];
      reject(@"session_creation_failed", @"Cannot create sessions", error);
    }
}

_networkController startSession should run in the background without throwing the warning/error and without blocking the main thread. Is there a way to do this?

Answer 1

You cannot make UIKit API calls on a background thread as it is not thread-safe.

You can remove the RN module methodQueue setup so your module's method will get called once again on a background thread and will not block the main thread. Whenever you need to make a call to [UIApplication setNetworkActivityIndicatorVisible:] (or any other method that requires the main queue), you can dispatch it to the main thread, for example:

dispatch_async(dispatch_get_main_queue(), ^{
    [UIApplication.sharedApplication setNetworkActivityIndicatorVisible:YES];
});

In case you need to resolve the promise only after setting the activity indicator, you can also put your calls to resolve/reject inside the same block after changing the activity indicator visibility.