SC callbacks must by default happen on the main thread unless the app
specifies a different queue.  This fixes a crash in reachability sample.
@@ -165,7 +165,7 @@ static NSLock* _allReachabilityOperationsLock;
_runLoopsScheduled.attach([NSMutableDictionary new]);
_reachabilityFlagsValid.attach([NSConditionLock new]);
_weakRef.attach([[SCNetworkReachabilityWeakRef alloc] initWithObject:self]);
- _callbackQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
+ _callbackQueue = dispatch_get_main_queue();
// clang-format off
CFRunLoopSourceContext ctxt{
