What I need

I want to convert an async system API to a synchronized one and call it inside SceneDelegate’s continueUserActivity.

Note: I fully understand that iOS will kill the app if I don’t return within a few seconds. But in my case I have to wait till the system API completion in order to continue this function.

What’s working

I have the following code:

// Imagine this is a system async API that (1) has to be called on main thread, and (2) it also completes on main thread.
- (void)systemAsyncAPI:(void(^)(void))completion {
  dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    completion();
  });
}

// Convert the system API from async to sync
- (void)synchronizedSystemAPI {
  __block BOOL completed = NO;
  
  [self systemAsyncAPI:^{
    completed = YES;
  }];
  
  while (!completed) {
    NSLog(@"Tick");
    [NSRunLoop.currentRunLoop runUntilDate:[NSDate dateWithTimeIntervalSinceNow:1]];
  }
  NSLog(@"completed");
}

Now when I use this code inside SceneDelegate’s willConnectToSession:

- (void)scene:(UIScene *)scene willConnectToSession:(UISceneSession *)session options:(UISceneConnectionOptions *)connectionOptions {
  [self synchronizedSystemAPI];
}

Everything works as expected, with the following log:

Tick
Tick
Tick
completed

What’s not working

Now, I need to call this synchronizedSystemAPI inside SceneDelegate‘s continueUserActivity instead of willConnectToSession.

- (void)scene:(UIScene *)scene continueUserActivity:(NSUserActivity *)userActivity {
  [self synchronizedSystemAPI];

  // Continue work after system API is complete
}

This code now has a deadlock, as it ticks forever:

Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
...

My understanding

Inspired by this answer, I think the reason is, willConnectToSession is called in old technique (predates GCD) that is not a dispatch item. continueUserActivity is part of a dispatch item on main queue, hence the deadlock, as explained in that linked answer.

How do I test the code?

An easy way to invoke continueUserActivity is via spotlight search:

1. add NSUserActivityTypes array in info.plist. Here I put a single item “foo”.

2. setup the activity:

- (void)scene:(UIScene *)scene willConnectToSession:(UISceneSession *)session options:(UISceneConnectionOptions *)connectionOptions {
  NSUserActivity *activity = [[NSUserActivity alloc] initWithActivityType:@"foo"];
  activity.title = @"foo title";
  activity.eligibleForSearch = YES;
  [activity becomeCurrent];
}

3. Run the app.
4. Go to home screen and search “foo title” in spotlight search.
5. You should be able to this app. Tap on it to resume
6. Notice continueUserActivity will be called.

And here is my full reproducible code. Don’t forget to add the activity item in Info.plist as described above.

@implementation SceneDelegate



// A system async API that has to be called on main thread, and it also completes on main thread.
- (void)systemAsyncAPI:(void(^)(void))completion {
  dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    completion();
  });
}

// Convert the above system API from async to sync
- (void)synchronizedSystemAPI {
  __block BOOL completed = NO;
  
  // Imagine this is a system API that must be called on main thread, and completes on main thread.
  [self systemAsyncAPI:^{
    completed = YES;
  }];
  
  while (!completed) {
    NSLog(@"Tick");
    [NSRunLoop.currentRunLoop runUntilDate:[NSDate dateWithTimeIntervalSinceNow:1]];
  }
  NSLog(@"completed");
}


- (void)scene:(UIScene *)scene willConnectToSession:(UISceneSession *)session options:(UISceneConnectionOptions *)connectionOptions {
  NSUserActivity *activity = [[NSUserActivity alloc] initWithActivityType:@"foo"];
  activity.title = @"foo title";
  activity.eligibleForSearch = YES;
  [activity becomeCurrent];
}

- (void)scene:(UIScene *)scene continueUserActivity:(NSUserActivity *)userActivity {
  [self synchronizedSystemAPI];
}
@end