Dispatch.main queue questions since I might be confused

This usually doesn't cause a crash. It spits out an error message to console and locks up your app for a bit in debug.

But unless you are actively creating concurrent queues and putting work on them, this isn't something you need to think about. Everything (more or less) runs on the main thread. If you are creating concurrent queues and putting work on them, then you need to be careful about what you are doing there. Don't just blindly start wrapping everything in dispatches.

Thanks for the input.

I am not an advanced user and do not user concurrent queues (at least not intentionally) at all. I am just learning about Dispatch queues now due to some unexpectedly crashes I am experiencing in my application.

Throughout my testing I have had Xcode's "Main Thread Checker" indicate a couple areas in my code where I needed to wrap around a Dispatch.main to ensure that code is running on the main thread so I am using Dispatch in these areas.

One more thing to share. I created a game which uses the GameKitHelper class to interface into Game Center. I set up a delegate to this GameKitHelper class to receive callback function calls when certain events occur. When a certain delegate function is called, my code will create and then present a new ViewController using this code:

let storyBoard = UIStoryboard(name: "Main", bundle: nil)
if let myVC: myViewController = storyBoard.instantiateViewController(withIdentifier: "myViewController") as? myViewController
{
  present(myVC, animated: true, completion: nil)
}

During testing, my application occasionally experiences a crash at the point the new ViewController should be displayed. Since I am inside a delegate callback function, which deals with UIStoryboard and creating a new ViewController to be presented (ie: displayed in the UI), then I am thinking I might need to have the code (above) wrapped in DispatchQueue.main.async to avoid the crash.

Would you expect an application to crash in the case described above when no Dispatch is being used ?

I print out the current thread name using "Thread.current.threadName". I do see most of the time the thread is the Main Thread, however, I did see one time a thread name was called "com.apple.root.default-qos" so I know it is not always the Main Thread. If the thread is not the Main Thread (for some reason) and the code above (with no Dispatch) is called then would a crash occur?

It is possible. But you can't just blindly "wrap" code in an async dispatch. Those are "asynchronous". You aren't "wrapping" the code. That is just an artifact of how you are expressing behaviour in the language. What you are doing is appending a block of code to a queue to be executed "later". It could be the same thread or a different thread. It could execute right away or after a few milliseconds. The code that follows it cannot assume that the dispatched block either has or has not run. You can do a sync dispatch, but that can be even trickier.

Performing actions in a delegate callback can also be tricky. That may be a case where dispatching onto the main thread is the correct solutions. I don't know. Sometimes it does work that way.

I updated all my delegate callback functions to ensure any UI updates are made on the main thread by placing the UI code logic in DispatchQueue.main.async. With my debug logging I can see the UI code is now being executed after the current function (and any parent calling functions) have completed. So far, it is ok for my UI logic to take place at that point so I might be good. I will start testing and ccross my fingers that my crashes go away so I can finally submit my application for review.

Thanks again John for your time.

My crashes have disappeared ! Thanks for all the help !

Yep sure do