While working on a codebase with a mix of structured concurrency and good old DispatchQueues I stumbled on something. Most of the time this would not be a problem, but in my case, it was important that events would happen in order. Let’s jump to an example for illustrating what I am talking about.
Firstly, let’s add an extension to Task which adds a static function for running operations on the main actor. Similar to the existing Task.detached. This made sense since I needed to push the Task to main actor in multiple places.
This extension was used in a class which observes state using non-async await code. If the state changed, it would trigger a closure.
The idea is that when the state changes, we jump to the main actor from the beginning, since it will always end up updating state in a @MainActor annotated class. In this example, a view model.
Looks great, we use Task.main which calls the operation on the main actor which in turn sets the updated state to a main actor guarded view model. If I run the code and observe isMainThread print statements, then it prints: true, false, true. Here is where the gotcha is. I expected it to be true, true, true since I made sure Task uses @MainActor and also the view model is @MainActor. Here is the change I needed to do for getting the expected true, true, true in console log.
I was missing the @MainActor from the closure’s definition. What happened was that task was forced to main actor, but the operation closure did not specify any isolation, and Swift happily switched to the global executor instead.
If I rethink about what happened, then most of the time this would not be a problem, but in my specific case the timing was important. It was important that when we jump to the main actor, then no other code path should not change the state in the view model.
All in all, this is just something to keep in mind when mixing structured concurrency with DispatchQueues.
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