1

Hi swift forum!

I want to understand the ideas behind the following situations:

Situation 1

The method is marked as @MainActor in the protocol, but the implementation removed @MainActor.

protocol MainActorProtocol {
    @MainActor func someFunc()
}

class NoMainActor: MainActorProtocol {

	func someFunc() {
		print("NoMainActor", Thread.current)
	}
}

A variable of protocol type, why does it switch to the main thread at runtime ?
Why does the compiler allow to omit @MainActor in the implementation if the thread change occurs anyway ?

Task.detached {
    let protocolTypeNoMainActor: MainActorProtocol = NoMainActor()
    // RESULT: NoMainActor <_NSMainThread: 0x600001704000>{number = 1, name = main}
    await protocolTypeNoMainActor.someFunc()
}

Situation 2

A method in the protocol is marked as @MainActor, but @MainActor was removed in the implementation.
The variable has a type of a specific class, for some reason the method still switches to the main thread and requires await to call it, although the implementation does not have @MainActor

Task.detached {
    let noMainActor: NoMainActor = NoMainActor()
    // RESULT: NoMainActor <_NSMainThread: 0x600001710000>{number = 1, name = main}
    await noMainActor.someFunc()
}

Could you please explain the logic behind how this works ?