How does the Xcode 's document knows my custom view is a MainActor

CityView is a custom View in a demo project , But the Xcode's documents says it was a @MainActor , and CityView isn't defined as @MainActor in project .

How does the Xcode's help document knows it was a @MainActor?

Is it means anything conform to View protocol is a @MainActor?

Is it means anything conform to View protocol is a @MainActor?

Yes, because the View protocol is marked @MainActor.

@MainActor @preconcurrency
protocol View { ... }

Types that conform to it are implcitly marked @MainActor too. Note that being isolated to the main actor is not a requirement of the protocol. You can opt out by adding nonisolated:

nonisolated struct CityView: View { ... }

Before Swift 6.2, you can do

struct CityView: View, _RemoveGlobalActorIsolation { ... }