The place View.process will get its main-actor isolation from – Ole Begemann

SwiftUI’s .process modifier inherits its actor context from the encircling perform. For those who name .process inside a view’s physique property, the async operation will run on the primary actor as a result of View.physique is (semi-secretly) annotated with @MainActor. Nonetheless, should you name .process from a helper property or perform that isn’t @MainActor-annotated, the async operation will run within the cooperative thread pool.

Right here’s an instance. Discover the 2 .process modifiers in physique and helperView. The code is equivalent in each, but solely one in every of them compiles — in helperView, the decision to a main-actor-isolated perform fails as a result of we’re not on the primary actor in that context:

import SwiftUI

@MainActor func onMainActor() {
  print("on MainActor")
}

struct ContentView: View {
  var physique: some View {
    VStack {
      helperView
      Textual content("in physique")
        .process {
          // We will name a @MainActor func with out await
          onMainActor()
        }
    }
  }

  var helperView: some View {
    Textual content("in helperView")
      .process {
        // ❗️ Error: Expression is 'async' however shouldn't be marked with 'await'
        onMainActor()
      }
  }
}

This conduct is attributable to two (semi-)hidden annotations within the SwiftUI framework:

1. The View protocol annotates its physique property with @MainActor. This transfers to all conforming sorts.

2. View.process annotates its motion parameter with @_inheritActorContext, inflicting it to undertake the actor context from its use web site.

Sadly, none of those annotations are seen within the SwiftUI documentation, making it very obscure what’s happening. The @MainActor annotation on View.physique is current in Xcode’s generated Swift interface for SwiftUI (Bounce to Definition of View), however that function doesn’t work reliably for me, and as we’ll see, it doesn’t present the entire fact, both.

To actually see the declarations the compiler sees, we have to take a look at SwiftUI’s module interface file. A module interface is sort of a header file for Swift modules. It lists the module’s public declarations and even the implementations of inlinable capabilities. Module interfaces use regular Swift syntax and have the .swiftinterface file extension.

SwiftUI’s module interface is positioned at:

[Path to Xcode.app]/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/System/Library/Frameworks/SwiftUI.framework/Modules/SwiftUI.swiftmodule/arm64e-apple-ios.swiftinterface

(There could be a number of .swiftinterface information in that listing, one per CPU structure. Choose any one in every of them. Professional tip for viewing the file in Xcode: Editor > Syntax Coloring > Swift allows syntax highlighting.)

Inside, you’ll discover that View.physique has the @MainActor(unsafe) attribute:

@accessible(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.0, *)
@_typeEraser(AnyView) public protocol View {
  // …
  @SwiftUI.ViewBuilder @_Concurrency.MainActor(unsafe) var physique: Self.Physique { get }
}

And also you’ll discover this declaration for .process, together with the @_inheritActorContext attribute:

@accessible(iOS 15.0, macOS 12.0, tvOS 15.0, watchOS 8.0, *)
extension SwiftUI.View {
  #if compiler(>=5.3) && $AsyncAwait && $Sendable && $InheritActorContext
    @inlinable public func process(
      precedence: _Concurrency.TaskPriority = .userInitiated,
      @_inheritActorContext _ motion: @escaping @Sendable () async -> Swift.Void
    ) -> some SwiftUI.View {
      modifier(_TaskModifier(precedence: precedence, motion: motion))
    }
  #endif
  // …
}

Armed with this information, the whole lot makes extra sense:

• When used inside physique, process inherits the @MainActor context from physique.
• When used exterior of physique, there is no such thing as a implicit @MainActor annotation, so process will run its operation on the cooperative thread pool by default. (Until the view comprises an @ObservedObject or @StateObject property, which one way or the other makes your complete view @MainActor. However that’s a unique subject.)

The lesson: should you use helper properties or capabilities in your view, take into account annotating them with @MainActor to get the identical semantics as physique.

By the way in which, be aware that the actor context solely applies to code that’s positioned immediately contained in the async closure, in addition to to synchronous capabilities the closure calls. Async capabilities select their very own execution context, so any name to an async perform can change to a unique executor. For instance, should you name URLSession.knowledge(from:) inside a main-actor-annotated perform, the runtime will hop to the worldwide cooperative executor to execute that technique. See SE-0338: Make clear the Execution of Non-Actor-Remoted Async Capabilities for the exact guidelines.

I perceive Apple’s impetus to not present unofficial API or language options within the documentation lest builders get the preposterous thought to make use of these options in their very own code!

Nevertheless it makes understanding so a lot tougher. Earlier than I noticed the annotations within the .swiftinterface file, the conduct of the code firstly of this text by no means made sense to me. Hiding the main points makes issues seem to be magic once they truly aren’t. And that’s not good, both.