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

SwiftUI’s .process modifier inherits its actor context from the encompassing perform. In case you name .process inside a view’s physique property, the async operation will run on the principle actor as a result of View.physique is (semi-secretly) annotated with @MainActor. Nonetheless, in case 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 an identical in each, but solely certainly one of them compiles — in helperView, the decision to a main-actor-isolated perform fails as a result of we’re not on the principle 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 will not be marked with 'await'
        onMainActor()
      }
  }
}

This habits is brought on by two (semi-)hidden annotations within the SwiftUI framework:

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

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

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 (Soar 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 essentially 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 features. Module interfaces use regular Swift syntax and have the .swiftinterface file extension.

SwiftUI’s module interface is situated 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 might be a number of .swiftinterface information in that listing, one per CPU structure. Choose any certainly one of them. Professional tip for viewing the file in Xcode: Editor > Syntax Coloring > Swift permits 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 data, all the things makes extra sense:

• When used inside physique, process inherits the @MainActor context from physique.
• When used exterior of physique, there isn’t any implicit @MainActor annotation, so process will run its operation on the cooperative thread pool by default.

• Except the view accommodates an @ObservedObject or @StateObject property, which makes the complete view @MainActor by way of this obscure rule for property wrappers whose wrappedValue property is sure to a world actor:

  A struct or class containing a wrapped occasion property with a world actor-qualified wrappedValue infers actor isolation from that property wrapper

  Replace Might 1, 2024: SE-0401: Take away Actor Isolation Inference brought on by Property Wrappers removes the above rule when compiling in Swift 6 language mode. This can be a good change as a result of it makes reasoning about actor isolation less complicated. Within the Swift 5 language mode, you’ll be able to decide into the higher habits with the -enable-upcoming-feature DisableOutwardActorInference compiler flags. I like to recommend you do.

The lesson: in case you use helper properties or features 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 straight contained in the async closure, in addition to to synchronous features the closure calls. Async features select their very own execution context, so any name to an async perform can change to a unique executor. For instance, in case you name URLSession.information(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!

However it makes understanding so a lot tougher. Earlier than I noticed the annotations within the .swiftinterface file, the habits of the code initially of this text by no means made sense to me. Hiding the small print makes issues look like magic after they truly aren’t. And that’s not good, both.