HomeiOS DevelopmentThe place View.job will get its main-actor isolation from – Ole Begemann

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

SwiftUI’s .job modifier inherits its actor context from the encompassing operate. In the event you name .job 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. Nevertheless, in the event you name .job from a helper property or operate that isn’t @MainActor-annotated, the async operation will run within the cooperative thread pool.

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

Xcode showing the compiler diagnostic 'Expression is 'async' but is not marked with await'
We are able to name a main-actor-isolated operate from inside physique, however not from a helper property.
import SwiftUI

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

struct ContentView: View {
  var physique: some View {
    VStack {
      Textual content("in physique")
        .job {
          // We are able to name a @MainActor func with out await

  var helperView: some View {
    Textual content("in helperView")
      .job {
        // ❗️ Error: Expression is 'async' however isn't marked with 'await'

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.job 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 (Leap to Definition of View), however that characteristic doesn’t work reliably for me, and as we’ll see, it doesn’t present the entire fact, both.

Xcode showing the generated interface for SwiftUI’s View protocol. The @MainActor annotation on View.body is selected.
View.physique is annotated with @MainActor in Xcode’s generated interface for SwiftUI.

To actually see the declarations the compiler sees, we have to have 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 might be a number of .swiftinterface recordsdata in that listing, one per CPU structure. Decide any considered one 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:

@out there(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 .job, together with the @_inheritActorContext attribute:

@out there(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 job(
      precedence: _Concurrency.TaskPriority = .userInitiated,
      @_inheritActorContext _ motion: @escaping @Sendable () async -> Swift.Void
    ) -> some SwiftUI.View {
      modifier(_TaskModifier(precedence: precedence, motion: motion))
  // …

Xcode showing the declaration for the View.task method in the SwiftUI.swiftinterface file. The @_inheritActorContext annotation is selected.
SwiftUI’s module interface file reveals the @_inheritActorContext annotatation on View.job.

Armed with this data, all the pieces makes extra sense:

  • When used inside physique, job inherits the @MainActor context from physique.
  • When used exterior of physique, there isn’t a implicit @MainActor annotation, so job will run its operation on the cooperative thread pool by default. (Until the view incorporates an @ObservedObject or @StateObject property, which one way or the other makes your entire view @MainActor. However that’s a distinct subject.)

The lesson: in the event 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, word that the actor context solely applies to code that’s positioned straight 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 operate can swap to a distinct executor. For instance, in the event you name URLSession.knowledge(from:) inside a main-actor-annotated operate, the runtime will hop to the worldwide cooperative executor to execute that methodology. 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 habits of the code at the start of this text by no means made sense to me. Hiding the main points makes issues appear to be magic after they truly aren’t. And that’s not good, both.



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