[Firestore] Label '@MainActor' isolated APIs of 'FirestoreQuery'

[ncooke3]

This PR addresses the following errors in Swift 6 mode:

// FirestoreQuery.swift @available(iOS 14.0, macOS 11.0, macCatalyst 14.0, tvOS 14.0, watchOS 7.0, *) @propertyWrapper public struct FirestoreQuery<T>: DynamicProperty { @StateObject private var firestoreQueryObservable: FirestoreQueryObservable<T> // <--snip--> public var wrappedValue: T { firestoreQueryObservable.items // 💥 Main actor-isolated property 'firestoreQueryObservable' can not be referenced from a nonisolated context } /// A binding to the request's mutable configuration properties @MainActor @preconcurrency public var projectedValue: Configuration { get { firestoreQueryObservable.configuration // 💥 Main actor-isolated property 'firestoreQueryObservable' can not be referenced from a nonisolated context } nonmutating set { firestoreQueryObservable.objectWillChange.send() // 💥 Main actor-isolated property 'firestoreQueryObservable' can not be referenced from a nonisolated context firestoreQueryObservable.configuration = newValue // 💥 Main actor-isolated property 'firestoreQueryObservable' can not be referenced from a nonisolated context } } // <--snip--> }

The issue is that firestoreQueryObservable is decorated with @StateObject which is labeled @MainActor in Xcode 16.1:

@MainActor @frozen @propertyWrapper @preconcurrency public struct StateObject<ObjectType> : DynamicProperty where ObjectType : ObservableObject { ... }

So the property is isolated to the main actor and cannot be referenced from a non-main-actor-isolated context.

Backwards compatibility

I found that adding @MainActor causes a warning in Xcode 16+ for cases where @FirestoreQuery is used inside a non-isolated class. The warning is not ideal (and a little unexpected, see https://www.jessesquires.com/blog/2024/06/05/swift-concurrency-non-sendable-closures/), but I think it's unlikely developers will face them as the @FirestoreQuery is likely going to be used in a SwiftUI view which has the correct @MainActor isolation by default.

For Xcode 15, this PR doesn't seem to make a difference in behavior.

class MyNonIsolatedClass { @FirestoreQuery(  collectionPath: "fruits",  predicates: [  .where("isFavourite", isEqualTo: true),  ]  ) fileprivate var fruitResults: [Fruit] nonisolated func nonisolatedFunc() { // Xcode 15.2 // Doesn't compile currently with or without `@MainActor @preconcurrency` // 💥 Main actor-isolated property 'fruitResults' can not be referenced from a nonisolated context // Xcode 16.1 // Currently, without this PR's `@MainActor @preconcurrency`, this PR causes a warning: // ⚠️ Main actor-isolated property 'fruitResults' can not be referenced from a nonisolated context; this is an error in the Swift 6 language mode // Without @preconcurrency, this PR causes an error: // 💥 Main actor-isolated property 'fruitResults' can not be referenced from a nonisolated context // With @preconcurrency, this PR causes a warning: // ⚠️ Main actor-isolated property 'fruitResults' can not be referenced from a nonisolated context; this is an error in the Swift 6 language mode // Xcode 16.2 b2 // Same behavior as 16.1 print(fruitResults) } func defaultIsolationFunc() { // Xcode 15.2 // No error/warn, with @preconcurrency or not // Xcode 16.1 // Same as Xcode 15.2 // Xcode 16.2 b2 // Same as Xcode 16.1 print(fruitResults) } }

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Commit: ff8b117
Last updated: Fri Nov 22 08:34 PST 2024
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