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An asynchronous initializer in Swift allows a type to suspend its instantiation process while awaiting the completion of asynchronous operations. By marking an init declaration with the async keyword, the initializer integrates directly into Swift’s structured concurrency model, enabling the resolution of stored properties through non-blocking, concurrent tasks.

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Syntax

The async keyword is placed immediately after the initializer’s parameter list. If the initializer can also throw errors, async must precede the throws keyword. 
// Mock functions to satisfy compiler requirements
func resolveConfiguration(for id: String) async -> [String: Any] {
    return ["status": "resolved"]
}

func resolveStrictConfiguration(for id: String) async throws -> [String: Any] {
    return ["status": "strict_resolved"]
}

struct AsyncEntity {
    let identifier: String
    let configuration: [String: Any]

    // Standard async initializer
    init(id: String) async {
        self.identifier = id
        self.configuration = await resolveConfiguration(for: id)
    }

    // Async and throwing initializer
    init(id: String, strict: Bool) async throws {
        self.identifier = id
        self.configuration = try await resolveStrictConfiguration(for: id)
    }
}

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Invocation

Because the initializer contains potential suspension points, instantiating the type requires the await keyword and must occur within an asynchronous context (such as a Task, an asynchronous function, or another asynchronous initializer). 
func createEntity() async throws -> AsyncEntity {
    // Await the initialization process
    let entity = try await AsyncEntity(id: "101", strict: true)
    return entity
}

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Technical Mechanics and Compiler Rules

1. Definite Initialization and Suspension Points Swift’s definite initialization rules apply to async initializers, but the compiler permits suspension points (await) before all stored properties are fully initialized. The compiler enforces memory safety by ensuring that self cannot escape or have its instance methods called until Phase 1 of initialization (where all stored properties have an initial value) is complete, even across multiple suspension points. To compute values before Phase 1 completes, static methods or global functions must be used. 
class StateContainer {
    let primaryState: Int
    let secondaryState: Int

    // Static methods used to prevent calling instance methods before Phase 1 completes
    static func computePrimary() async -> Int { return 1 }
    static func computeSecondary() async -> Int { return 2 }
    
    func register() {
        // Internal registration logic
    }

    init() async {
        // Valid: Awaiting static methods before all properties are initialized
        self.primaryState = await StateContainer.computePrimary()
        self.secondaryState = await StateContainer.computeSecondary()
        
        // Phase 1 complete. 'self' is now fully initialized and instance methods can be called.
        self.register() 
    }
}
2. Actor Isolation When an actor defines an asynchronous initializer, the initialization process interacts specifically with the actor’s isolation domain.
  • Prior to the completion of Phase 1 initialization, the async initializer is treated as nonisolated.
  • Once all stored properties are initialized, the remainder of the initializer implicitly transitions to being isolated on the actor’s executor. This prevents data races if the initializer continues to mutate actor state after the initial allocation.
3. Delegation (self.init and super.init) Asynchronous initializers can delegate to other initializers.
  • A synchronous initializer cannot delegate to an asynchronous initializer.
  • An asynchronous initializer can delegate to either a synchronous or an asynchronous initializer. If delegating to an asynchronous initializer, the delegation call must be prefixed with await.
import Foundation

// Mock function to satisfy compiler requirements
func fetchNodeData() async -> Data {
    return Data()
}

struct Node {
    let id: UUID
    let data: Data

    init(data: Data) {
        self.id = UUID()
        self.data = data
    }

    init() async {
        let fetchedData = await fetchNodeData()
        // Delegating to a synchronous initializer from an async context
        self.init(data: fetchedData) 
    }
}
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