laconicman/asynctosyncbridge

Why this exists

It isn’t common to wrap an async function with a completion handler — usually, we do the reverse. But this becomes useful when some public API can’t change yet while your implementation is moving to async/await. This package provides small, focused helpers to:

• Run an async operation in a Task.
• Deliver results to a completion handler.
• Choose where the completion runs (MainActor or a specific DispatchQueue).
• Avoid boilerplate and reduce the risk of incorrect callback queues.

Features

• Unlabeled operation closure for ergonomic, trailing closure syntax (just like Swift’s native Task initializers)
• MainActor delivery for UI‑safe callbacks using await MainActor.run { ... }
• DispatchQueue delivery when you need a specific GCD queue
• Overloads for:

- async throwing returning a value → Result<T, Error> completion - async throwing returning Void → Error? completion - async returning a value → (T) -> Void completion - async returning Void → () -> Void completion

• @Sendable closures and T: Sendable constraints to help prevent data races across concurrency boundaries
• Public APIs are marked @inlinable for maximal inlining and performance

Use cases

The library can be helpful when:

1. Apple frameworks mandate completion handlers (WidgetKit, some UIKit patterns) - example
2. Public API compatibility is non-negotiable (SDKs, frameworks)
3. Gradual migration from legacy codebases with mixed paradigms
4. Objective-C interoperability requirements (completion handlers bridge better than async/await)
5. Testing infrastructure hasn’t fully adopted Swift Concurrency yet

The library eliminates boilerplate, ensures thread safety with Sendable constraints, and defaults to MainActor delivery for UI safety—making it significantly better than ad-hoc Task wrapping.

Quick start

Add package dependency

MainActor delivery (no Dispatch dependency)

import AsyncToSyncBridge

// Suppose you have "Modern Concurrency" API
func didReceiveRemoteNotification(userInfo:[AnyHashable: Any]) async -> UIBackgroundFetchResult {
    // ...
    return .newData
}

// But you need to use it inside a synchronous completion-based API, i.e. `UIApplicationDelegate`
func application(_ application: UIApplication,
                 didReceiveRemoteNotification userInfo: [AnyHashable: Any],
                 fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
    // With trailing closure, this looks like native Task initializers:
    Task {
        await didReceiveRemoteNotification(userInfo:userInfo)
    } completion: { value in
        completionHandler(value)
    }
}

And other overloads for other types of completions are available:

// Async throwing returning a value → Result<T, Error> on MainActor Task { try await doWorkReturningValue() } completion: { (result: Result<MyType, Error>) in // runs on MainActor }

// Async throwing returning Void → Error? on MainActor Task { try await doWorkThrowingVoid() } completion: { (error: Error?) in // runs on MainActor; error is nil on success }

// Async returning a value → (T) -> Void on MainActor Task { await doWorkReturningValue() } completion: { value in // runs on MainActor when finished }

// Async returning Void → () -> Void on MainActor Task { await doWorkVoid() } completion: { // runs on MainActor when finished }

DispatchQueue Delivery

If you must call your completion handler on a specific DispatchQueue (for example, a background queue, or for legacy code):

Implementation notes

• Public API is marked @inlinable for performance and cross-module optimization.
• Trailing closure syntax makes adoption seamless and familiar for Swift developers.
• The difference between MainActor and .main queue is clearly documented and enforced at the API level.

Task(queue: .main) {
    try await doWorkReturningValue()
} completion: { (result: Result<MyType, Error>) in
    // runs on DispatchQueue.main (GCD), NOT MainActor
}

Note:

• DispatchQueue.main.async is not the same as await MainActor.run {}.
• Use the MainActor overloads for UI updates and actor isolation.
• Use the DispatchQueue variants for legacy queue requirements.

Package Metadata

Repository: laconicman/asynctosyncbridge

Default branch: main

README: README.md