Async/Await function without a return | Swift 5.5

user_13893465 0
Viewed 2197

How can I remove the Bool return from my function without getting the error:

Generic parameter 'T' could not be inferred

This is the function:

private func syncDataStore() async throws -> Bool {
    try await withUnsafeThrowingContinuation { continuation in
        Amplify.DataStore.stop { (result) in
            switch(result) {
            case .success:
                Amplify.DataStore.start { (result) in
                    switch(result) {
                    case .success:
                        print("DataStore started")
                        continuation.resume(returning: true)
                    case .failure(let error):
                        print("Error starting DataStore:\(error)")
                        continuation.resume(throwing: error)
                    }
                }
            case .failure(let error):
                print("Error stopping DataStore:\(error)")
                continuation.resume(throwing: error)
            }
        }
    }
}

This is what I tried to do but I get the error mentioned above:

private func syncDataStore() async throws {
    try await withUnsafeThrowingContinuation { continuation in
        Amplify.DataStore.stop { (result) in
            switch(result) {
            case .success:
                Amplify.DataStore.start { (result) in
                    switch(result) {
                    case .success:
                        print("DataStore started")
                        continuation.resume()
                    case .failure(let error):
                        print("Error starting DataStore:\(error)")
                        continuation.resume(throwing: error)
                    }
                }
            case .failure(let error):
                print("Error stopping DataStore:\(error)")
                continuation.resume(throwing: error)
            }
        }
    }
}

Honestly I don't know why it's complaining, no returns are there and it's not tie to any model or anything...

swiftui swift
4 Answers

This is what worked the best:

try await withUnsafeThrowingContinuation { (continuation: UnsafeContinuation<Void, Error>) in
user_138934650

The trick is to cast return value of withUnsafeThrowingContinuation to Void, like so:

try await withUnsafeThrowingContinuation { continuation in
    someAsyncFunction() { error in
        if let error = error { continuation.resume(throwing: error) }
        else { continuation.resume() }
    }
} as Void
user_172260950

Let's look at signature

/// Suspends the current task,
/// then calls the given closure with the an unsafe throwing continuation for the current task.
///
/// - Parameter fn: A closure that takes an `UnsafeContinuation` parameter.
/// You must resume the continuation exactly once.
///
/// - Returns: The value passed to the continuation by the closure.
///
/// If `resume(throwing:)` is called on the continuation,
/// this function throws that error.
@available(macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0, *)
public func withUnsafeThrowingContinuation<T>(_ fn: (UnsafeContinuation<T, Error>) -> Void) async throws -> T

as it is seen withUnsafeThrowingContinuation is function with generics on return value which type is detected from continuation

So the solution for your case can be as follows:

private func syncDataStore() async throws {
    _ = try await withUnsafeThrowingContinuation { continuation in

        // ...

        continuation.resume(returning: true)

        // ...
    }
}
user_122990300

try this in your "private func syncDataStore() async throws {...}":

return try await withUnsafeThrowingContinuation {....}
user_119698170
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