How to sync an async function in Swift

Issue #547

func sync<T>(_ work: (@escaping ([T]) -> Void) -> Void) -> [T] {
    let semaphore = DispatchSemaphore(value: 1)
    var results = [T]()
    work { values in
        results = values
        semaphore.signal()
    }

    return results
}

sync({ completion in 
    service.load(completion)
})