A function declared as rethrows that synchronously executes a throwing block in a dispatch_queue.

gistfile1.swift

import Dispatch

@_silgen_name("dispatch_sync") private func os_dispatch_sync(queue: dispatch_queue_t, @noescape _ block: dispatch_block_t)
@_silgen_name("dispatch_barrier_sync") private func os_dispatch_barrier_sync(queue: dispatch_queue_t, @noescape _ block: dispatch_block_t)

private func dispatch_sync_rethrow_wrapper<Return>(queue: dispatch_queue_t, @noescape dispatcher: (dispatch_queue_t, dispatch_block_t) -> Void, @noescape body: () throws -> Return, @noescape onError: ErrorType throws -> Void) rethrows -> Return {
    var ret: Return!
    var innerError: ErrorType?

    os_dispatch_sync(queue) {
        do {
            ret = try body()
        } catch {
            innerError = error
        }
    }

    if let innerError = innerError {
        try onError(innerError)
    }

    return ret
}

// A function declared as rethrows that synchronously executes a throwing
// block in a dispatch_queue.
//
// Based on Ransak's answer to https://forums.developer.apple.com/message/19685
public func dispatch_sync<Return>(queue: dispatch_queue_t, @noescape _ body: () throws -> Return) rethrows -> Return {
    return try dispatch_sync_rethrow_wrapper(queue, dispatcher: os_dispatch_sync, body: body, onError: { throw $0 })
}

public func dispatch_barrier_sync<Return>(queue: dispatch_queue_t, @noescape _ body: () throws -> Return) rethrows -> Return {
    return try dispatch_sync_rethrow_wrapper(queue, dispatcher: os_dispatch_barrier_sync, body: body, onError: { throw $0 })
}

// Some error
struct SomeError: ErrorType {}

// Some queue:
let queue = dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0)

// try required for throwing closure:
do {
    let value = try dispatch_sync(queue) { () -> Int in
        throw SomeError()
    }
    print("returned \(value)")
} catch {
    print("caught \(error)")
}

// try not required for non-throwing closure:
let value = dispatch_sync(queue) { () -> Int in
    print("hello")
    return 5
}

print("returned \(value)")