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

gistfile1.swift

import Foundation

// 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
func perform_sync(queue: dispatch_queue_t, block: () throws -> Void) rethrows {
    func impl(queue: dispatch_queue_t, block: () throws -> Void, block2:((ErrorType) throws -> ()) ) rethrows {
        var blockError: ErrorType? = nil
        dispatch_sync(queue) {
            do {
                try block()
            } catch {
                blockError = error
            }
        }
        if let blockError = blockError {
            try block2(blockError)
        }
    }
    try impl(queue, block: block, block2: { throw $0 })
}


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

// try required for throwing closure:
do {
    try perform_sync(queue) {
        throw NSError(domain: "foo", code: 0, userInfo: nil)
    }
} catch {
    print("caught \(error)")
}

// try not required for non-throwing closure:
perform_sync(queue) {
    print("hello")
}