How to dispatch on main queue synchronously without a deadlock?

Question

I need to dispatch a block on the main queue, synchronously. I don’t know if I’m currently running on the main thread or no. The naive solution looks like this:

Answer 1

For syncing on the main queue or on the main thread (that is not the same) I use:

import Foundation

private let mainQueueKey    = UnsafeMutablePointer.alloc(1)
private let mainQueueValue  = UnsafeMutablePointer.alloc(1)


public func dispatch_sync_on_main_queue(block: () -> Void)
{
    struct dispatchonce  { static var token : dispatch_once_t = 0  }
    dispatch_once(&dispatchonce.token,
    {
        dispatch_queue_set_specific(dispatch_get_main_queue(), mainQueueKey, mainQueueValue, nil)
    })

    if dispatch_get_specific(mainQueueKey) == mainQueueValue
    {
        block()
    }
    else
    {
        dispatch_sync(dispatch_get_main_queue(),block)
    }
}

extension NSThread
{
    public class func runBlockOnMainThread(block: () -> Void )
    {
        if NSThread.isMainThread()
        {
            block()
        }
        else
        {
            dispatch_sync(dispatch_get_main_queue(),block)
        }
    }

    public class func runBlockOnMainQueue(block: () -> Void)
    {
        dispatch_sync_on_main_queue(block)
    }
}