How to create dispatch queue in Swift 3

Question

Creating a concurrent queue

let concurrentQueue = DispatchQueue(label: "queuename", attributes: .concurrent)
concurrentQueue.sync {

}  

Create a serial queue

let serialQueue = DispatchQueue(label: "queuename")
serialQueue.sync { 

}

Get main queue asynchronously

DispatchQueue.main.async {

}

Get main queue synchronously

DispatchQueue.main.sync {

}

To get one of the background thread

DispatchQueue.global(qos: .background).async {

}

Xcode 8.2 beta 2:

To get one of the background thread

DispatchQueue.global(qos: .default).async {

}

DispatchQueue.global().async {
    // qos' default value is ´DispatchQoS.QoSClass.default`
}

If you want to learn about using these queues .See this answer

Compiles under >=Swift 3. This example contains most of the syntax that we need.

QoS - new quality of service syntax

weak self - to disrupt retain cycles

if self is not available, do nothing

async global utility queue - for network query, does not wait for the result, it is a concurrent queue, the block (usually) does not wait when started. Exception for a concurrent queue could be, when its task limit has been previously reached, then the queue temporarily turns into a serial queue and waits until some previous task in that queue completes.

async main queue - for touching the UI, the block does not wait for the result, but waits for its slot at the start. The main queue is a serial queue.

Of course, you need to add some error checking to this...

DispatchQueue.global(qos: .utility).async { [weak self] () -> Void in

    guard let strongSelf = self else { return }

    strongSelf.flickrPhoto.loadLargeImage { loadedFlickrPhoto, error in

        if error != nil {
            print("error:\(error)")
        } else {
            DispatchQueue.main.async { () -> Void in
                activityIndicator.removeFromSuperview()
                strongSelf.imageView.image = strongSelf.flickrPhoto.largeImage
            }
        }
    }
}

Compiled in XCode 8, Swift 3 https://github.com/rpthomas/Jedisware

 @IBAction func tap(_ sender: AnyObject) {

    let thisEmail = "emailaddress.com"
    let thisPassword = "myPassword" 

    DispatchQueue.global(qos: .background).async {

        // Validate user input

        let result = self.validate(thisEmail, password: thisPassword)

        // Go back to the main thread to update the UI
        DispatchQueue.main.async {
            if !result
            {
                self.displayFailureAlert()
            }

        }
    }

}

Since the OP question has already been answered above I just want to add some speed considerations:

It makes a lot of difference what priority class you assign to your async function in DispatchQueue.global.

I don't recommend running tasks with the .background thread priority especially on the iPhone X where the task seems to be allocated on the low power cores.

Here is some real data from a computationally intensive function that reads from an XML file (with buffering) and performs data interpolation:

Device name / .background / .utility / .default / .userInitiated / .userInteractive

1. iPhone X: 18.7s / 6.3s / 1.8s / 1.8s / 1.8s
2. iPhone 7: 4.6s / 3.1s / 3.0s / 2.8s / 2.6s
3. iPhone 5s: 7.3s / 6.1s / 4.0s / 4.0s / 3.8s

Note that the data set is not the same for all devices. It's the biggest on the iPhone X and the smallest on the iPhone 5s.

I did this and this is especially important if you want to refresh your UI to show new data without user noticing like in UITableView or UIPickerView.

    DispatchQueue.main.async
 {
   /*Write your thread code here*/
 }