Executing on the Main (UI) Thread

 
DispatchQueue.main.async() {
    // Code to execute on main thread
}
 

(Xcode 8, Swift 3.0)

Completion Handler on Background Thread

Example

 
// Function with a completion handler parameter
func doWork(completion: @escaping (String) -> () ) {
    DispatchQueue.global(qos: .userInitiated).async {
        completion("Work is all done!")
    }
}

// Function that calls your completion handler function
func startWork() {
    print("Starting work")
    
    doWork { (result) -> () in
        print(result)
    }
    
    print("Waiting for doWork to finish.")
}
 

Console printout

Starting work
Waiting for doWork to finish.
Work is all done!

Quality of Service Parameter Enum

• userInteractive - Need it done almost instantaneously. Highest priority.
• userInitiated - Need it done in a few seconds or less.
• utility - Can wait a few seconds to a few minutes.
• background - Work will take significant time. Like minutes or hours. Lowest priority.

Notes

Closures all follow the same format:

() -> ()

For example, you see this format here:

doWork { (result) -> () in

But there are a few more ways you can write this:

doWork { (result) -> Void in

doWork { (result) in

doWork { (result: String) in

doWork() { result in

doWork { result in

These all work the same. The last one is probably the bare minimum you can get away with.

(Swift 3)