Grand central dispatch – use ‘sync’ to schedule queues and use with What is the difference between work items marked by wait?

I'm learning Apple's GCD and watching the video concurrent programming with GCD in swift 3

At 16:00 in this video, a flag of dispatchworkitem is called Wait, functions and charts all show what I think of myqueue Sync (execute:) exactly

So my question is; What's the difference?

myQueue.sync { sleep(1); print("sync") }

And:

myQueue.async(flags: .wait) { sleep(1); print("wait") }
// NOTE: This Syntax doesn't compile,I'm not sure where the `.wait` flag moved to.
// `.wait` Seems not to be in the DispatchWorkItemFlags enum.

It seems that these two methods block the current thread while waiting for the named queue:

>Complete any current or previous work (if continuous) > complete a given block / work item

My understanding of this must be somewhere. What did I miss?

Solution

. Wait is not a flag in dispatchworkitemflags, that's why

myQueue.async(flags: .wait) { sleep(1); print("wait") }

Do not compile

Wait() is a method of dispatchworkitem, which is just a wrapper_ block_ wait().

/*!
 * @function dispatch_block_wait
 *
 * @abstract
 * Wait synchronously until execution of the specified dispatch block object has
 * completed or until the specified timeout has elapsed.

Simple example:

let myQueue = DispatchQueue(label: "my.queue",attributes: .concurrent)
let workItem = DispatchWorkItem {
    sleep(1)
    print("done")
}
myQueue.async(execute: workItem)
print("before waiting")
workItem.wait()
print("after waiting")

dispatchMain()