Posting NSNotification on the main thread

Yes you can.

Generally you want the NSNotifications to be sent on the main , especially if they trigger UI activities like dismissing a modal login dialog.

Delivering Notifications To Particular Threads

Regular notification centers deliver notifications on the thread in which the notification was posted. Distributed notification centers deliver notifications on the main thread. At times, you may require notifications to be delivered on a particular thread that is determined by you instead of the notification center. For example, if an object running in a background thread is listening for notifications from the user interface, such as a window closing, you would like to receive the notifications in the background thread instead of the main thread. In these cases, you must capture the notifications as they are delivered on the default thread and redirect them to the appropriate thread.

Yes

This is - you are getting into the main thread and posting your notification. Can't get any safer than that.

YES

Swift 2 syntax

dispatch_async(dispatch_get_main_queue()) {
    NSNotificationCenter.defaultCenter().postNotificationName("updateSpinner", object: nil, userInfo: ["percent":15])
}

Swift 3 syntax

DispatchQueue.main.async {
    NotificationCenter.default.post(name: "updateSpinner", object: nil, userInfo: ["percent":15])
}

Somewhere along the line this became possible with:

addObserver(forName:object:queue:using:)

which is here, but the whole point is the queue object.

The operation queue to which block should be added. If you pass nil, the block is run synchronously on the posting thread.

So how do you get the queue that corresponds to the main runloop?

let mainQueue = OperationQueue.main

Note: this is when you are subscribing to notifications, so you do it once and you're done. Doing it on every single call is terribly redundant.