Neato OnMain swift keyword.

OnMain.swift

import Foundation

func onMain(closure: () -> ()) {
    if NSThread.isMainThread() {
        closure()
    }
    else {
        dispatch_async(dispatch_get_main_queue(), closure)
    }
}

func delay(delay: NSTimeInterval, closure: () -> Void) {
    dispatch_after(
        dispatch_time(
            DISPATCH_TIME_NOW,
            Int64(delay * Double(NSEC_PER_SEC))
        ),
        dispatch_get_main_queue(), closure)
}

Swift 3:
func onMain(closure: @escaping () -> ()) {
if Thread.isMainThread {
closure()
}
else {
DispatchQueue.main.async(execute: closure)
}
}