Using dispatch_async with self
You should condition spinning off this action on the non-nullity, not test for it after you've already initiated it:
if let hostView = self.hostViewController?.view { DispatchQueue.main.async { hostView.addSubview(self.commandField) }} else { // handle nil hostView }You should never unwrap an optional outside of an if let, or testing it first. Doing this should also resolve yer weak self issue.
The dispatch_async syntax has changed with Swift 3:
DispatchQueue.main.async { hostView.addSubview(self.commandField)}