Demo for view controller's life cycle

The application include a main view and and a (Demo View)

You can add the demo view/remove the demo view by clicking the buttons

in the meantime, check out the logs and find out how the view responds
to the add/remove event

Also - try to simulate the memory warning when the view is shown/not
shown, this will reveal how the viewDidUnload works and how the
loadView can be triggered for multiple times.