title: hide and show left navigation bar button on demand in iOS-7
body: <p>I added my left navigation bar button using the storyboard. but I want it to hide when I first load the screen. And then in response to something else, I want it to show. The navigation bar has a method for hiding the back button. But there is no method for hiding/showing the left button. Is there a simple way for doing this? Or do I have to use two methods: the hiding method creates an empty button and the showing method creates the correct button? The button in question is just the <code>Add</code> template that iOS provides (which makes it easy to just use the one in the storyboard than to create my own).</p>