What's the relationship between UINavigationController and UINavigationItem
I'm becoming confused these days using UINavigationController;
I'd like to customize the UINavigationController,changing its background image and adding buttons;
I tried this in 2 projects. In the first I did this by using

 `self.navigationController.navigationItem.leftBarButtonItem = customItem;`
but in the secoenter code herend this method doesn't work.I have to use 

    self.navigationItem.leftBarButtonItem = customItem;
if a ViewController is a rootViewController of a NavigationController,it has two properties:

    self.navigationController
    self.navigationItem

and the Navigation has 3 properties:

    self.navigationController.navigationController
    self.navigationController.navigationBar
    self.navigationController.navigationItem
why the NavigationController also has a property NavigationController?

why sometimes I use

    self.navigationController.navigationItem

but sometimes I have to use

    self.navigationItem

I'm really confused.
What's the relationship between UINavigationController and UINavigationItem
===

I'm becoming confused these days using UINavigationController;
I'd like to customize the UINavigationController,changing its background image and adding buttons;
I tried this in 2 projects. In the first I did this by using

 `self.navigationController.navigationItem.leftBarButtonItem = customItem;`
but in the secoenter code herend this method doesn't work.I have to use 

    self.navigationItem.leftBarButtonItem = customItem;
if a ViewController is a rootViewController of a NavigationController,it has two properties:

    self.navigationController
    self.navigationItem

and the Navigation has 3 properties:

    self.navigationController.navigationController
    self.navigationController.navigationBar
    self.navigationController.navigationItem
why the NavigationController also has a property NavigationController?

why sometimes I use

    self.navigationController.navigationItem

but sometimes I have to use

    self.navigationItem

I'm really confused.