bar上back button的圖片

* 設定navation bar上back button的圖片，而且不顯示文字

ViewController.swift

override func viewDidLoad() {
        super.viewDidLoad()

        self.navigationItem.backBarButtonItem = UIBarButtonItem(title: "", style: .Plain, target: nil, action: nil)

        let image = UIImage(named: "back")
        self.navigationController?.navigationBar.backIndicatorImage = image
        self.navigationController?.navigationBar.backIndicatorTransitionMaskImage = image
    }

* 設定整個App navigation bar上back button的圖片

AppDelegate.swift

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {

        let image = UIImage(named: "back")
        UINavigationBar.appearance().backIndicatorImage = image
        UINavigationBar.appearance().backIndicatorTransitionMaskImage = image

        return true
    }