需要在NavigationBar上面添加图片，文字，按钮等内容
搜：
swift draw on navigationbar
swift custom navigation bar
参考：
搜：
swift uibarbuttonitem example
参考：
试了各种：
//4. add button
let addImage = UIImage(named:"navi_add")
// let addNaviBarButtonItem:UIBarButtonItem = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.Add, target: self, action: Selector("toggleAddDropdownList"))
let addNaviBarButtonItem:UIBarButtonItem = UIBarButtonItem(image: addImage, style: UIBarButtonItemStyle.Plain, target: self, action: Selector("toggleAddDropdownList"))
let searchImage = UIImage(named:"navi_search")
let searchNaviBarButtonItem:UIBarButtonItem = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.Search, target: self, action: Selector("jumpToSearch"))
let mainNaviBarRightItems:[UIBarButtonItem] = [addNaviBarButtonItem, searchNaviBarButtonItem]
// self.navigationController?.navigationItem.setRightBarButtonItems(mainNaviBarRightItems, animated: true)
//
//mainViewVC.navigationController?.navigationItem.leftBarButtonItem = switchTeamButton
//self.navigationBar.backItem?.hidesBackButton = false
//self.navigationItem.setRightBarButtonItems(mainNaviBarRightItems, animated: true)
//self.navigationItem.setRightBarButtonItem(addNaviBarButtonItem, animated: true)
self.navigationController?.navigationItem.setRightBarButtonItem(addNaviBarButtonItem, animated: true)
self.navigationItem.setRightBarButtonItem(addNaviBarButtonItem, animated: true)都没用。
后来才发现，之前是在AppDelegate：
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {或者NavigationController：
class MainNavigationController: UINavigationController {
override func viewDidLoad() {中调用navigationItem，是不行的
以及在ViewController的viewDidLoad中：
class MainViewController: UITabBarController, UIGestureRecognizerDelegate {
override func viewDidLoad() {但是却想要操作：self.navigationController
结果得到的是nil：
print("self.navigationController=\(self.navigationController)") //self.navigationController=nil最后是在：
ViewController中的viewDidLoad操作self.navigationItem：
print("self.navigationItem=\(self.navigationItem)")//self.navigationItem=<UINavigationItem: 0x7f8c205132a0>即可通过代码：
let addNaviBarButtonItem:UIBarButtonItem = UIBarButtonItem(image: UIImage(named:"navi_add"), style: UIBarButtonItemStyle.Plain, target: self, action: Selector("toggleAddDropdownList"))
self.navigationItem.setRightBarButtonItem(addNaviBarButtonItem, animated: true)添加出来对应的按钮了：
另外，再去添加其它按钮：
//use system add button
//let addNaviBarButtonItem:UIBarButtonItem = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.Add, target: self, action: Selector("toggleAddDropdownList"))
//add custom image add button
let addNaviBarButtonItem:UIBarButtonItem = UIBarButtonItem(image: UIImage(named:"navi_add"), style: UIBarButtonItemStyle.Plain, target: self, action: Selector("toggleAddDropdownList"))
//self.navigationItem.setRightBarButtonItem(addNaviBarButtonItem, animated: true)
let searchNaviBarButtonItem:UIBarButtonItem = UIBarButtonItem(image: UIImage(named:"navi_search"), style: UIBarButtonItemStyle.Plain, target: self, action: Selector("jumpToSearch"))
let mainNaviBarRightItems:[UIBarButtonItem] = [addNaviBarButtonItem, searchNaviBarButtonItem]
self.navigationItem.setRightBarButtonItems(mainNaviBarRightItems, animated: true)
let switchTeamNaviBarButtonItem = UIBarButtonItem(customView: switchTeamButton)
self.navigationItem.setLeftBarButtonItem(switchTeamNaviBarButtonItem, animated: true)即可添加到几个按钮了：
然后接着去解决：