UIBarButtonItem in navigation bar programmatically?

Custom button image without setting button frame:

You can use init(image: UIImage?, style: UIBarButtonItemStyle, target: Any?, action: Selector?) to initializes a new item using the specified image and other properties.

let button1 = UIBarButtonItem(image: UIImage(named: "imagename"), style: .plain, target: self, action: Selector("action")) // action:#selector(Class.MethodName) for swift 3
self.navigationItem.rightBarButtonItem  = button1

Check this Apple Doc. reference

UIBarButtonItem with custom button image using button frame

FOR Swift 3.0

    let btn1 = UIButton(type: .custom)
    btn1.setImage(UIImage(named: "imagename"), for: .normal)
    btn1.frame = CGRect(x: 0, y: 0, width: 30, height: 30)
    btn1.addTarget(self, action: #selector(Class.Methodname), for: .touchUpInside)
    let item1 = UIBarButtonItem(customView: btn1)

    let btn2 = UIButton(type: .custom)
    btn2.setImage(UIImage(named: "imagename"), for: .normal)
    btn2.frame = CGRect(x: 0, y: 0, width: 30, height: 30)
    btn2.addTarget(self, action: #selector(Class.MethodName), for: .touchUpInside)
    let item2 = UIBarButtonItem(customView: btn2)  

    self.navigationItem.setRightBarButtonItems([item1,item2], animated: true)

FOR Swift 2.0 and older

let btnName = UIButton()
btnName.setImage(UIImage(named: "imagename"), forState: .Normal)
btnName.frame = CGRectMake(0, 0, 30, 30)
btnName.addTarget(self, action: Selector("action"), forControlEvents: .TouchUpInside)

//.... Set Right/Left Bar Button item
let rightBarButton = UIBarButtonItem()
rightBarButton.customView = btnName
self.navigationItem.rightBarButtonItem = rightBarButton

Or simply use init(customView:) like

 let rightBarButton = UIBarButtonItem(customView: btnName)
 self.navigationItem.rightBarButtonItem = rightBarButton

For System UIBarButtonItem

let camera = UIBarButtonItem(barButtonSystemItem: .Camera, target: self, action: Selector("btnOpenCamera"))
self.navigationItem.rightBarButtonItem = camera

For set more then 1 items use rightBarButtonItems or for left side leftBarButtonItems

let btn1 = UIButton()
btn1.setImage(UIImage(named: "img1"), forState: .Normal)
btn1.frame = CGRectMake(0, 0, 30, 30)
btn1.addTarget(self, action: Selector("action1:"), forControlEvents: .TouchUpInside)
let item1 = UIBarButtonItem()
item1.customView = btn1

let btn2 = UIButton()
btn2.setImage(UIImage(named: "img2"), forState: .Normal)
btn2.frame = CGRectMake(0, 0, 30, 30)
btn2.addTarget(self, action: Selector("action2:"), forControlEvents: .TouchUpInside)
let item2 = UIBarButtonItem()
item2.customView = btn2

self.navigationItem.rightBarButtonItems = [item1,item2]

Using setLeftBarButtonItem or setRightBarButtonItem

let btn1 = UIButton()
btn1.setImage(UIImage(named: "img1"), forState: .Normal)
btn1.frame = CGRectMake(0, 0, 30, 30)
btn1.addTarget(self, action: Selector("action1:"), forControlEvents: .TouchUpInside)
self.navigationItem.setLeftBarButtonItem(UIBarButtonItem(customView: btn1), animated: true);

For swift >= 2.2 action should be #selector(Class.MethodName) ... for e.g. btnName.addTarget(self, action: #selector(Class.MethodName), forControlEvents: .TouchUpInside)

It's much easier with Swift 4 or Swift 4.2

inside your ViewDidLoad method, define your button and add it to the navigation bar.

override func viewDidLoad() {
    super.viewDidLoad()

    let logoutBarButtonItem = UIBarButtonItem(title: "Logout", style: .done, target: self, action: #selector(logoutUser))
    self.navigationItem.rightBarButtonItem  = logoutBarButtonItem

}

then you need to define the function that you mentioned inside action parameter as below

@objc func logoutUser(){
     print("clicked")
}

You need to add the @objc prefix as it's still making use of the legacy stuff (Objective C).

Just setup UIBarButtonItem with customView

For example:

  var leftNavBarButton = UIBarButtonItem(customView:yourButton)
  self.navigationItem.leftBarButtonItem = leftNavBarButton

or use setFunction:

  self.navigationItem.setLeftBarButtonItem(UIBarButtonItem(customView: yourButton), animated: true);

I just stumbled upon this question and here is an update for Swift 3 and iOS 10:

let testUIBarButtonItem = UIBarButtonItem(image: UIImage(named: "test.png"), style: .plain, target: self, action: nil)
self.navigationItem.rightBarButtonItem  = testUIBarButtonItem

It is definitely much faster than creating the UIButton with all the properties and then subsequently adding the customView to the UIBarButtonItem.

And if you want to change the color of the image from the default blue to e.g. white, you can always change the tint color:

test.tintColor = UIColor.white()

PS You should obviously change the selector etc. for your app :)