Change navigation bar bottom border color Swift

Putting @alessandro-orru's answer in one extension

extension UINavigationController {

    func setNavigationBarBorderColor(_ color:UIColor) {
        self.navigationBar.shadowImage = color.as1ptImage()
    }
}

extension UIColor {

    /// Converts this `UIColor` instance to a 1x1 `UIImage` instance and returns it.
    ///
    /// - Returns: `self` as a 1x1 `UIImage`.
    func as1ptImage() -> UIImage {
        UIGraphicsBeginImageContext(CGSize(width: 1, height: 1))
        setFill()
        UIGraphicsGetCurrentContext()?.fill(CGRect(x: 0, y: 0, width: 1, height: 1))
        let image = UIGraphicsGetImageFromCurrentImageContext() ?? UIImage()
        UIGraphicsEndImageContext()
        return image
    }
}

then in your view controller just add:

self.navigationController?.setNavigationBarBorderColor(UIColor.red)

Wonderful contributions from @TheoF, @Alessandro and @Pavel.

Here is what I did for...

Swift 4

extension UIColor {

    /// Converts this `UIColor` instance to a 1x1 `UIImage` instance and returns it.
    ///
    /// - Returns: `self` as a 1x1 `UIImage`.
    func as1ptImage() -> UIImage {
        UIGraphicsBeginImageContext(CGSize(width: 1, height: 1))
        setFill()
        UIGraphicsGetCurrentContext()?.fill(CGRect(x: 0, y: 0, width: 1, height: 1))
        let image = UIGraphicsGetImageFromCurrentImageContext() ?? UIImage()
        UIGraphicsEndImageContext()
        return image
    }
}

Using it in viewDidLoad():

/* In this example, I have a ViewController embedded in a NavigationController in IB. */

// Remove the background color.
navigationController?.navigationBar.setBackgroundImage(UIColor.clear.as1ptImage(), for: .default)

// Set the shadow color.
navigationController?.navigationBar.shadowImage = UIColor.gray.as1ptImage()

You have to adjust the shadowImage property of the navigation bar.

Try this one. I created a category on UIColor as an helper, but you can refactor the way you prefer.

extension UIColor {
    func as1ptImage() -> UIImage {
        UIGraphicsBeginImageContext(CGSizeMake(1, 1))
        let ctx = UIGraphicsGetCurrentContext()
        self.setFill()
        CGContextFillRect(ctx, CGRect(x: 0, y: 0, width: 1, height: 1))
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return image
    }
}

Option 1: on a single navigation bar

And then in your view controller (change the UIColor to what you like):

// We can use a 1px image with the color we want for the shadow image
self.navigationController?.navigationBar.shadowImage = UIColor.redColor().as1ptImage()

// We need to replace the navigation bar's background image as well 
// in order to make the shadowImage appear. We use the same 1px color tecnique
self.navigationController?.navigationBar.setBackgroundImage(UIColor.yellowColor‌​().as1ptImage(), forBarMetrics: .Default)    

Option 2: using appearance proxy, on all navigation bars

Instead of setting the background image and shadow image on each navigation bar, it is possible to rely on UIAppearance proxy. You could try to add those lines to your AppDelegate, instead of adding the previous ones in the viewDidLoad.

// We can use a 1px image with the color we want for the shadow image
UINavigationBar.appearance().shadowImage = UIColor.redColor().as1ptImage()

// We need to replace the navigation bar's background image as well 
// in order to make the shadowImage appear. We use the same 1px color technique
UINavigationBar.appearance().setBackgroundImage(UIColor.yellowColor().as1ptImage(), forBarMetrics: .Default)

Solution for Swift 4.0 - 5.2

Here is small extension for changing both Height and Color of bottom navbar line

extension UINavigationController
{
    func addCustomBottomLine(color:UIColor,height:Double)
    {
        //Hiding Default Line and Shadow
        navigationBar.setValue(true, forKey: "hidesShadow")
    
        //Creating New line
        let lineView = UIView(frame: CGRect(x: 0, y: 0, width:0, height: height))
        lineView.backgroundColor = color
        navigationBar.addSubview(lineView)
    
        lineView.translatesAutoresizingMaskIntoConstraints = false
        lineView.widthAnchor.constraint(equalTo: navigationBar.widthAnchor).isActive = true
        lineView.heightAnchor.constraint(equalToConstant: CGFloat(height)).isActive = true
        lineView.centerXAnchor.constraint(equalTo: navigationBar.centerXAnchor).isActive = true
        lineView.topAnchor.constraint(equalTo: navigationBar.bottomAnchor).isActive = true
    }
}

And after adding this extension, you can call this method on any UINavigationController (e.g. from ViewController viewDidLoad())

self.navigationController?.addCustomBottomLine(color: UIColor.black, height: 20)