How do I create a UITabBarController with a custom UITabBar class without using IB?

This is surprisingly hard! The best I've come up with is subclassing UITabBarController and then doing this in the init:

super.init(nibName: nil, bundle: nil)

object_setClass(self.tabBar, CustomTabBar.self)
(self.tabBar as? CustomTabBar)?.setup()

Unfortunately you can't set the class before the call to super.init (not in Swift anyway), and so by the time you change the class the init method has already been run and so won't be called on your custom subclass. To get around this, I've just added a setup() method to do all my customisation in.

Another option in Swift is to extend UITabBar and do something like this:

extension UITabBar {

    open override func willMove(toSuperview newSuperview: UIView?) {
        super.willMove(toSuperview: newSuperview)
        /// Customise in here.
    }

    // Modify the height.
    open override func sizeThatFits(_ size: CGSize) -> CGSize {
        return CGSize(width: size.width, height: 64.0)
    }
}

However this will affect all instances of UITabBar, so I prefer the first option.

I do not think it is possible.

Here is the Apple Documentation that talks about the tabBar property of UITabBarController.

You should never attempt to manipulate the UITabBar object itself stored in this property. If you attempt to do so, the tab bar view throws an exception. To configure the items for your tab bar interface, you should instead assign one or more custom view controllers to the viewControllers property. The tab bar collects the needed tab bar items from the view controllers you specify.

The tab bar view provided by this property is only for situations where you want to display an action sheet using the showFromTabBar: method of the UIActionSheet class.