How To Properly Add Child View Controller in iOS 8 With Swift

use following Extension for adding childViewController On View

extension UIViewController {   
func configureChildViewController(childController: UIViewController, onView: UIView?) {
    var holderView = self.view
    if let onView = onView {
        holderView = onView
    }
    addChildViewController(childController)
    holderView.addSubview(childController.view)
    constrainViewEqual(holderView, view: childController.view)
    childController.didMoveToParentViewController(self)
    childController.willMoveToParentViewController(self)
}


func constrainViewEqual(holderView: UIView, view: UIView) {
    view.translatesAutoresizingMaskIntoConstraints = false
    //pin 100 points from the top of the super
    let pinTop = NSLayoutConstraint(item: view, attribute: .Top, relatedBy: .Equal,
        toItem: holderView, attribute: .Top, multiplier: 1.0, constant: 0)
    let pinBottom = NSLayoutConstraint(item: view, attribute: .Bottom, relatedBy: .Equal,
        toItem: holderView, attribute: .Bottom, multiplier: 1.0, constant: 0)
    let pinLeft = NSLayoutConstraint(item: view, attribute: .Left, relatedBy: .Equal,
        toItem: holderView, attribute: .Left, multiplier: 1.0, constant: 0)
    let pinRight = NSLayoutConstraint(item: view, attribute: .Right, relatedBy: .Equal,
        toItem: holderView, attribute: .Right, multiplier: 1.0, constant: 0)

    holderView.addConstraints([pinTop, pinBottom, pinLeft, pinRight])
}}

Updated for Swift 5+

Just one line in your view controller to add child view controller.

Super scalable methods in the extension if you want to add it on any custom view.

 public extension UIViewController {

    /// Adds child view controller to the parent.
    ///
    /// - Parameter child: Child view controller.
    func add(_ child: UIViewController) {
        addChild(child)
        view.addSubview(child.view)
        child.didMove(toParent: self)
    }

    /// It removes the child view controller from the parent.
    func remove() {
        guard parent != nil else {
            return
        }
        willMove(toParent: nil)
        removeFromParent()
        view.removeFromSuperview()
    }
}

How to use:

Adding: In the view controller where you want to add the child view controller.

// let yourChildViewController = Load fro the storyboard or XIB
add(yourChildViewController)

Removing:

yourChildViewController.remove()

If you want the red controller to be the child controller, delete the yellow one, and control-drag from the container to the red controller. There's no need to add it in code, or do any resizing. The red controller will be set to the same size as the container in the storyboard.