Sometimes when add child view controller programmatically and just add subview fill all container. You have to add constraint again and again. This snip set code will help you add by one line of code.

AddConstraint.swift

//Add constraint with top left right bottom is (0, 0, 0, 0)
func addChildToContainer(parent container: UIView, child childView: UIView) {
    container.addSubview(childView)
    childView.translatesAutoresizingMaskIntoConstraints = false
    let views = Dictionary(dictionaryLiteral: ("childView", childView),("container", container))
    //Horizontal constraints
    let horizontalConstraints = NSLayoutConstraint.constraintsWithVisualFormat("H:|[childView]|", options: NSLayoutFormatOptions(rawValue: 0), metrics: nil, views: views)
    container.addConstraints(horizontalConstraints)
    
    //Vertical constraints
    let verticalConstraints = NSLayoutConstraint.constraintsWithVisualFormat("V:|[childView]|", options: NSLayoutFormatOptions(rawValue: 0), metrics: nil, views: views)
    container.addConstraints(verticalConstraints)
}


//Add constraint with top left right bottom is same with option UIEdgeInsets
func addChildToContainer(parent container: UIView, child childView: UIView, andInsets insets: UIEdgeInsets) {
    container.addSubview(childView)
    childView.translatesAutoresizingMaskIntoConstraints = false
    let views = Dictionary(dictionaryLiteral: ("childView", childView),("container", container))
    //Horizontal constraints
    let horizontalConstraints = NSLayoutConstraint.constraintsWithVisualFormat("H:|-\(insets.left)-[childView]-\(insets.right)-|", options: NSLayoutFormatOptions(rawValue: 0), metrics: nil, views: views)
    container.addConstraints(horizontalConstraints)
    
    //Vertical constraints
    let verticalConstraints = NSLayoutConstraint.constraintsWithVisualFormat("V:|-\(insets.top)-[childView]-\(insets.bottom)-|", options: NSLayoutFormatOptions(rawValue: 0), metrics: nil, views: views)
    container.addConstraints(verticalConstraints)
}