View constraints - fit a subview - ios

Question

I am adding a subview to a view and I want it to fill the height and width of the view. I am having difficulty with constraints. Any help is appreciated. This is what I have currently:
self.view.addSubview(self.mainView)
var leftSideConstraint = NSLayoutConstraint(item: self.mainView, attribute: .Left, relatedBy: .Equal, toItem: self.view, attribute: .Left, multiplier: 1.0, constant: 0.0)
var bottomConstraint = NSLayoutConstraint(item: self.mainView, attribute: .Bottom, relatedBy: .Equal, toItem: self.view, attribute: .Bottom, multiplier: 1.0, constant: 0.0)
var widthConstraint = NSLayoutConstraint(item: self.mainView, attribute: .Width, relatedBy: .Equal, toItem: self.view, attribute: .Width, multiplier: 1.0, constant: 0.0)
var heightConstraint = NSLayoutConstraint(item: self.mainView, attribute: .Height, relatedBy: .Equal, toItem: self.view, attribute: .Height, multiplier: 1.0, constant: 0.0)
self.view.addConstraints([leftSideConstraint, bottomConstraint, widthConstraint, heightConstraint])

Answer 1

Swift 5
Here is an elegant way with using UIView extension
extension UIView {
func addConstrained(subview: UIView) {
addSubview(subview)
subview.translatesAutoresizingMaskIntoConstraints = false
subview.topAnchor.constraint(equalTo: topAnchor).isActive = true
subview.leadingAnchor.constraint(equalTo: leadingAnchor).isActive = true
subview.trailingAnchor.constraint(equalTo: trailingAnchor).isActive = true
subview.bottomAnchor.constraint(equalTo: bottomAnchor).isActive = true
}
}
And then you add a subview wherever you need it with
yourContainerView.addConstrained(subview: yourSubview)

Answer 2

The following code should work:
let mainView = UIView() //Make sure your mainView is initialized
mainView.backgroundColor = UIColor.blueColor() //For test purpose
mainView.setTranslatesAutoresizingMaskIntoConstraints(false) //Don't forget this line
view.addSubview(mainView)
var leftSideConstraint = NSLayoutConstraint(item: mainView, attribute: .Left, relatedBy: .Equal, toItem: view, attribute: .Left, multiplier: 1.0, constant: 0.0)
var bottomConstraint = NSLayoutConstraint(item: mainView, attribute: .Bottom, relatedBy: .Equal, toItem: view, attribute: .Bottom, multiplier: 1.0, constant: 0.0)
var widthConstraint = NSLayoutConstraint(item: mainView, attribute: .Width, relatedBy: .Equal, toItem: view, attribute: .Width, multiplier: 1.0, constant: 0.0)
var heightConstraint = NSLayoutConstraint(item: mainView, attribute: .Height, relatedBy: .Equal, toItem: view, attribute: .Height, multiplier: 1.0, constant: 0.0)
view.addConstraints([leftSideConstraint, bottomConstraint, heightConstraint, widthConstraint])
As an alternative, you can use the Auto layout Visual Format Language:
let mainView = UIView() //Make sure your mainView is initialized
mainView.backgroundColor = UIColor.blueColor() //For test purpose
mainView.setTranslatesAutoresizingMaskIntoConstraints(false) //Don't forget this line
view.addSubview(mainView)
var viewsDict = ["mainView" : mainView]
var horizontalConstraints = NSLayoutConstraint.constraintsWithVisualFormat("H:|[mainView]|", options: NSLayoutFormatOptions(0), metrics: nil, views: viewsDict)
var verticalConstraints = NSLayoutConstraint.constraintsWithVisualFormat("V:|[mainView]|", options: NSLayoutFormatOptions(0), metrics: nil, views: viewsDict)
view.addConstraints(horizontalConstraints)
view.addConstraints(verticalConstraints)