Adding a UIView to a UITableViewController with NSLayoutConstraints programmatically with Swift 4

Question

I'm attempting to add a UIView with NSLayoutConstraints but I keep getting an uncaught exception of type NSException error. The main reason I'm doing it with code instead of in the storyboard is I want it to be above the Navigation bar on the UITableViewController.

Here is the code I currently have.

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)

    let mainWindow: UIWindow = UIApplication.shared.keyWindow!

    let newView = UIView()
    newView.backgroundColor = UIColor.darkAqua
    newView.translatesAutoresizingMaskIntoConstraints = false

    mainWindow.addSubview(newView)

    let viewHeight = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 100)
    let viewWidth = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.width, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 100)
    let viewLeft = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.left, relatedBy: NSLayoutRelation.equal, toItem: mainWindow, attribute: NSLayoutAttribute.left, multiplier: 1, constant: 40)
    let viewTop = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.top, relatedBy: NSLayoutRelation.equal, toItem: mainWindow, attribute: NSLayoutAttribute.top, multiplier: 1, constant: 0)

    newView.addConstraints([viewHeight, viewWidth, viewLeft, viewTop])
}

If I take out the viewLeft and viewTop it will run okay since the toItem: is nil in NSLayoutConstraint. But I think the problem has something to do with that toItem: I've tried using self.view and mainWindow but it keeps throwing the error.

I've seen lots of examples of how to do this and I'm pretty much copying them as far as I can tell, but none of them were Swift 4 so I'm wondering if something changed in Swift 4.

Any help would be great! Thank you.

Error Message:

libc++abi.dylib: terminating with uncaught exception of type NSException (lldb)

Answer 1

I tried with self.view instead of main window and with the different style of layout constraints. Just try this Jason Brady.

override func viewDidLoad() {
    super.viewDidLoad()

    let newView = UIView()
    newView.backgroundColor = UIColor.red
    newView.translatesAutoresizingMaskIntoConstraints = false

    self.view.addSubview(newView)


    newView.leftAnchor.constraint(equalTo: self.view.leftAnchor, constant: 40).isActive = true
    newView.topAnchor.constraint(equalTo: self.view.topAnchor, constant: 0).isActive = true
    newView.widthAnchor.constraint(equalToConstant: 100).isActive = true
    newView.heightAnchor.constraint(equalToConstant: 100).isActive = true
}