I need top constraint of unknow element - ios

Question

I have an app which is downloading image from server by clicking the button. After image have downloaded i create a new imageView and add it to the my contentView(UIView). I need to create the constraints - every new imageview need top constraint from previous one
func addNewImageToTheScrollView(img: UIImage?) {
if let imageResponse = img {
let imageView = UIImageView(image: imageResponse.crop(rect: CGRect(x: 0, y: imageResponse.size.height/2, width: self.contentView.frame.width, height: 200)))
self.contentView.addSubview(imageView)
imageView.translatesAutoresizingMaskIntoConstraints = false
let x = NSLayoutConstraint(item: imageView, attribute: .centerX, relatedBy: .equal, toItem: self.contentView, attribute: .centerX, multiplier: 1.0, constant: 0)
let y = NSLayoutConstraint(item: imageView, attribute: .top, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 30)
let width = NSLayoutConstraint(item: imageView, attribute: .width, relatedBy: .equal, toItem: self.contentView, attribute: .width, multiplier: 1.0, constant: 0)
let height = NSLayoutConstraint(item: imageView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 150)
self.contentView.addConstraints([x, y])
imageView.addConstraints([width, height])
}
}
If i comment the constraint code, it will be work fine unless every new imageView will be on the same place, on the top of the View. Now whit this constraint code i have such code issue after downloading
2017-07-02 14:50:01.018 ImageFromServerTest[11516:1080948] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'NSLayoutConstraint for >: A multiplier of 0 or a nil second item together with a location for the first attribute creates an illegal constraint of a location equal to a constant. Location attributes must be specified in pairs.'

Answer 1

Whenever you are working with scrollViews, there are 2 thumb rules for it:-
Give the scrollView leading, trailing, bottom, and top constraint with respect to the superview, that is self.view
#IbOutlet weak var scrollView: UIScrollView!
let leading = NSLayoutConstraint(item: scrollView, attribute: .leading, relatedBy: .equal, toItem: self.view, attribute: .leadingMargin, multiplier: 1.0, constant: 0)
let trailing = NSLayoutConstraint(item: scrollView, attribute: .trailing, relatedBy: .equal, toItem: self.view, attribute: .trailingMargin, multiplier: 1.0, constant: 0)
let top = NSLayoutConstraint(item: scrollView, attribute: .top, relatedBy: .equal, toItem: self.view, attribute: .top, multiplier: 1.0, constant: 0)
let bottom = NSLayoutConstraint(item: scrollView, attribute: .bottom, relatedBy: .equal, toItem: self.view, attribute: .bottom, multiplier: 1.0, constant: 0)
Add constraints to the contentView with respect to the scrollView
#IbOutlet weak var contentView: UIView!
let leading = NSLayoutConstraint(item: contentView, attribute: .leading, relatedBy: .equal, toItem: scrollView, attribute: .leading, multiplier: 1.0, constant: 0)
let trailing = NSLayoutConstraint(item: contentView, attribute: .trailing, relatedBy: .equal, toItem: scrollView, attribute: .trailing, multiplier: 1.0, constant: 0)
let top = NSLayoutConstraint(item: contentView, attribute: .top, relatedBy: .equal, toItem: scrollView, attribute: .top, multiplier: 1.0, constant: 0)
//increase the constant according to how much long you need the scrollview to be
let bottom = NSLayoutConstraint(item: contentView, attribute: .bottom, relatedBy: .equal, toItem: scrollView, attribute: .bottom, multiplier: 1.0, constant: 0)
Now add your subviews constraints (labels, images) with respect to the contentView
For example- You received your first image, so we will maintain an array of UIImageViews outside your function.
var imageViews = [UIImageViews]() //declared outside the function
//received an image here
var imageView = UIImageView() // define the frame according to yourself using frame init method
imageView.image = image
if imageViews.isEmpty { // first image view
//add constraints to first image view
let x = NSLayoutConstraint(item: imageView, attribute: .centerX, relatedBy: .equal, toItem: contentView, attribute: .centerX, multiplier: 1.0, constant: 0)
let y = NSLayoutConstraint(item: imageView, attribute: .top, relatedBy: .equal, toItem: contentView, attribute: .top, multiplier: 1.0, constant: 30)
let width = NSLayoutConstraint(item: imageView, attribute: .width, relatedBy: .equal, toItem: self.contentView, attribute: .width, multiplier: 1.0, constant: 0)
let height = NSLayoutConstraint(item: imageView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 150)
}
else { //second, third, fourth image view... so on
let x = NSLayoutConstraint(item: imageView, attribute: .centerX, relatedBy: .equal, toItem: contentView, attribute: .centerX, multiplier: 1.0, constant: 0)
let y = NSLayoutConstraint(item: imageView, attribute: .top, relatedBy: .equal, toItem: imageViews[imageViews.count - 1], attribute: .bottom, multiplier: 1.0, constant: 30)
let width = NSLayoutConstraint(item: imageView, attribute: .width, relatedBy: .equal, toItem: self.contentView, attribute: .width, multiplier: 1.0, constant: 0)
let height = NSLayoutConstraint(item: imageView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 150)
}
imageViews.append(imageView)
}
Hope you got an idea now, how to proceed with this problem. If having more than 4 or 5 imageviews, you'll probably want to check the count of the array and increase the contentView of the scrollView accordingly. you can do so by using
self.scrollView.contentSize = CGSize(width, height)

Answer 2

I believe you have a problem here:
let y = NSLayoutConstraint(item: imageView, attribute: .top, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 30)
"toItem" parameter should have some value. Also first parameter should be imageView.topAnchor. Probably it should look something like this:
let y = NSLayoutConstraint(item: imageView.topAnchor, attribute: .top, relatedBy: .equal, toItem: self.contentView.topAnchor, attribute: .notAnAttribute, multiplier: 1.0, constant: 30)