Adding Constraint messed up view - swift

Question

I have a view with a UICollectionView and a UISegmentedControl.
I want to change constraints so the segment controller won't overlap collection view, like in this picture:
This is my code :
override func viewDidLoad()
{
super.viewDidLoad()
self.navigationItem.leftBarButtonItem = nil
self.tabBarController?.tabBar.isHidden = true
self.SegmentController.setTitle(SegmentAtext, forSegmentAt: 0)
self.SegmentController.setTitle(SegmentBtext, forSegmentAt: 1)
self.view.bringSubview(toFront: SegmentController)
self.LoadProducts(productsToShow: SegmentAtype)
}
SO I add this command:
self.ProductsCollection.topAnchor.constraint(equalTo: SegmentController.bottomAnchor, constant: 10).isActive = true
But the result is worse:
Now the segment controller is almost completely hidden!
How do I fix this?
Edit:
My viewDidLayoutSubviews function:
override func viewDidLayoutSubviews()
{
ProductsCollection.translatesAutoresizingMaskIntoConstraints = false
let topConstraint = NSLayoutConstraint(item: ProductsCollection, attribute: .top, relatedBy: .equal, toItem: self.view, attribute: .top, multiplier: 1, constant: 20)
let bottomConstraint = NSLayoutConstraint(item: ProductsCollection, attribute: .bottom, relatedBy: .equal, toItem: self.view, attribute: .bottom, multiplier: 1, constant: -50) //leaving space for search field
let leadingConstraint = NSLayoutConstraint(item: ProductsCollection, attribute: .leading, relatedBy: .equal, toItem: self.view, attribute: .leading, multiplier: 1, constant: 0)
let trailingConstraint = NSLayoutConstraint(item: ProductsCollection, attribute: .trailing, relatedBy: .equal, toItem: self.view, attribute: .trailing, multiplier: 1, constant: 0)
self.view.addConstraints([topConstraint, bottomConstraint, leadingConstraint, trailingConstraint])
}
Notice:
my viewDidLayoutSubviews is implemented in the super view, which does not contain a UISegmentedControl. the UISegmentedControl is contained in an inheriting view.
Edit: An updated view

Answer 1

If you would want the UISegmentedControl to be centred on the screen and below it to have your collection view you would do
NSLayoutConstraint.activate([
segmentedControl.topAnchor.constraint(equalTo: view.topAnchor),
segmentedControl.centerXAnchor.constraint(equalTo: view.centerXAnchor),
collectionView.topAnchor.constraint(equalTo: segmentedControl.bottomAnchor),
collectionView.leftAnchor.constraint(equalTo: view.leftAnchor),
collectionView.rightAnchor.constraint(equalTo: view.rightAnchor),
collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor)
])
So we have segmented controls top be top of the view, and center it,
then top of collection view is bottom of segmented control (you can add a constant for padding if needed) and left right and bottom to the view