macOS NSView - How to give dynamic margin to left and right sides - swift

Question

I have two views A(View Controller's view), B(is a NSView and subview of A). View B pinned to top, trailing, bottom and leading of view A. When I drag window view B grows according to auto layout constrains. Which is perfectly fine. At one point I want view B to stop growing and provide margin at leading, trailing between view A and B.
I did play around widthAnchor, so the margin is grows only at right(trailing) side and which is obvious. How can I give equal margin to left(leading) side as well.
B.widthAnchor.constraint(lessThanOrEqualToConstant: 1000)
Code:
override func viewDidLoad() {
super.viewDidLoad()
view.translatesAutoresizingMaskIntoConstraints = false
let B = CustomNSView(frame: .zero)
B.translatesAutoresizingMaskIntoConstraints = false
//detailView.autoresizingMask = [.width, .maxXMargin, .maxYMargin]
// detailView.frame.size.width = 600
view.addSubview(B)
NSLayoutConstraint.activate([
B.topAnchor.constraint(equalTo: view.topAnchor),
//B.trailingAnchor.constraint(equalTo: view.trailingAnchor),
//B.widthAnchor.constraint(lessThanOrEqualToConstant: 1000),
B.bottomAnchor.constraint(equalTo: view.bottomAnchor),
B.leadingAnchor.constraint(equalTo: view.leadingAnchor),
])
}
Appreciate your inputs,
Thanks.

Answer 1

One way would be to save the leading and trailing layout constraints and then change their constants when the view exceeds B's max width:
var bLeading: NSLayoutConstraint?
var bTrailing: NSLayoutConstraint?
var bMaxWidth: CGFloat = 1000
override func viewDidLoad() {
super.viewDidLoad()
let b = CustomNSView()
view.addSubview(b)
b.translatesAutoresizingMaskIntoConstraints = false
b.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
b.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
bLeading = b.leadingAnchor.constraint(equalTo: view.leadingAnchor)
bTrailing = b.trailingAnchor.constraint(equalTo: view.trailingAnchor)
bLeading?.isActive = true
bTrailing?.isActive = true
}
override func viewWillLayout() {
let margin = view.bounds.width - bMaxWidth
if margin > 0 {
bLeading?.constant = margin
bTrailing?.constant = -margin
} else {
bLeading?.constant = 0
bTrailing?.constant = 0
}
}