Proper way to set UIScrollView with constraints programmatically? - ios

Question

I'm trying to create a simple horizontal scroll between two view controllers positioned side by side programmatically using constraints. What I have here appears to work but everything I've read says that I don't need to configure the size of the contentSize if the constraints are set correctly--autolayout will do that for me. But when I remove scrollView.contentSize = CGSize(width: view.bounds.width * 2, height: view.bounds.height) from viewWillLayoutSubviews, it doesn't scroll. Where did I go wrong?
class ViewController0: UIViewController {
let scrollView = UIScrollView()
let contentView = UIView()
let page1 = ViewController1()
let page2 = ViewController2()
override func loadView() {
setView()
addScrollView()
fillScrollView()
}
override func viewDidLoad() {
super.viewDidLoad()
}
override func viewWillLayoutSubviews() {
super.viewWillLayoutSubviews()
scrollView.contentSize = CGSize(width: view.bounds.width * 2, height: view.bounds.height)
}
func setView() {
view = UIView()
view.frame = UIScreen.main.bounds
view.backgroundColor = UIColor.blue
}
func addScrollView() {
scrollView.bounces = false
scrollView.isPagingEnabled = true
scrollView.backgroundColor = UIColor.brown
scrollView.translatesAutoresizingMaskIntoConstraints = false
view.addSubview(scrollView)
scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
scrollView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
scrollView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
scrollView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
contentView.backgroundColor = UIColor.green
contentView.translatesAutoresizingMaskIntoConstraints = false
scrollView.addSubview(contentView)
contentView.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor).isActive = true
contentView.topAnchor.constraint(equalTo: scrollView.topAnchor).isActive = true
contentView.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 1).isActive = true
contentView.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 2).isActive = true
}
func fillScrollView() {
addChildViewController(page1)
page1.didMove(toParentViewController: self)
page1.view.translatesAutoresizingMaskIntoConstraints = false
contentView.addSubview(page1.view)
page1.view.leadingAnchor.constraint(equalTo: contentView.leadingAnchor).isActive = true
page1.view.topAnchor.constraint(equalTo: contentView.topAnchor).isActive = true
page1.view.bottomAnchor.constraint(equalTo: contentView.bottomAnchor).isActive = true
page1.view.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 1).isActive = true
addChildViewController(page2)
page2.didMove(toParentViewController: self)
page2.view.translatesAutoresizingMaskIntoConstraints = false
contentView.addSubview(page2.view)
page2.view.leadingAnchor.constraint(equalTo: page1.view.trailingAnchor).isActive = true
page2.view.topAnchor.constraint(equalTo: contentView.topAnchor).isActive = true
page2.view.bottomAnchor.constraint(equalTo: contentView.bottomAnchor).isActive = true
page2.view.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 1).isActive = true
}
}

Answer 1

this should work
class ViewController0: UIViewController {
let scrollView = UIScrollView()
let page1 = ViewController1()
let page2 = ViewController1()
override func loadView() {
setView()
addScrollView()
setupPage()
}
func setView() {
view = UIView()
view.frame = UIScreen.main.bounds
view.backgroundColor = UIColor.blue
}
func addScrollView() {
scrollView.backgroundColor = UIColor.brown
scrollView.translatesAutoresizingMaskIntoConstraints = false
view.addSubview(scrollView)
scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
scrollView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
scrollView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
scrollView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
}
func setupPage() {
page1.view.translatesAutoresizingMaskIntoConstraints = false
scrollView.addSubview(page1.view)
addChildViewController(page1)
page1.didMove(toParentViewController: self)
page2.view.translatesAutoresizingMaskIntoConstraints = false
scrollView.addSubview(page2.view)
addChildViewController(page2)
page2.didMove(toParentViewController: self)
let views: [String: UIView] = ["view": view, "page1": page1.view, "page2": page2.view]
let verticalConstraints = NSLayoutConstraint.constraints(withVisualFormat: "V:|[page1(==view)]|", options: [], metrics: nil, views: views)
let horizontalConstraints = NSLayoutConstraint.constraints(withVisualFormat: "H:|[page1(==view)][page2(==view)]|", options: [.alignAllTop, .alignAllBottom], metrics: nil, views: views)
NSLayoutConstraint.activate(verticalConstraints + horizontalConstraints)
}
}