Swift: I'm not able to make autolayout constraints work for my UIScrollView

Question

I'm trying to make a simple layout that should scroll vertically when views are too many to fit on the screen.

So this is what I have so far:

I created a scrollview and a container view like that

let mainScrollView: UIScrollView = {

    let scrollView = UIScrollView()

    scrollView.isUserInteractionEnabled = true
    scrollView.translatesAutoresizingMaskIntoConstraints = false

    return scrollView
}()

let containerView: UIView = {

    let view = UIView()

    view.backgroundColor = .lightGray
    view.translatesAutoresizingMaskIntoConstraints = false

    return view
}()

Then I added the scrollview to the main view, added the container view to the scrollview and a couple of labels to the container view

view.addSubview(mainScrollView)
mainScrollView.addSubview(containerView)

containerView.addSubview(firstLabel)
containerView.addSubview(secondLabel)

I pinned the scrollview to the main view, and the container view to the scrollview. After that I started adding the labels inside the container view like this

mainScrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
mainScrollView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
mainScrollView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
mainScrollView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true

containerView.topAnchor.constraint(equalTo: mainScrollView.layoutMarginsGuide.topAnchor).isActive = true
containerView.bottomAnchor.constraint(equalTo: mainScrollView.layoutMarginsGuide.bottomAnchor).isActive = true
containerView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
containerView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true

firstLabel.centerXAnchor.constraint(equalTo: containerView.centerXAnchor).isActive = true
firstLabel.topAnchor.constraint(equalTo: containerView.topAnchor, constant: 30).isActive = true
firstLabel.widthAnchor.constraint(equalToConstant: 50).isActive = true
firstLabel.heightAnchor.constraint(equalToConstant: 20).isActive = true

secondLabel.centerXAnchor.constraint(equalTo: containerView.centerXAnchor).isActive = true
secondLabel.topAnchor.constraint(equalTo: firstLabel.bottomAnchor, constant: 750).isActive = true
secondLabel.widthAnchor.constraint(equalToConstant: 50).isActive = true
secondLabel.heightAnchor.constraint(equalToConstant: 20).isActive = true

The problem is that for some reason the scrollview is not able to calculate it's height and it doesn't scroll vertically. The second label remains invisible because it's too "low" on the screen.

I tried setting the bottom constraint of the container view to the bottom of the second label but I'm going nowhere honestly.

What am I missing? How can I set constraints to make the scrollview scroll with autolayout (without setting a specific height of the container view)?

Answer 1

Scrollview scrolls in either direction. So any view added to it would need some additional constraints to let the Scrollview calculate its content size.

You have pinned the container view to scroll view. If you are looking at a vertically scrolling view like a table view then you need to also set the width of the scroll to the container view so that it can calculate the width.

Next for height, its automatically calculated based on the UI elements added in the container view. Ensure that all of the labels have leading, trailing to the container and vertical spacing to each other and top of the container view. This will let the scroll view know the height needed.

    self.scrollView.translatesAutoresizingMaskIntoConstraints = false
    self.contentView.translatesAutoresizingMaskIntoConstraints = false
    self.label1.translatesAutoresizingMaskIntoConstraints = false
    self.label2.translatesAutoresizingMaskIntoConstraints = false
    self.label3.translatesAutoresizingMaskIntoConstraints = false

    self.contentView.addSubview(self.label1)
    self.contentView.addSubview(self.label2)
    self.contentView.addSubview(self.label3)

    self.scrollView.addSubview(self.contentView)
    self.view.addSubview(self.scrollView)

    NSLayoutConstraint.activate([
        self.scrollView.topAnchor.constraint(equalTo: self.view.topAnchor),
        self.scrollView.trailingAnchor.constraint(equalTo: self.view.trailingAnchor),
        self.scrollView.bottomAnchor.constraint(equalTo: self.view.bottomAnchor),
        self.scrollView.leadingAnchor.constraint(equalTo: self.view.leadingAnchor)
    ])

    NSLayoutConstraint.activate([
        self.contentView.topAnchor.constraint(equalTo: self.scrollView.topAnchor),
        self.contentView.trailingAnchor.constraint(equalTo: self.scrollView.trailingAnchor),
        self.contentView.bottomAnchor.constraint(equalTo: self.scrollView.bottomAnchor),
        self.contentView.leadingAnchor.constraint(equalTo: self.scrollView.leadingAnchor),
        self.contentView.widthAnchor.constraint(equalTo: self.scrollView.widthAnchor)
    ])

    NSLayoutConstraint.activate([
        self.label1.leadingAnchor.constraint(equalTo: self.contentView.leadingAnchor),
        self.label2.leadingAnchor.constraint(equalTo: self.contentView.leadingAnchor),
        self.label3.leadingAnchor.constraint(equalTo: self.contentView.leadingAnchor),
        self.label1.trailingAnchor.constraint(equalTo: self.contentView.trailingAnchor),
        self.label2.trailingAnchor.constraint(equalTo: self.contentView.trailingAnchor),
        self.label3.trailingAnchor.constraint(equalTo: self.contentView.trailingAnchor)
    ])

    NSLayoutConstraint.activate([
        self.label1.topAnchor.constraint(equalTo: self.contentView.topAnchor),
        self.label2.topAnchor.constraint(equalTo: self.label1.bottomAnchor),
        self.label3.topAnchor.constraint(equalTo: self.label2.bottomAnchor),
        self.label3.bottomAnchor.constraint(equalTo: self.contentView.bottomAnchor)
    ])

And when run, it won't complain about any constraint issues either. Same can be done for horizontal scroll.

Answer 2

Thank you very much @GoodSp33d, you pointed me to the right direction.

I added the width constraint as you said like that:

containerView.widthAnchor.constraint(equalTo: mainScrollView.widthAnchor).isActive = true

And the key point was to update the bottom constraint after all:

containerView.bottomAnchor.constraint(equalTo: secondLabel.bottomAnchor).isActive = true

Here is the full working code:

mainScrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
mainScrollView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
mainScrollView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
mainScrollView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true

containerView.topAnchor.constraint(equalTo: mainScrollView.topAnchor).isActive = true
containerView.trailingAnchor.constraint(equalTo: mainScrollView.trailingAnchor).isActive = true
containerView.bottomAnchor.constraint(equalTo: mainScrollView.bottomAnchor).isActive = true
containerView.leadingAnchor.constraint(equalTo: mainScrollView.leadingAnchor).isActive = true
containerView.widthAnchor.constraint(equalTo: mainScrollView.widthAnchor).isActive = true

firstLabel.centerXAnchor.constraint(equalTo: containerView.centerXAnchor).isActive = true
firstLabel.topAnchor.constraint(equalTo: containerView.topAnchor, constant: 30).isActive = true
firstLabel.widthAnchor.constraint(equalToConstant: 50).isActive = true
firstLabel.heightAnchor.constraint(equalToConstant: 20).isActive = true

secondLabel.centerXAnchor.constraint(equalTo: containerView.centerXAnchor).isActive = true
secondLabel.topAnchor.constraint(equalTo: firstLabel.bottomAnchor, constant: 550).isActive = true
secondLabel.widthAnchor.constraint(equalToConstant: 50).isActive = true
secondLabel.heightAnchor.constraint(equalToConstant: 20).isActive = true

containerView.bottomAnchor.constraint(equalTo: secondLabel.bottomAnchor).isActive = true

Thanks for the help.