Autolayout without intrinsic size - uiview

Question

Can anyone tell me how Autolayout knows the size of a UIView which does not have an intrinsic size and only one dimension has been constrained?
For example, I have a plain view containing a set of subviews. The view is constrained to the leading and trailing margins of the parent and centered vertically. Autolayout is able to ask the view for its height for a given width but I don't know how. I need to layout the subviews by hand but I cannot see which function/property I need to implement so Autolayout can tell me the width and ask for the corresponding height.
Any one know?

Answer 1

As a general rule, the more complex the layout, the more reason to use auto-layout.
However, if you really want to "hand layout" your subviews and calculate the intrinsic size, you can do it by overriding intrinsicContentSize.
So, for example, if you calculate your subviews sizes / positions in layoutSubviews(), you should also calculate the height there, call invalidateIntrinsicContentSize(), and return your calculated height.
Here's a quick example. We add two subviews, one above the other. We give the top view a 4:1 ratio and the bottom view a 3:1 ratio.
class MyIntrinsicView: UIView {
var myHeight: CGFloat = 0
let view1: UIView = {
let v = UIView()
v.backgroundColor = .red
return v
}()
let view2: UIView = {
let v = UIView()
v.backgroundColor = .green
return v
}()
override init(frame: CGRect) {
super.init(frame: frame)
commonInit()
}
required init?(coder: NSCoder) {
super.init(coder: coder)
commonInit()
}
func commonInit() -> Void {
addSubview(view1)
addSubview(view2)
backgroundColor = .blue
}
override var intrinsicContentSize: CGSize {
var sz = super.intrinsicContentSize
sz.height = myHeight
return sz
}
override func layoutSubviews() {
super.layoutSubviews()
// let's say view 1 needs a 4:1 ratio and
// view 2 needs a 3:1 ratio
view1.frame = CGRect(x: 0, y: 0, width: bounds.width, height: bounds.width / 4.0)
view2.frame = CGRect(x: 0, y: view1.frame.maxY, width: bounds.width, height: bounds.width / 3.0)
myHeight = view1.frame.height + view2.frame.height
invalidateIntrinsicContentSize()
}
}
Using this example view controller, where we give the view 60-pts leading and trailing, and only centerY constraints:
class QuickExampleViewController: UIViewController {
let testView = MyIntrinsicView()
override func viewDidLoad() {
super.viewDidLoad()
testView.translatesAutoresizingMaskIntoConstraints = false
view.addSubview(testView)
let g = view.safeAreaLayoutGuide
NSLayoutConstraint.activate([
testView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 60.0),
testView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -60.0),
testView.centerYAnchor.constraint(equalTo: g.centerYAnchor),
])
}
}
We get this output: