systemLayoutSizeFitting总是返回零

Question

Based on Apple's doc , systemLayoutSizeFitting is supposed to respect current constraints on the UIView element when returning the optimal size. However, whenever I run following code, I would get {0, 0} for UIView.layoutFittingCompressedSize and {1000, 1000} for UIView.layoutFittingExpandedSizeSize input.

let mainView = UIView(frame: CGRect(origin: .zero, size: CGSize(width: 375, height: 50)))
mainView.backgroundColor = .red
PlaygroundPage.current.liveView = mainView

let subview = UIView()
subview.backgroundColor = .yellow
mainView.addSubview(subview)
subview.snp.makeConstraints { make in
    make.width.equalToSuperview().dividedBy(3.0)
    make.left.top.bottom.equalToSuperview()
}
mainView.setNeedsLayout()
mainView.layoutIfNeeded()

subview.frame

subview.systemLayoutSizeFitting(UIView.layoutFittingCompressedSize)

I noticed that if I change the width constraint to something constants, then I'd get a valid value from systemLayoutSizeFitting . Trying to understand why such behavior is happening and if it's possible to get a right value from systemLayoutSizeFittingSize(_ size: CGSize) .

Answer 1

Documentation seems rather lacking on this one.

It appears that .systemLayoutSizeFitting is highly dependent on the .intrinsicContentSize of the element. In the case of a UIView , it has no intrinsic content size (unless you've overridden it).

So, if the related constraint is a percentage of another constraint, .systemLayoutSizeFitting(UIView.layoutFittingCompressedSize) will return {0, 0} . I gather this is because the related constraint could change (to Zero), therefor the minimum value is, in fact, Zero.

If you change your .width constraint to a constant (such as mainView.frame.width * 0.3333 ) then you'll get a valid size value, as the constant width constraint becomes the intrinsic width.

If your subview is a UILabel , for example, that element will have an intrinsic size, and .systemLayoutSizeFitting should return a size value that you'd expect.

Here's an example using a UILabel that will demonstrate:

import UIKit
import PlaygroundSupport

let mainView = UIView(frame: CGRect(origin: .zero, size: CGSize(width: 375, height: 50)))
mainView.backgroundColor = .red
PlaygroundPage.current.liveView = mainView

let v = UILabel()
v.text = "Testing"
v.translatesAutoresizingMaskIntoConstraints = false
v.backgroundColor = .green
mainView.addSubview(v)

NSLayoutConstraint.activate([
    v.widthAnchor.constraint(equalTo: mainView.widthAnchor, multiplier: 3.0 / 10.0),
    v.leftAnchor.constraint(equalTo: mainView.leftAnchor),
    v.topAnchor.constraint(equalTo: mainView.topAnchor),
    v.bottomAnchor.constraint(equalTo: mainView.bottomAnchor),
    ])

mainView.setNeedsLayout()
mainView.layoutIfNeeded()

v.frame

v.systemLayoutSizeFitting(UIView.layoutFittingCompressedSize)