Introduction to UIStackView.Distribution.fillProportionally property

I've developed a pet project called MyCalculator, and the image below shows how its interface looked like.

I used stackviews to lay out the buttons and labels in rows. I used a root stackview, which contains two sub stackviews: one contains the labels to display the current operation and its result and another one contains all the digit and operator buttons.

First, I set the Distribution property of the root stackview to Fill Proportionally and defined a constraint to make the height of its first sub stackview proportionally smaller than its second sub stackview. Their height ratio is 2:6. I did it by Control+click the first stackview and drag to the second stackview then select Equal Heights from the popup menu. After that, I changed the Modifier in the constraint from 1 to 2:6.

Second, I set the Distribution property of the first stackview to Fill Proportionally because it contains two labels, one to display the entering operation and another one to display its result. The entering-operation label should be smaller than the result label. The system will display them based on their intrinsic content sizes (different font sizes, different intrinsic content sizes).

Third, I set the Distribution property of the second stackview to Fill Equally so that the digit and operator buttons have equal sizes. When using Fill Equally distribution, the system does not need to know the intrinsic content size of the stackview's contents. It can calculate the stackview's size based on the device screen's size.

Problem 1

The above solution worked well until I added a scroll view (UIScrollView) for the entering-operation label so that it's scrollable. But, the entering-operation label disappeared when I run the project.

   

 The reason that the label didn't show up is because the scroll view does not have intrinsic content size so the parent stackview can't calculate its height. I even switched the scroll view with a view (UIView object) and it still didn't work because the UIView object does not have intrinsic content size either. However, it worked when I replaced the scroll view with a stackview. It seems like the parent stackview can guess the inner stack view's size base on the size of its contents.

Then, I created a custom scroll view and override the intrinsicContentSize variable to return a value as following:

import Foundation

import UIKit

public class CustomScrollView : UIScrollView {

    

    @IBInspectable

    var height: CGFloat = 40.0

    

    public override var intrinsicContentSize : CGSize {

        return CGSize(width: 100, height: height)

    }

}

And it worked. However, the height of the entering-operation label is fixed to 40. I want it to dynamically scale according to the screen size so I added a constraint to define its height proportional to the result label's in the ratio of 2:4. Its height then scale dynamically according to the screen's size but the second sub stack view (we talked about above) stretched to the entire screen and overlapped it.

I don't know why. Logically, it should have worked because it did when there were only the two labels without the scroll view. I feel like the system didn't consider the first sub stackview when calculating the size of the second sub stackview.

What's interesting is the second sub stackview no longer overlapped the first sub stackview when I moved it up above the first sub stackview.

My assumption is the system calculated the size of the first sub stackview before the size of the second sub stackview as it found the first sub stack view from the array first. Then, the first sub stack view's size is included in the calculation of the second sub stackview's size.

But, why it worked when I used a stackview instead of a scroll view even if I didn't move the second sub stackview up, as I tested above? Perhaps, it was coded that way for the stackview in the UIKit. It tries to calculate the size of multiple hierarchical level stackview only when each container view is of type UIStackView.

Finally, I found a workaround. I created CustomStackView class by overriding the intrinsicContentSize variable to return any value similar to the CustomScrollView class above and then I modified the first sub stackview to be of type CustomStackView.