Auto-Layout: "center horizontally" does not seem to resolve inequality ambiguity of width/x-position, within a UICollectionViewCell

Question

Suppose I have a UIView called inner, which is a subview of a UICollectionViewCell's Content View in Interface Builder. The collection view uses flow layout and uses collectionView(_:layout:sizeForItemAt:) to determine the size of the cell at layout time.

I want inner to take up the entire width of the cell, but if the cell's width is less than 6 then inner should be centered horizontally in the cell and have a width of 6 (clipsToBounds is false on the cell and the cell's Content View).

The horizontal constraints below "should" be sufficient to do this, but Interface Builder shows me two errors:

• Missing Constraints: Need constraints for: X position, width
• Inequality Constraint Ambiguity: inner.leading <= leading

Proposed horizontal constraints:

1. inner.centerX = centerX
2. inner.trailing >= trailing
3. inner.leading <= leading
4. inner.width >= 6

Why isn't this enough? Does it have something to do with the sizeForItemAt:?

Answer 1

Interface Builder's complaints might be spurious. It's better to run the app and see what happens in the View Debugger. I was able to achieve your goal like this:

• inner leading <= superview leading

• inner trailing >= superview trailing

• inner centerX == superview centerX

• inner width == 6 at a priority of 999

The results look right. The view debugger didn't complain, so I presume my settings are valid.