How to create a custom UIView that defines a baseline?

From the UIView docs:

viewForBaselineLayout

Returns a view used to satisfy baseline constraints.

- (UIView *)viewForBaselineLayout

Return Value

The view the constraint system should use to satisfy baseline constraints

I've just used Hopper disassembler to look at UILabel in iOS 8.1, and it implements methods _baselineOffsetFromBottom and _firstBaselineOffsetFromTop, which are in turn called by -[UIView nsli_lowerAttribute:intoExpression:withCoefficient:forConstraint:], so UIKit has private methods similar to the one OS X has, they are just not exposed to the public.

_baselineOffsetFromBottom and _firstBaselineOffsetFromTop are implemented by UIView (returning 0), UILabel and UITextView. There's also a class named _UIGlintyStringView that implements _baselineOffsetFromBottom; no other UIKit classes have these methods.

Btw when the baseline of a view changes, it performs something like:

__UITagLayoutConstraintsForConstantChangeForSelectedAttributes(self, __UILayoutAttributeIsBaselineAttribute)

Doesn't seem like there's anything extra-special that couldn't be exposed to the public; perhaps they didn't feel like there's a need or wanted to discourage people from writing their own labels. UILabel is a fairly complicated class involving a custom CoreAnimation layer (_UILabelLayer) and a whole lot of trickery, including quite a bit of code for accessibility support.