How do you change and animate constraints: isActive, constant, layoutIfNeeded?
The gist
There are three ways to change layout at runtime, from cheap to expensive: change the constant of a stored constraint, swap the priorities of two optional constraints, or deactivate one set and activate another. Recreating constraints on every change, especially in layoutSubviews, is an anti-pattern: it is constraint churn and extra work for the engine.
The animation is the same in all three cases: flush pending layout, change the constraints, and call layoutIfNeeded() on the common ancestor inside the animation block. What animates is the result of the layout pass, not the constraint itself.
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