Should classes be overridden in transitions?

[anselmbradford]

Transitions are composited with BaseTransition so that each specific transition has a set of CSS classes, that are then passed into the BaseTransition, which handles adding and removing them when the transition completes, etc. So, for example, MoveTransition has classes such as .u-move-left, which defines the CSS to transition to from the current state.

I noticed ExpandableTransition allows overriding of its transition classes.

I would argue that the classes in a transition should be immutable and the only ones that are overridable are the ones in BaseTransition, which should never be directly instantiated. This would be so that the utility classes associated with a particular transition are always the same. In the case of u-expandable-*, when would those classes ever be used and not overridden? It appears they would always be replaced since there isn't an implementation of u-expandable-* that has CSS defining the states of the transition. A setup may be to have a MaxHeightTransition (since this is what Expandables transition, which separates it from a MoveTransition, which works on transforms), and if custom classes are needed on top of that to create an ExpandableTransition inside the cf-expandable module (not here) that composites on top of the MaxHeightTransition and applies the o-expandable_* classes.