IssueTracker

PerformanceMetricsState.getForHierarchy returns a MetricsStateHolder which is just a wrapper around PerformanceMetricsState?.

As I started using it, I did this:

// Named it "state" because I'm calling "get" on something a class which as a name ending in State.
val state = PerformanceMetricsState.getForHierarchy(view)
state.state?.let { state ->
 state.addState("foo", "bar")
}

So.. I'm getting a state, then getting a state from it or maybe not, and if I do have a state then I can add a state to it!

The other thing that's a bit weird about this is how PerformanceMetricsState is a separate class but it's directly coupled to JankState, i.e. MetricsStateHolder.state will always be set if I already called JankState.createAndTrack() for that window, but it'll be null if I didn't. This conditional nullability isn't super clear.

Suggestions:

1. Consider hiding MetricsStateHolder and having getForHierarchy directly return PerformanceMetricsState?
2. Consider a name changes so that we're not adding "state" (addState) to an object already named "state" (PerformanceMetricsState).
3. Consider either breaking the coupling entirely or making the coupling more explicit. Breaking the coupling would mean deleting the PerformanceMetricsState.create() function that JankStat is calling, and having MetricsStateHolder.getForHierarchy() automatically create it on first call. Making the coupling more explicit might mean making a PerformanceMetricsState instance directly available on a JankState instance.