trait collections and ListSectionControllers

Has anyone done anything with changing trait collections (eg iPad splitviews) and section controllers and maybe formed some best practices or neat solutions around it?

In particular, I often need to update the minimum{Interitem,Line}Spacing values depending on the trait collection. So I made the following protocol:

```swift
protocol TraitCollectionAdaptable {
func traitCollectionDidChange(from previousTraitCollection: UITraitCollection?, to traitCollection: UITraitCollection)
}

extension ListAdapter {
// Call this from UIViewController.traitCollectionDidChange to propagate changes to supporting section controllers
func updateTraitCollectionsForVisibleSectionControllers(from previousTraitCollection: UITraitCollection?, to traitCollection: UITraitCollection) {
for controller in visibleSectionControllers().flatMap({ $0 as? TraitCollectionAdaptable }) {
controller.traitCollectionDidChange(from: previousTraitCollection, to: traitCollection)
}
}
}
```

I could make the IGListSectionControllers conform to `UITraitEnvironment`, but then I have to support a `traitCollection` property. Obviously, the downside to this approach is that you have to remember to call the update method in order to inform the section controllers that things have changed.

Does anyone have a better approach here?