As Element Call grows in complexity, it has become a pain point that our business logic remains so tightly coupled to the UI code. In particular, this has made testing difficult, and the complex semantics of React hooks are not a great match for arbitrary business logic. Here, I show the beginnings of what it would look like for us to adopt the MVVM pattern. I've created a CallViewModel and TileViewModel that expose their state to the UI as rxjs Observables, as well as a couple of helper functions for consuming view models in React code.
This should contain no user-visible changes, but we need to watch out for regressions particularly around focus switching and promotion of speakers, because this was the logic I chose to refactor first.
reorderTiles was programmed to only place a tile in the speaker section if that tile's previous position was off-screen. But for speakers that started off-screen, this would cause them to oscillate in and out of the speaker section on each render, because the speaker section is, of course, on-screen. The solution I've gone with here is to avoid referencing the previous position, and instead go by the computed natural ordering, which ought to be more stable.
