Start refactoring some business logic into view models

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.

src/state/TileViewModel.ts

@@ -0,0 +1,53 @@
+/*
+Copyright 2023 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { LocalParticipant, RemoteParticipant } from "livekit-client";
+import { RoomMember } from "matrix-js-sdk/src/matrix";
+
+export abstract class TileViewModel {
+  // TODO: Properly separate the data layer from the UI layer by keeping the
+  // member and LiveKit participant objects internal. The only LiveKit-specific
+  // thing we need to expose here is a TrackReference for the video, everything
+  // else should be simple strings, flags, and callbacks.
+  public abstract readonly id: string;
+  public abstract readonly member: RoomMember | undefined;
+  public abstract readonly sfuParticipant: LocalParticipant | RemoteParticipant;
+}
+
+// Right now it looks kind of pointless to have user media and screen share be
+// represented by two classes rather than a single flag, but this will come in
+// handy when we go to move more business logic out of VideoTile and into this
+// file
+
+export class UserMediaTileViewModel extends TileViewModel {
+  public constructor(
+    public readonly id: string,
+    public readonly member: RoomMember | undefined,
+    public readonly sfuParticipant: LocalParticipant | RemoteParticipant,
+  ) {
+    super();
+  }
+}
+
+export class ScreenShareTileViewModel extends TileViewModel {
+  public constructor(
+    public readonly id: string,
+    public readonly member: RoomMember | undefined,
+    public readonly sfuParticipant: LocalParticipant | RemoteParticipant,
+  ) {
+    super();
+  }
+}