Add screensharing behind a feature flag

ts/hooks/useActivateSpeakerViewOnPresenting.ts

@@ -0,0 +1,29 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { useEffect } from 'react';
+import { usePrevious } from '../util/hooks';
+
+type RemoteParticipant = {
+  hasRemoteVideo: boolean;
+  presenting: boolean;
+  title: string;
+  uuid?: string;
+};
+
+export function useActivateSpeakerViewOnPresenting(
+  remoteParticipants: ReadonlyArray<RemoteParticipant>,
+  isInSpeakerView: boolean,
+  toggleSpeakerView: () => void
+): void {
+  const presenterUuid = remoteParticipants.find(
+    participant => participant.presenting
+  )?.uuid;
+  const prevPresenterUuid = usePrevious(presenterUuid, presenterUuid);
+
+  useEffect(() => {
+    if (prevPresenterUuid !== presenterUuid && !isInSpeakerView) {
+      toggleSpeakerView();
+    }
+  }, [isInSpeakerView, presenterUuid, prevPresenterUuid, toggleSpeakerView]);
+}