refactor(workspace): split workspace interface and implementation (#5463)

@affine/workspace -> (@affine/workspace, @affine/workspace-impl)

packages/frontend/workspace-impl/src/local/awareness.ts

@@ -0,0 +1,62 @@
+import type { AwarenessProvider } from '@affine/workspace';
+import type { Awareness } from 'y-protocols/awareness.js';
+import {
+  applyAwarenessUpdate,
+  encodeAwarenessUpdate,
+} from 'y-protocols/awareness.js';
+
+type AwarenessChanges = Record<'added' | 'updated' | 'removed', number[]>;
+
+type ChannelMessage =
+  | { type: 'connect' }
+  | { type: 'update'; update: Uint8Array };
+
+export function createBroadcastChannelAwarenessProvider(
+  workspaceId: string,
+  awareness: Awareness
+): AwarenessProvider {
+  const channel = new BroadcastChannel('awareness:' + workspaceId);
+
+  function handleAwarenessUpdate(changes: AwarenessChanges, origin: unknown) {
+    if (origin === 'remote') {
+      return;
+    }
+
+    const changedClients = Object.values(changes).reduce((res, cur) =>
+      res.concat(cur)
+    );
+
+    const update = encodeAwarenessUpdate(awareness, changedClients);
+    channel.postMessage({
+      type: 'update',
+      update: update,
+    } satisfies ChannelMessage);
+  }
+
+  function handleChannelMessage(event: MessageEvent<ChannelMessage>) {
+    if (event.data.type === 'update') {
+      const update = event.data.update;
+      applyAwarenessUpdate(awareness, update, 'remote');
+    }
+    if (event.data.type === 'connect') {
+      channel.postMessage({
+        type: 'update',
+        update: encodeAwarenessUpdate(awareness, [awareness.clientID]),
+      } satisfies ChannelMessage);
+    }
+  }
+
+  return {
+    connect() {
+      channel.postMessage({
+        type: 'connect',
+      } satisfies ChannelMessage);
+      awareness.on('update', handleAwarenessUpdate);
+      channel.addEventListener('message', handleChannelMessage);
+    },
+    disconnect() {
+      awareness.off('update', handleAwarenessUpdate);
+      channel.removeEventListener('message', handleChannelMessage);
+    },
+  };
+}