RTC: Add session activity notifications

Collaborators had no way to know when others joined, left, or saved
the post. This adds snackbar notices for all four events, broadcast
via a new `lastSaveEvent` field on the Yjs awareness state.

Notification types can be toggled via the
`editor.collaborationNotifications` WordPress filter.

Author: shekharnwagh

packages/core-data/src/awareness/post-editor-awareness.ts

@@ -36,6 +36,10 @@ export class PostEditorAwareness extends BaseAwarenessState< PostEditorState > {
 	protected equalityFieldChecks = {
 		...baseEqualityFieldChecks,
 		editorState: this.areEditorStatesEqual,
+		lastSaveEvent: (
+			a?: PostEditorState[ 'lastSaveEvent' ],
+			b?: PostEditorState[ 'lastSaveEvent' ]
+		) => a?.savedAt === b?.savedAt,
 	};
 
 	public constructor(

packages/core-data/src/awareness/types.ts

@@ -33,12 +33,22 @@ export interface EditorState {
 	selection: SelectionState;
 }
 
+/**
+ * Represents a save event broadcast via the awareness system.
+ * `status` is the WordPress post status at the time of the save.
+ */
+export interface SaveEvent {
+	savedAt: number;
+	status: string;
+}
+
 /**
  * The post editor state extends the base state with information used to render
  * presence indicators in the post editor.
  */
 export interface PostEditorState extends BaseState {
 	editorState?: EditorState;
+	lastSaveEvent?: SaveEvent;
 }
 
 /**

packages/core-data/src/hooks/use-post-editor-awareness-state.ts

@@ -1,7 +1,7 @@
 /**
  * External dependencies
  */
-import { useEffect, useState } from '@wordpress/element';
+import { useCallback, useEffect, useState } from '@wordpress/element';
 
 /**
  * Internal dependencies
@@ -156,3 +156,36 @@ export function useIsDisconnected(
 	return usePostEditorAwarenessState( postId, postType )
 		.isCurrentCollaboratorDisconnected;
 }
+
+/**
+ * Hook that returns a callback to broadcast a save event to all collaborators
+ * via the awareness system. When called, it sets the `lastSaveEvent` field on
+ * the local awareness state so that other collaborators can display a notification.
+ *
+ * @param postId   The ID of the post.
+ * @param postType The type of the post.
+ */
+export function useBroadcastSaveEvent(
+	postId: number | null,
+	postType: string | null
+): ( status: string ) => void {
+	return useCallback(
+		( status: string ) => {
+			if ( null === postId || null === postType ) {
+				return;
+			}
+
+			const awareness =
+				getSyncManager()?.getAwareness< PostEditorAwareness >(
+					`postType/${ postType }`,
+					postId.toString()
+				);
+
+			awareness?.setLocalStateField( 'lastSaveEvent', {
+				savedAt: Date.now(),
+				status,
+			} );
+		},
+		[ postId, postType ]
+	);
+}

packages/core-data/src/private-apis.js

@@ -6,6 +6,7 @@ import { RECEIVE_INTERMEDIATE_RESULTS } from './utils';
 import {
 	useActiveCollaborators,
 	useResolvedSelection,
+	useBroadcastSaveEvent,
 } from './hooks/use-post-editor-awareness-state';
 import { lock } from './lock-unlock';
 
@@ -15,4 +16,5 @@ lock( privateApis, {
 	RECEIVE_INTERMEDIATE_RESULTS,
 	useActiveCollaborators,
 	useResolvedSelection,
+	useBroadcastSaveEvent,
 } );