add whenSynced and isSynced property with refined logic

src/utils/Doc.js

@@ -72,13 +72,57 @@ export class Doc extends Observable {
     this.shouldLoad = shouldLoad
     this.autoLoad = autoLoad
     this.meta = meta
+    /**
+     * This is set to true when the persistence provider loaded the document from the database or when the `sync` event fires.
+     * Note that not all providers implement this feature. Provider authors are encouraged to fire the `load` event when the doc content is loaded from the database.
+     *
+     * @type {boolean}
+     */
     this.isLoaded = false
+    /**
+     * This is set to true when the connection provider has successfully synced with a backend.
+     * Note that when using peer-to-peer providers this event may not provide very useful.
+     * Also note that not all providers implement this feature. Provider authors are encouraged to fire
+     * the `sync` event when the doc has been synced (with `true` as a parameter) or if connection is
+     * lost (with false as a parameter).
+     */
+    this.isSynced = false
+    /**
+     * Promise that resolves once the document has been loaded from a presistence provider.
+     */
     this.whenLoaded = promise.create(resolve => {
       this.on('load', () => {
         this.isLoaded = true
         resolve(this)
       })
     })
+    const provideSyncedPromise = () => promise.create(resolve => {
+      /**
+       * @param {boolean} isSynced
+       */
+      const eventHandler = (isSynced) => {
+        if (isSynced === undefined || isSynced === true) {
+          this.off('sync', eventHandler)
+          resolve()
+        }
+      }
+      this.on('sync', eventHandler)
+    })
+    this.on('sync', isSynced => {
+      if (isSynced === false && this.isSynced) {
+        this.whenSynced = provideSyncedPromise()
+      }
+      this.isSynced = isSynced === undefined || isSynced === true
+      if (!this.isLoaded) {
+        this.emit('load', [])
+      }
+    })
+    /**
+     * Promise that resolves once the document has been synced with a backend.
+     * This promise is recreated when the connection is lost.
+     * Note the documentation about the `isSynced` property.
+     */
+    this.whenSynced = provideSyncedPromise()
   }
 
   /**