Integrate Document Overlay with the SDK

packages/firestore/src/local/document_overlay_cache.ts

@@ -43,6 +43,15 @@ export interface DocumentOverlayCache {
     key: DocumentKey
   ): PersistencePromise<Overlay | null>;
 
+  /**
+   * Gets the saved overlay mutation for the given document keys. Skips keys for
+   * which there are no overlays.
+   */
+  getOverlays(
+    transaction: PersistenceTransaction,
+    keys: DocumentKeySet
+  ): PersistencePromise<OverlayMap>;
+
   /**
    * Saves the given document mutation map to persistence as overlays.
    * All overlays will have their largest batch id set to `largestBatchId`.

packages/firestore/src/local/indexeddb_document_overlay_cache.ts

@@ -81,6 +81,24 @@ export class IndexedDbDocumentOverlayCache implements DocumentOverlayCache {
       });
   }
 
+  getOverlays(
+    transaction: PersistenceTransaction,
+    keys: DocumentKeySet
+  ): PersistencePromise<OverlayMap> {
+    const result = newOverlayMap();
+    const promises: Array<PersistencePromise<void>> = [];
+    keys.forEach(key => {
+      promises.push(
+        this.getOverlay(transaction, key).next(overlay => {
+          if (overlay !== null) {
+            result.set(key, overlay);
+          }
+        })
+      );
+    });
+    return PersistencePromise.waitFor(promises).next(() => result);
+  }
+
   saveOverlays(
     transaction: PersistenceTransaction,
     largestBatchId: number,