handle subscribe result delta flag

src/client.proto.json

@@ -629,6 +629,10 @@
                     "was_recovering": {
                       "type": "bool",
                       "id": 12
+                    },
+                    "delta": {
+                      "type": "bool",
+                      "id": 13
                     }
                   },
                   "reserved": [

src/subscription.ts

@@ -27,6 +27,7 @@ export class Subscription extends (EventEmitter as new () => TypedEventEmitter<S
   private _resubscribeAttempts: number;
   private _promiseId: number;
   private _delta: string;
+  private _delta_negotiated: boolean;
   private _token: string;
   private _data: any | null;
   private _getData: null | ((ctx: SubscriptionDataContext) => Promise<any>);
@@ -62,6 +63,7 @@ export class Subscription extends (EventEmitter as new () => TypedEventEmitter<S
     this._inflight = false;
     this._refreshTimeout = null;
     this._delta = '';
+    this._delta_negotiated = false;
     this._prevValue = null;
     this._setOptions(options);
     // @ts-ignore – we are hiding some symbols from public API autocompletion.
@@ -225,6 +227,11 @@ export class Subscription extends (EventEmitter as new () => TypedEventEmitter<S
       this._offset = result.offset || 0;
       this._epoch = result.epoch || '';
     }
+    if (result.delta) {
+      this._delta_negotiated = true;
+    } else {
+      this._delta_negotiated = false;
+    }
 
     this._setState(SubscriptionState.Subscribed);
     // @ts-ignore – we are hiding some methods from public API autocompletion.
@@ -455,7 +462,7 @@ export class Subscription extends (EventEmitter as new () => TypedEventEmitter<S
   }
 
   private _handlePublication(pub: any) {
-    if (this._delta) {
+    if (this._delta && this._delta_negotiated) {
       // @ts-ignore – we are hiding some methods from public API autocompletion.
       const {newData, newPrevValue} = this._centrifuge._codec.applyDeltaIfNeeded(pub, this._prevValue)
       pub.data = newData;