Implement ObservableQuery#isDifferentFromLastResult.

This commit is a more conservative version of
e66027c

We still need to make a deep clone of observableQuery.lastResult in order
to determine if future results are different (see #3992), but we can
restrict the use of that snapshot to a single method, rather than
replacing observableQuery.lastResult with the snapshot.

Should help with #4054.

packages/apollo-client/src/core/ObservableQuery.ts

@@ -77,6 +77,7 @@ export class ObservableQuery<
   private subscriptionHandles: Subscription[];
 
   private lastResult: ApolloQueryResult<TData>;
+  private lastResultSnapshot: ApolloQueryResult<TData>;
   private lastError: ApolloError;
 
   constructor({
@@ -201,7 +202,7 @@ export class ObservableQuery<
     }
 
     const result = {
-      data: cloneDeep(data),
+      data,
       loading: isNetworkRequestInFlight(networkStatus),
       networkStatus,
     } as ApolloQueryResult<TData>;
@@ -216,11 +217,24 @@ export class ObservableQuery<
 
     if (!partial) {
       this.lastResult = { ...result, stale: false };
+      this.lastResultSnapshot = cloneDeep(this.lastResult);
     }
 
     return { ...result, partial } as ApolloCurrentResult<TData>;
   }
 
+  // Compares newResult to the snapshot we took of this.lastResult when it was
+  // first received.
+  public isDifferentFromLastResult(newResult: ApolloQueryResult<TData>) {
+    const { lastResultSnapshot: snapshot } = this;
+    return !(
+      snapshot && newResult &&
+      snapshot.networkStatus === newResult.networkStatus &&
+      snapshot.stale === newResult.stale &&
+      isEqual(snapshot.data, newResult.data)
+    );
+  }
+
   // Returns the last result that observer.next was called with. This is not the same as
   // currentResult! If you're not sure which you need, then you probably need currentResult.
   public getLastResult(): ApolloQueryResult<TData> {
@@ -585,7 +599,8 @@ export class ObservableQuery<
 
     const observer: Observer<ApolloQueryResult<TData>> = {
       next: (result: ApolloQueryResult<TData>) => {
-        this.lastResult = cloneDeep(result);
+        this.lastResult = result;
+        this.lastResultSnapshot = cloneDeep(result);
         this.observers.forEach(obs => obs.next && obs.next(result));
       },
       error: (error: ApolloError) => {

packages/apollo-client/src/core/QueryManager.ts

@@ -12,7 +12,6 @@ import {
   getQueryDefinition,
   isProduction,
   hasDirectives,
-  isEqual,
 } from 'apollo-utilities';
 
 import { QueryScheduler } from '../scheduler/scheduler';
@@ -605,15 +604,9 @@ export class QueryManager<TStore> {
           }
 
           if (observer.next) {
-            const isDifferentResult = !(
-              lastResult &&
-              resultFromStore &&
-              lastResult.networkStatus === resultFromStore.networkStatus &&
-              lastResult.stale === resultFromStore.stale &&
-              isEqual(lastResult.data, resultFromStore.data)
-            );
-
-            if (isDifferentResult || previouslyHadError) {
+            if (previouslyHadError ||
+                !observableQuery ||
+                observableQuery.isDifferentFromLastResult(resultFromStore)) {
               try {
                 observer.next(resultFromStore);
               } catch (e) {