Allow a new Apollolink to be set after ApolloClient instantiation

Provides support for setting a new `ApolloLink` (or link chain)
after `new ApolloClient()` has been called, using a public
`ApolloClient.setLink()` method.

CHANGELOG.md

@@ -144,6 +144,9 @@
 - Add a default `gc` implementation to `ApolloCache`.  <br/>
   [@justinwaite](https://github.com/justinwaite) in [#5974](https://github.com/apollographql/apollo-client/pull/5974)
 
+- Apollo Client now supports setting a new `ApolloLink` (or link chain) after `new ApolloClient()` has been called, using the `ApolloClient.setLink()` method.  <br/>
+  [@hwillson](https://github.com/hwillson) in [#6193](https://github.com/apollographql/apollo-client/pull/6193)
+
 ### Bug Fixes
 
 - `useMutation` adjustments to help avoid an infinite loop / too many renders issue, caused by unintentionally modifying the `useState` based mutation result directly.  <br/>

src/ApolloClient.ts

@@ -555,4 +555,11 @@ export class ApolloClient<TCacheShape> implements DataProxy {
   public setLocalStateFragmentMatcher(fragmentMatcher: FragmentMatcher) {
     this.localState.setFragmentMatcher(fragmentMatcher);
   }
+
+  /**
+   * Define a new ApolloLink (or link chain) that Apollo Client will use.
+   */
+  public setLink(newLink: ApolloLink) {
+    this.link = this.queryManager.link = newLink;
+  }
 }

src/__tests__/ApolloClient.ts

@@ -2271,4 +2271,33 @@ describe('ApolloClient', () => {
       expect((client.cache as any).data.data).toEqual({});
     });
   });
+
+  describe('setLink', () => {
+    it('should override default link with newly set link', async () => {
+      const client = new ApolloClient({
+        cache: new InMemoryCache()
+      });
+      expect(client.link).toBeDefined();
+
+      const newLink = new ApolloLink(operation => {
+        return new Observable(observer => {
+          observer.next({
+            data: {
+              widgets: [
+                { name: 'Widget 1'},
+                { name: 'Widget 2' }
+              ]
+            }
+          });
+          observer.complete();
+        });
+      });
+
+      client.setLink(newLink);
+
+      const { data } = await client.query({ query: gql`{ widgets }` });
+      expect(data.widgets).toBeDefined();
+      expect(data.widgets.length).toBe(2);
+    });
+  });
 });