feat(core): Allow fetch exchange to handle subscriptions

.changeset/tricky-poets-clap.md

@@ -0,0 +1,5 @@
+---
+'@urql/core': minor
+---
+
+Allow subscriptions to be handled by the `fetchExchange` when `fetchSubscriptions` is turned on.

packages/core/src/client.ts

@@ -100,6 +100,16 @@ export interface ClientOptions {
    * @see {@link https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API} for the Fetch API spec.
    */
   fetch?: typeof fetch;
+  /** Allows a subscription to be executed using a `fetch` API request.
+   *
+   * @remarks
+   * If your API supports the `text/event-stream` and/or `multipart/mixed` response protocol, and you use
+   * this protocol to handle subscriptions, then you may switch this flag to `true`.
+   *
+   * This means you won’t have to create a {@link subscriptionExchange} to handle subscriptions with an
+   * external transport, and will instead be able to use GraphQL over HTTP transports.
+   */
+  fetchSubscriptions?: boolean;
   /** A list of `Exchange`s that will be used to create the `Client`'s execution pipeline.
    *
    * @remarks

packages/core/src/exchanges/fetch.ts

@@ -31,7 +31,11 @@ export const fetchExchange: Exchange = ({ forward, dispatchDebug }) => {
     const fetchResults$ = pipe(
       ops$,
       filter(operation => {
-        return operation.kind === 'query' || operation.kind === 'mutation';
+        return (
+          operation.kind !== 'teardown' &&
+          (operation.kind !== 'subscription' ||
+            !!operation.context.fetchSubscriptions)
+        );
       }),
       mergeMap(operation => {
         const body = makeFetchBody(operation);
@@ -87,7 +91,11 @@ export const fetchExchange: Exchange = ({ forward, dispatchDebug }) => {
     const forward$ = pipe(
       ops$,
       filter(operation => {
-        return operation.kind !== 'query' && operation.kind !== 'mutation';
+        return (
+          operation.kind === 'teardown' ||
+          (operation.kind === 'subscription' &&
+            !operation.context.fetchSubscriptions)
+        );
       }),
       forward
     );

packages/core/src/internal/fetchOptions.ts

@@ -121,7 +121,9 @@ export const makeFetchOptions = (
 ): RequestInit => {
   const headers: HeadersInit = {
     accept:
-      'application/graphql-response+json, application/graphql+json, application/json, text/event-stream, multipart/mixed',
+      operation.kind === 'subscription'
+        ? 'text/event-stream, multipart/mixed'
+        : 'application/graphql-response+json, application/graphql+json, application/json, text/event-stream, multipart/mixed',
   };
   const extraOptions =
     (typeof operation.context.fetchOptions === 'function'

packages/core/src/types.ts

@@ -473,6 +473,16 @@ export interface OperationContext {
    * @see {@link https://developer.mozilla.org/en-US/docs/Web/API/fetch} for a description of this object.
    */
   fetchOptions?: RequestInit | (() => RequestInit);
+  /** Allows the `fetchExchange` to handle subscriptions.
+   *
+   * @remarks
+   * When set to `true`, subscriptions are allowed to be handled by the {@link fetchExchange} and will
+   * be sent using a `fetch` call as GraphQL over HTTP requests.
+   * This may be enabled on {@link ClientOptions.fetchSubscriptions} when your API supports the
+   * `text/event-stream` and `multipart/mixed` response protocols and is able to use them to
+   * respond with subscription results.
+   */
+  fetchSubscriptions?: boolean;
   /** The request and caching strategy instructing cache exchanges how to treat cached results.
    *
    * @remarks