feat: add beforeFetch option to `RelayNetworkLayer(middlewares, { b…

…eforeFetch })`.

BREAKING CHANGE: If you use subscriptions in your app and provide
`subscribeFn` as a second argument for RelayNetworkLayer. Now you should
to provide it as a named option: ```new RelayNetworkLayer(middlewares, {
subscribeFn: ...fn.code... });`.

README.md

@@ -12,6 +12,8 @@ with various middlewares which can manipulate requests/responses on the fly (cha
 
 Network Layer for Relay Classic can be found [here](https://github.com/nodkz/react-relay-network-layer).
 
+Migration guide from v1 to v2 can be found [here](https://github.com/nodkz/react-relay-network-modern/releases/tag/v2.0.0).
+
 `ReactRelayNetworkModern` can be used in browser, react-native or node server for rendering. Under the hood this module uses global `fetch` method. So if your client is too old, please import explicitly proper polyfill to your code (eg. `whatwg-fetch`, `node-fetch` or `fetch-everywhere`).
 
 Install
@@ -166,13 +168,24 @@ const network = new RelayNetworkLayer([
 
     return res;
   }
-]); // as second arg you may pass SubscribeFunction
+], opts); // as second arg you may pass advanced options for RRNL
 
 const source = new RecordSource();
 const store = new Store(source);
 const environment = new Environment({ network, store });
 ```
 
+### Advanced options (2nd argument after middlewares)
+RelayNetworkLayer may accept additional options:
+```js
+const middlewares = []; // array of middlewares
+const options = {}; // optional advanced options
+const network = new RelayNetworkLayer(middlewares, options);
+```
+Available options:
+- **subscribeFn** - if you use subscriptions in your app, you may provide this function which will be passed to [RelayNetwork](https://github.com/facebook/relay/blob/master/packages/relay-runtime/network/RelayNetwork.js).
+- **beforeFetch** - if you need synchronously render app on the client side from already generated payload by server (SSR) this options is for you. Some details can be found [here](https://github.com/facebook/relay/issues/2232).
+
 ### Server-side rendering (SSR)
 For performant server-side rendering with `node 8` and above recommended to use special build of this network layer. Also you may execute `graphql` directly without http-request if you write custom middleware. All this recommendations present in following example:
 

src/RelayNetworkLayer.js

@@ -6,21 +6,37 @@ import fetchWithMiddleware from './fetchWithMiddleware';
 import type {
   Middleware,
   FetchFunction,
+  FetchHookFunction,
   SubscribeFunction,
   RNLExecuteFunction,
 } from './definition';
 
+type RelayNetworkLayerOpts = {|
+  subscribeFn?: SubscribeFunction,
+  beforeFetch?: FetchHookFunction,
+|};
+
 export default class RelayNetworkLayer {
   _middlewares: Middleware[];
   execute: RNLExecuteFunction;
-  fetchFn: FetchFunction;
-  subscribeFn: ?SubscribeFunction;
+  +fetchFn: FetchFunction;
+  +subscribeFn: ?SubscribeFunction;
+  beforeFetch: ?FetchHookFunction;
 
-  constructor(middlewares: Array<?Middleware>, subscribeFn?: SubscribeFunction) {
+  constructor(middlewares: Array<?Middleware>, opts?: RelayNetworkLayerOpts) {
     this._middlewares = Array.isArray(middlewares) ? (middlewares: any) : [middlewares];
-    this.subscribeFn = subscribeFn;
+
+    if (opts) {
+      this.subscribeFn = opts.subscribeFn;
+      this.beforeFetch = opts.beforeFetch;
+    }
 
     this.fetchFn = (operation, variables, cacheConfig, uploadables) => {
+      if (this.beforeFetch) {
+        const res = this.beforeFetch(operation, variables, cacheConfig, uploadables);
+        if (res) return res;
+      }
+
       const req = new RelayRequest(operation, variables, cacheConfig, uploadables);
       return fetchWithMiddleware(req, this._middlewares.filter(o => !!o));
     };

src/definition.js

@@ -95,6 +95,12 @@ export type FetchFunction = (
   cacheConfig: CacheConfig,
   uploadables: ?UploadableMap
 ) => ObservableFromValue<QueryPayload>;
+export type FetchHookFunction = (
+  operation: ConcreteBatch,
+  variables: Variables,
+  cacheConfig: CacheConfig,
+  uploadables: ?UploadableMap
+) => void | ObservableFromValue<QueryPayload>;
 // See SubscribeFunction type declaration in relay-runtime/network/RelayNetworkTypes.js
 export type SubscribeFunction = (
   operation: ConcreteBatch,