Relay all-in

This file describes experimental and more advanced Relay features. It can be very unstable due to its nature so be careful. Here be dragons!

TODO:

There are different tradeoffs across completeness, consistency, and performance, and there isn't one approach that is appropriate for every app. Relay focuses on cases where consistency matters: if you don't need consistency then a simpler/lighter solution can be more appropriate. (source)

Useful Links (learning resources):

Relay Config (relay.config.js)

Relay supports configuration via CLI but also via configuration files using official NPM package relay-config. Configuration files work only when you install this package. Relay Config relies on cosmiconfig to do its bidding. It’s configured to load from:

  • a relay key in package.json
{
  "relay": {
    "src": "./src"
  }
}
  • a relay.config.json file
{
  "src": "./src"
}
  • or a relay.config.js file
module.exports = {
  src: './src',
};

It accepts all the same configuration as the CLI does. Additionally, when using the relay.config.js file, a configuration entry like the language plugin also accepts an actual function:

const typescript = require('relay-compiler-language-typescript');
module.exports = {
  language: typescript,
};

In the future, other entries such as persistedQueries and customScalars could also be configured as such and allow for projec specific setup.

See: https://github.com/facebook/relay/commit/d3ec68ec137f7d72598a6f28025e94fba280e86e

Future of QueryRenderer/useQuery pattern

In general we're planning to move away from the QueryRenderer/useQuery pattern, which we're referring to as "fetch-on-render". This design makes behavior unpredictable (rendering can happen arbitrarily due to changes in parent components, suspense can cause re-renders and doesn't guarantee cleanup). The alternative is "fetch-then-render" - perform your data-fetching based on some event (user interaction, navigation, timer, app initialization) and then consume that result during render. Then "how do i refetch?" has the same answer as "how do i fetch?". Expect to see more API changes in this direction.

Source: https://github.com/facebook/relay/issues/2864#issuecomment-535108266

Deferred results

Custom Relay Compiler

Most of the people are OK with the default OSS version of Relay Compiler. However, it can be sometimes beneficial to write your own Relay Compiler in order to achieve some advanced features (custom behavior or persisting queries to your server for example). Facebook also uses internally their own Relay Compiler implementation. Here is one example of "why" (source: https://github.com/facebook/relay/commit/f1e2e79462d593d73efb80727bc5dd56b1c43cf6#commitcomment-36337550).

The default config generates the flow types inline in the generated files, so something like:

meeting: {
  response: 'GOING' | 'NOT_GOING'
}

This can in some cases introduce a bunch of noise if the generated files are checked in and the schema changes frequently. For that purpose, we instead generate something like:

import type {MeetingResponse} from 'MeetingResponse.enums';
meeting: {
  response: MeetingResponse
}

Doing that in OSS as well would increase the number of generated files and also add the question of where to put these files and how to import them.

RelayResponseNormalizer: handleStrippedNulls

Relay is able to recover completely missing fields in the response. You can use this knowledge to optimize JSON response from the server. Let's say this is our incoming payload from the server:

{
  "data": {
    "allLocations": {
      "edges": [
        { "node": { "id": "san-francisco_ca_us", "name": "San Francisco" } },
        { "node": { "id": "boston_ma_us", "name": "Boston" } },
        { "node": { "id": "washington_dc_us", "name": "Washington, D.C." } }
      ]
    }
  }
}

Traditionally, server would return something like this in case of failure (or just missing data):

{
  "data": {
    "allLocations": {
      "edges": [
        { "node": { "id": "san-francisco_ca_us", "name": "San Francisco" } },
        { "node": { "id": "boston_ma_us", "name": null } },
        { "node": { "id": "washington_dc_us", "name": null } }
      ]
    }
  },
  "errors": ...
}

But it's not necessary to send the nullable fields at all. Afterall, server knows what fields were requested. RelayResponseNormalizer by default recovers from this state so you can send response like this from the server (see the missing names):

{
  "data": {
    "allLocations": {
      "edges": [
        { "node": { "id": "san-francisco_ca_us", "name": "San Francisco" } },
        { "node": { "id": "boston_ma_us" } },
        { "node": { "id": "washington_dc_us" } }
      ]
    }
  },
  "errors": ...
}

Relay will show you this warning in this console (dev mode only):

Warning: RelayResponseNormalizer(): Payload did not contain a value for field name: name. Check that you are parsing with the same query that was used to fetch the payload.

See: https://github.com/facebook/relay/blob/76fef685f70a5aa09cd180ce0f2ef6b6d3f4f7e8/packages/relay-runtime/store/RelayResponseNormalizer.js#L75

Refetch container

https://facebook.github.io/relay/docs/en/refetch-container.html

When refetch is called and the refetchQuery is executed, Relay doesn't actually use the result of the query to re-render the component. All it does is normalize the payload into the store and fire any relevant subscriptions. This means that if the fetched data is unrelated to the data that the mounted container is subscribed to (e.g. using a totally different node id that doesn't have any data overlaps), then the component won't re-render.

Refetch containers are only really meant to be used when you are changing variables in the component fragment. If you don't want or need to include variables in the fragment, you could go one level up and set new variables directly in the QueryRenderer (using props or state).

https://github.com/facebook/relay/issues/2244#issuecomment-355054944

RelayNetworkLogger

TODO: https://github.com/facebook/relay/issues/2674 !

import RelayNetworkLogger from 'relay-runtime/lib/RelayNetworkLogger';
import fetchFunction from './fetchFunction';
import subscribeFunction from './subscribeFunction';
const fetch = __DEV__ ? RelayNetworkLogger.wrapFetch(fetchFunction) : fetchFunction;
const subscribe = __DEV__ ? RelayNetworkLogger.wrapSubscribe(subscribeFunction) : subscribeFunction;
const network = Network.create(fetch, subscribe);
const source = new RecordSource();
const store = new Store(source);
const env = new Environment({
  network,
  store,
});
export default env;

RelayObservable.onUnhandledError

You can override default behavior of unhandled errors when using Relay Observable:

import { Observable } from 'relay-runtime';
if (__DEV__) {
  Observable.onUnhandledError((error, isUncaughtThrownError) => {
    console.error(error);
  });
}
Observable.create( ... )

Default implementation:

if (__DEV__) {
  // Default implementation of HostReportErrors() in development builds.
  // Can be replaced by the host application environment.
  RelayObservable.onUnhandledError((error, isUncaughtThrownError) => {
    declare function fail(string): void;
    if (typeof fail === 'function') {
      // In test environments (Jest), fail() immediately fails the current test.
      fail(String(error));
    } else if (isUncaughtThrownError) {
      // Rethrow uncaught thrown errors on the next frame to avoid breaking
      // current logic.
      setTimeout(() => {
        throw error;
      });
    } else if (typeof console !== 'undefined') {
      // Otherwise, log the unhandled error for visibility.
      // eslint-disable-next-line no-console
      console.error('RelayObservable: Unhandled Error', error);
    }
  });
}
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