Local Schema

Official docs: https://relay.dev/docs/en/local-state-management

Relay has a built-in support for local-only schema which allows you to work with GraphQL in-memory without sending requests to the server. First define local schema (schema.local.graphql):

"""
Extend type: https://graphql.github.io/graphql-spec/draft/#sec-Object-Extensions
"""
extend type Article {
draft: String!
}
"""
Or add new query: https://github.com/facebook/relay/issues/1656#issuecomment-382461183
"""
extend type Query {
errors: [Error!]
}
type Error {
id: ID!
message: String!
}

Run Relay Compiler as usual (no special option required):

$ relay-compiler --src ./packages --schema ./packages/schema.graphql --verbose

File schema.local.graphql must be somewhere in --src folder with the *.graphql extension. You should be good to go - just fetch the fields as usual. You have to commit local update to fill these fields and types:

Relay.commitLocalUpdate(environment, store => {
const articleID = 'f9496862-4fb7-4a09-bc05-a9a3ce2cb7b3'; // ID of the `Article` type you want to update
store.get(articleID).setValue('My custom draft text', 'draft');
// or create new types:
const root = store.getRoot();
const errRecord = store.create('ID_1', 'Error');
errRecord.setValue('ID_1', 'id');
errRecord.setValue('My custom error message', 'message');
root.setLinkedRecords([errRecord, ...], 'errors');
});

More info here: http://facebook.github.io/relay/docs/en/relay-store.html

Another example:

extend type PNRInfo {
successMessage: String
}

All local schemas are being auto-discovered thanks to *.graphql file extension. You can now fetch and render this success message somewhere in the application. Propagation of this message is trivial (you have to fetch the PNRInfo ID):

Relay.commitLocalUpdate(environment, store => {
store
.get(response.id) // unique opaque ID identifying PNRInfo record
.setValue(
'Request has been successfully sent.', // the actual message
'successMessage', // client field name
);
});

Please note that server may introduce the same field successMessage which will conflict with the client one (new kind of BC break). Luckily, Relay will recognize this BC break and it will throw an error:

ERROR:
Field "PNRInfo.successMessage" already exists in the schema. It cannot also be defined in this type extension.

Related resources:

Interesting helper from @sibelius:

export const setLocal = (query: GraphQLTaggedNode, localData: object) => {
const request = getRequest(query);
const operation = createOperationDescriptor(request, {});
env.commitPayload(operation, localData);
env.retain(operation.root);
};

(source)

Client field via @__clientField(handle: " ... ")

Directive @__clientField is a special directive used by Relay client to create virtual (local) field. But, before we move on:

This directive is not intended for use by developers directly. To set a field handle in product code use a compiler plugin (source)

That's in fact exactly what @connection does behind the scenes - it translates itself into something like:

@__clientField(handle: "connection", key: "UserFriends_friends", filters: ["orderby"])

Let's ignore this transform for now - it's possible to use it even directly with the warning in mind. It's possible to use it for the computation of our client field value from other server field:

fragment Example on Article {
body @__clientField(handle: "draft")
# this is a client field and it will contain uppercased `body` value
draft
}

This obviously means that you need to define your local client schema (schema.local.graphql):

extend type Article {
draft: String
}

And you have to create the handler which is being registered when you are creating new Relay environment (https://facebook.github.io/relay/docs/en/relay-environment.html#adding-a-handlerprovider):

const DraftHandler = {
update(store, payload) {
const record = store.get(payload.dataID);
const content = record.getValue(payload.fieldKey);
record.setValue(content.toUpperCase(), 'draft');
// Set the original value to handleKey, otherwise the field with @__clientField directive will be undefined
record.setValue(content, payload.handleKey);
},
};

Don't forget to run Relay compiler after you add these changes. More info: https://medium.com/@matt.krick/replacing-redux-with-relay-47ed085bfafe

How does the payload look like?

/**
* A payload that is used to initialize or update a "handle" field with
* information from the server.
*/
export type HandleFieldPayload = {|
// The arguments that were fetched.
+args: Variables,
// The __id of the record containing the source/handle field.
+dataID: DataID,
// The (storage) key at which the original server data was written.
+fieldKey: string,
// The name of the handle.
+handle: string,
// The (storage) key at which the handle's data should be written by the
// handler.
+handleKey: string,
|};
// args: {show: "ALL"}
// dataID: "TGVhZDpiOTZiZWMxMC0yYmVlLTExZWEtYTg5Yi04MWU2NGJlOGY5OTk="
// fieldKey: "labels(show:"ALL")"
// handle: "list"
// handleKey: "__list_test_key_list"

Another use-case is to affect behavior of your field (that's what @connection does). You can for example write some "sort" handler which will sort your arrays in the Relay store. Technically, this @__clientField annotations gives you separated space (payload.handleKey) in the Relay store so you can do whatever you want.

LocalQueryRenderer