This is a graphql-code-generator plugin that generates type policies for every field that uses custom scalars.
Overview
Given a config like:
generates:
integration/graphql-types.ts:
config:
scalars:
Date: "src/dates#Date"
scalarTypePolicies:
Date: "src/dates#dateTypePolicy"
And a schema that uses Date
fields:
scalar Date
type Author {
name: String!
date: Date
}
This plugin generates the following snippet in the output file:
import { dateTypePolicy } from "src/dates";
export const scalarTypePolicies = {
Author: { fields: { date: dateTypePolicy } },
};
Where the dateTypePolicy
const
should implement the Apollo Client FieldPolicy
.
Which you can then pass to Apollo's InMemoryCache
:
new InMemoryCache({ typePolicies: scalarTypePolicies })
And you can implement a field policy like:
export const dateTypePolicy: FieldPolicy<Date, string> = {
merge: (_, incoming) => {
if (isNullOrUndefined(incoming)) {
return incoming;
} else if (incoming instanceof Date) {
return incoming;
} else {
return parseISO(incoming as string);
}
},
};
Note that this will handle reading data from the cache; to handle submitting custom scalars (i.e. Date
) as variables / input to mutations, your custom scalars should implement toJSON()
, which Apollo will implicitly calls when putting them on the wire.