Rewrite GraphQL Live Query to subscription.

LiveQueryPolyfill.ts

/**
 * This is a Live Query polyfill, as graphql-js does not strictly have support for that. Instead, we leave it up to the
 * client to recognize a Live Query is requested with the `@live` directive on a `query` operation and here we
 * transform it into a `live` subscription instead.
 *
 * Consider a schema like the following:
 *
 * ```graphql
    type Badge {
        id: ID!
    }
    type Query {
        current: Badge!
    }
    type Subscription {
        live: Query!
    }
 * ```
 *
 * …and an incoming query like the following:
 *
 * ```graphql
    query BadgeQuery @live {
        current {
            id
        }
    }
 * ```
 *
 * This polyfill will then rewrite it to be:
 *
 * ```graphql
    subscription BadgeQuery {
        live {
            current {
                id
            }
        }
    }
 * ```
 */
function rewriteLiveQuery(ast: DocumentNode): { document: DocumentNode, live: boolean } {
    let live = false;
    const document = visit(ast, {
        OperationDefinition: {
            leave(operationNode) {
                if (operationNode.operation === "query" && live) {
                    return {
                        ...operationNode,
                        // Change the operation to be a subscription
                        operation: "subscription",
                        // Insert a `live` field selection in place of the existing selection set…
                        selectionSet: {
                            kind: "SelectionSet",
                            selections: [{
                                kind: "Field",
                                name: {
                                    kind: "Name",
                                    value: "live",
                                },
                                // …and add the existing selection set to the new `live` field selection.
                                selectionSet: operationNode.selectionSet,
                            }],
                        },
                    };
                }
            },
        },
        Directive(directiveNode) {
            if (directiveNode.name.value === "live") {
                live = true;
                // Delete the `live` directive
                // eslint-disable-next-line no-null/no-null
                return null;
            }
        },
        SelectionSet() {
            // Ensure we don't find directives anywhere else in the query
            return false;
        },
    });
    return {
        document,
        live,
    };
}