feat: Support NestJS filter function for subscriptions (#2977)

.changeset/fluffy-queens-lick.md

@@ -0,0 +1,5 @@
+---
+'@graphql-yoga/nestjs': minor
+---
+
+Support for NestJS @Subscription.filter method in YogaDriver.

packages/nestjs/__tests__/fixtures/graphql/cats/cats.resolvers.ts

@@ -53,4 +53,17 @@ export class CatsResolvers {
       yield { greetings: hi };
     }
   }
+
+  @Subscription('filteredGreetings', {
+    filter: (payload, variables) => {
+      return payload.filteredGreetings
+        .toLowerCase()
+        .startsWith(variables.firstLetter.toLowerCase());
+    },
+  })
+  async *filteredGreetings() {
+    for (const hi of ['Hi', 'Bonjour', 'Hola', 'Ciao', 'Zdravo']) {
+      yield { filteredGreetings: hi };
+    }
+  }
 }

packages/nestjs/__tests__/fixtures/graphql/cats/cats.types.graphql

@@ -10,6 +10,7 @@ type Mutation {
 type Subscription {
   catCreated: Cat
   greetings: String
+  filteredGreetings(firstLetter: String!): String
 }
 
 type Cat {

packages/nestjs/__tests__/subscriptions.spec.ts

@@ -54,3 +54,33 @@ event: complete
 "
 `);
 });
+
+it("should execute Nest Subscription decorator's filter function on each emitted value", async () => {
+  const sub = await fetch(url, {
+    method: 'POST',
+    headers: {
+      'content-type': 'application/json',
+    },
+    body: JSON.stringify({
+      query: /* GraphQL */ `
+        subscription {
+          filteredGreetings(firstLetter: "H")
+        }
+      `,
+    }),
+  });
+
+  await expect(sub.text()).resolves.toMatchInlineSnapshot(`
+":
+
+event: next
+data: {"data":{"filteredGreetings":"Hi"}}
+
+event: next
+data: {"data":{"filteredGreetings":"Hola"}}
+
+event: complete
+
+"
+`);
+});

packages/nestjs/src/index.ts

@@ -1,7 +1,7 @@
 import type { Express, Request as ExpressRequest, Response as ExpressResponse } from 'express';
 import type { FastifyInstance, FastifyReply, FastifyRequest } from 'fastify';
 import { printSchema } from 'graphql';
-import { createYoga, YogaServerInstance, YogaServerOptions } from 'graphql-yoga';
+import { createYoga, filter, pipe, YogaServerInstance, YogaServerOptions } from 'graphql-yoga';
 import type { ExecutionParams } from 'subscriptions-transport-ws';
 import { Injectable, Logger } from '@nestjs/common';
 import {
@@ -143,6 +143,28 @@ export abstract class AbstractYogaDriver<
       return reply;
     });
   }
+
+  public subscriptionWithFilter<TPayload, TVariables, TContext>(
+    instanceRef: unknown,
+    filterFn: (
+      payload: TPayload,
+      variables: TVariables,
+      context: TContext,
+    ) => boolean | Promise<boolean>,
+    // disable next error, the original function in @nestjs/graphql is also untyped
+    // eslint-disable-next-line @typescript-eslint/ban-types
+    createSubscribeContext: Function,
+  ) {
+    return async (...args: [TPayload, TVariables, TContext]) => {
+      return pipe(
+        await createSubscribeContext()(...args),
+        filter((payload: TPayload) =>
+          // typecast the spread sliced args to avoid error TS 2556, see https://github.com/microsoft/TypeScript/issues/49802
+          filterFn.call(instanceRef, payload, ...(args.slice(1) as [TVariables, TContext])),
+        ),
+      );
+    };
+  }
 }
 
 @Injectable()