made mercurius's implicitly injected pubsub subscribe/publish methods type safe

[adrtivv]

The rationale is that mercurius implicitly injects the pubsub field into the context received by the graphql resolvers. For making the context type-safe we need to define a custom context type to annotate its typescript type correctly for usage in the graphql resolvers and its in that context type where we can annotate the typescript type for pubsub object correctly for usage in the graphql resolvers by having the option of providing strict event_name-payload types to the pubsub.subscribe and pubsub.publish methods.

Usage is shown in the example.ts file present in the root directory of this stackblitz example with proper comments.

This implementation is inspired by this implementation of pubsub by graphql-yoga but its made to work with mercurius's implementation of pubsub.

Ideally mercurius should provide an explicit way of either correctly passing the types for these event_name-payload that are to be used in pubsub.subscribe and pubsub.publish functions or it should provide an explicit way of initializing the pubsub object instead of implicitly injecting it into the graphql context.

[adrtivv]

ahh i was fixing some formatting, anyway you can fix the minor details

[mcollina]

Can you add tests for the types? We use tsd.

[adrtivv]

Can you add tests for the types? We use tsd.

Not free right now, will try later.

[voxpelli]

Some comments. I'm not a maintainer of this module though, just an active user.

[voxpelli]

Unless you do this, this is a breaking change, as PubSub will require TPubSubPublishArgsByKey to be specified:

Suggested change

	export interface PubSub<
	TPubSubPublishArgsByKey extends PubSubPublishArgsByKey,
	> {
	export interface PubSub<
	TPubSubPublishArgsByKey extends PubSubPublishArgsByKey = PubSubPublishArgsByKey,
	> {

[voxpelli]

The subscribe is a wrapper for the emitter / mqemitter method on and publish is a wrapper for the emitter / mqemitter method emit:

export type Message = Record<string, any> & { topic: string }

on(topic: string, listener: (message: Message, done: () => void) => void, callback?: () => void): this
emit(message: Message, callback?: (error?: Error) => void): void

There is some mismatch here, both in the old and in the new types. Eg: the publish callback should be: callback?: (error?: Error) => void

I think the emitter option of the constructor should also be specified to ensure a match between it and what's sent here:

mercurius/index.d.ts

Line 449 in a45fd77

emitter?: object;

(Ideally the publish() would be reimplemented as public(topic, payload, [callback]) and have some way to resolve to a promise rather than the callback, so that an await pubsub.publish('topic1', 'value1)` would be possible, but that's out of scope of this PR)

[voxpelli]

Does this work properly when TKey is something like 'key1' | 'key2'?

[voxpelli]

This should probably mimic eg MercuriusContext and be one can extend, rather than (or in addition) to being needed to specify like PubSub<MyTopics> – that way it will work automatically in all the places that use the PubSub type.

The name should also be in the style of the other interfaces.

Suggested change

	export type PubSubPublishArgsByKey = {
	[key: string]: any;
	};
	export interface PubSubTopics {
	[topic: string]: any;
	}