graphql-firestore-subscriptions

_graphql-firestore-subscriptions_ implements the PubSubEngine interface from the graphql-subscriptions package.

Unlike other databases, Google's Firestore comes across with real time updates. Therefore, it is not required to publish events to a queue or a pub-sub.
However, there is still something to do to get the data to the clients. In graphql-firestore-subscriptions those tasks are called handlers. They are subscribing a specific topic and broadcast whatever you want over an AsyncIterator which is compatible with graphql-subscriptions.

Usage

First of all, you have to install the _graphql-firestore-subscriptions_ package using yarn or npm by calling either yarn add graphql-firestore-subscriptions or npm i --save graphql-firestore-subscriptions.

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``typescript
import PubSub from 'graphql-firestore-subscriptions';

const ps = new PubSub();
`

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A handler gets two arguments:

- The broadcast function itself to send new data
- An object with options

Note, that the handler MUST return a _unsubscribe_ function.

`typescript
ps.registerHandler(() => {
// subscribe to a topic
return () => {
// unsubscribe
};
});
`

The unsubscribe function can either return void or a boolean value. If a boolean value is returned by the unsubscribe function, the PubSubEngine will throw an error if the return value is falsey.

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Unlike other _graphql-subscriptions_, graphql-firestore-subscriptions requires a handler for each topic you are about to subscribe.
To make the handler-creation as easy as possibile graphql-firestore-subscriptions comes across with a bunch of utility functions.

The following example shows a simple fall-through handler which takes document changes of a collection to broadcast this changes immediately.

`typescript
import PubSub, { createFallThroughHandler } from 'graphql-firestore-subscriptions';
import db from '../path/to/firestore/conenction';

// ...

enum Topic {
NEW_COMMENT = 'NEW_COMMENT',
}

ps.registerHandler(
...createFallThroughHandler(db, {
topic: Topic.NEW_COMMENT,
collection: 'comment',
filter: ['added'],
})
);
`

You can also create multiple fall-through handlers at once:

`typescript
import PubSub, { createFallThroughHandlerFromMap } from 'graphql-firestore-subscriptions';
import db from '../path/to/firestore/connection';

// ...

enum Topic {
NEW_COMMENT = 'NEW_COMMENT',
UPDATE_COMMENT = 'UPDATE_COMMENT',
}

createFallThroughHandlerFromMap(db, {
[Topic.NEW_COMMENT]: {
collection: 'comment',
filter: ['added'],
},
[Topic.UPDATE_COMMENT]: {
collection: 'comment',
filter: ['modified'],
},
}).forEach((topic, handler) => ps.registerHandler(topic, handler));
`

See API for additional information about how createFallThroughHandlerFromMap / createFallThroughHandler work.

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`typescript
import PubSub from 'graphql-firestore-subscriptions';
import db from '../path/to/firestore/connection';

enum Topic {
NEW_COMMENT = 'NEW_COMMENT',
}

const ps = new PubSub();

ps.registerHandler(Topic.NEW_COMMENT, broadcast =>
// Note, that onSnapshot returns a unsubscribe function which
// returns void.
db.collection('comments').onSnapshot(snapshot => {
snapshot
.docChanges()
.filter(change => change.type === 'added')
.map(item => broadcast(item.doc.data()));
})
);

const iterator = ps.asyncIterator(Topic.NEW_COMMENT);
const addedComment = await iterator.next();

// ...
`

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Define a _GraphQL_ schema with a Subscription type.

`graphql
schema {
query: Query
mutation: Mutation
subscription: Subscription
}

type Subscription {
newComment: Comment
}

type Comment {
message: String
}
`

Now, implement the resolver:

`typescript
export const resolvers = {
Subscription: {
newComment: {
subscribe: () => ps.asyncIterator(Topic.NEW_COMMENT),
},
},
};
`

Calling asyncIterator(topics: string | string[]) will subscribe to the given topics and will return an AsyncIterator bound to the PubSubEngine of graphql-firestore-subscriptions.
Everytime, a handler calls the obtained broadcast-function, the PubSubEngine of graphql-firestore-subscriptions will publish the event.

API

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`typescript
function createFallThroughHandler(
fs: Firestore,
overwriteOptions: FallThroughHandlerOptions
): [string, Handler];
`

#### Options

| Name | Type | Description |
| -------------- | ----------------------------------------------------- | ------------------------------------------------------------- |
| topic\* | string | - |
| collection\* | string | The firebase collection |
| transform | TransformStrategy | (change: DocumentChange) => any | Called to transform the broadcast-payload |
| filter | (change: DocumentChange) => boolean | Called to filter document changes before they are broadcasted |

> \* required

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`typescript
function createFallThroughHandlerFromMap(
fs: Firestore,
options: FallThroughHandlerFromMapOptions
): [string, Handler][];
`

#### Options

| Name | Type | Description |
| ----- | ------------------------- | ------------------------------------------------------------ |
| topic | [topic: string]: Object | See createFallThroughHandler#Options for a complete overview |

Contribute

Something is broken? The documentation is incorrect? You're missing a feature? ...and you wanna help? That's great.

The following steps are describing the way from an idea / bug / ... to a pull-request.

1. Fork this repository
1. Apply the changes
1. Write tests (you can execute the current tests by calling npm run test:unit OR npm run test:unit:watch`)
1. If necessary, update the documentation
1. Open a pull-request
1. :tada: