# apollo-link

## Purpose

`apollo-link` is a standard interface for modifying control flow of GraphQL requests and fetching GraphQL results, designed to provide a simple GraphQL client that is capable of extensions.
The targeted use cases of `apollo-link` are highlighted below:

* fetch queries directly without normalized cache
* network interface for Apollo Client
* network interface for Relay Modern
* fetcher for

Apollo Link is the interface for creating new links in your application.

The client sends a request as a method call to a link and can recieve one or more (in the case of subscriptions) responses from the server. The responses are returned using the Observer pattern.

![Apollo Link Chain](https://cdn-images-1.medium.com/max/1600/1*62VLGUaU-9ULCoBCGvgdkQ.png)

Results from the server can be provided by calling `next(result)` on the observer. In the case of a network/transport error (not a GraphQL Error) the `error(err)` method can be used to indicate a response will not be recieved. If multiple responses are not supported by the link, `complete()` should be called to inform the client no further data will be provided.

In the case of an intermediate link, a second argument to `request(operation, forward)` is the link to `forward(operation)` to. `forward` returns an observable and it can be returned directly or subscribed to.

```js
forward(operation).subscribe({
  next: result => {
    handleTheResult(result)
  },
  error: error => {
    handleTheNetworkError(error)
  },
});
```

## Implementing a basic custom transport

```js
import { ApolloLink, Observable } from 'apollo-link';

export class CustomApolloLink extends ApolloLink {
  request(operation /*, forward*/) {
    //Whether no one is listening anymore
    let unsubscribed = false;

    return new Observable(observer => {
      somehowGetOperationToServer(operation, (error, result) => {
        if (unsubscribed) return;
        if (error) {
          //Network error
          observer.error(error);
        } else {
          observer.next(result);
          observer.complete(); //If subscriptions not supported
        }
      });

      function unsubscribe() {
        unsubscribed = true;
      }

      return unsubscribe;
    });
  }
}
```

## Installation

`npm install apollo-link --save`