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70 lines
2.3 KiB
Markdown
70 lines
2.3 KiB
Markdown
# apollo-link
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## Purpose
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`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.
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The targeted use cases of `apollo-link` are highlighted below:
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* fetch queries directly without normalized cache
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* network interface for Apollo Client
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* network interface for Relay Modern
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* fetcher for
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Apollo Link is the interface for creating new links in your application.
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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.
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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.
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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.
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```js
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forward(operation).subscribe({
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next: result => {
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handleTheResult(result)
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},
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error: error => {
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handleTheNetworkError(error)
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},
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});
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```
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## Implementing a basic custom transport
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```js
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import { ApolloLink, Observable } from 'apollo-link';
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export class CustomApolloLink extends ApolloLink {
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request(operation /*, forward*/) {
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//Whether no one is listening anymore
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let unsubscribed = false;
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return new Observable(observer => {
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somehowGetOperationToServer(operation, (error, result) => {
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if (unsubscribed) return;
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if (error) {
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//Network error
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observer.error(error);
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} else {
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observer.next(result);
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observer.complete(); //If subscriptions not supported
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}
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});
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function unsubscribe() {
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unsubscribed = true;
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}
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return unsubscribe;
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});
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}
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}
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```
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## Installation
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`npm install apollo-link --save`
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