sgauthier
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59 lines
1.8 KiB
Markdown
59 lines
1.8 KiB
Markdown
---
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title: apollo-link-context
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description: Easily set a context on your operation, which is used by other links further down the chain.
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---
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The `setContext` function takes a function that returns either an object or a promise that returns an object to set the new context of a request.
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It receives two arguments: the GraphQL request being executed, and the previous context. This link makes it easy to perform async look up of things like authentication tokens and more!
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```js
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import { setContext } from "apollo-link-context";
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const setAuthorizationLink = setContext((request, previousContext) => ({
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headers: {authorization: "1234"}
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}));
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const asyncAuthLink = setContext(
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request =>
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new Promise((success, fail) => {
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// do some async lookup here
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setTimeout(() => {
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success({ token: "async found token" });
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}, 10);
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})
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);
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```
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## Caching lookups
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Typically async actions can be expensive and may not need to be called for every request, especially when a lot of request are happening at once. You can setup your own caching and invalidation outside of the link to make it faster but still flexible!
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Take for example a user auth token being found, cached, then removed on a 401 response:
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```js
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import { setContext } from "apollo-link-context";
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import { onError } from "apollo-link-error";
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// cached storage for the user token
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let token;
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const withToken = setContext(() => {
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// if you have a cached value, return it immediately
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if (token) return { token };
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return AsyncTokenLookup().then(userToken => {
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token = userToken;
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return { token };
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});
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});
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const resetToken = onError(({ networkError }) => {
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if (networkError && networkError.name ==='ServerError' && networkError.statusCode === 401) {
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// remove cached token on 401 from the server
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token = null;
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}
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});
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const authFlowLink = withToken.concat(resetToken);
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```
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