84 lines
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84 lines
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<!--replace-end-7--><!--replace-end-4--><!--replace-end-1--></head><body><div class="ui fluid container universe"><!--replace-start-2--><!--replace-start-3--><!--replace-start-6--><div class="ui text container" id="zettel-container" style="position: relative"><div class="zettel-view"><article class="ui raised attached segment zettel-content"><div class="pandoc"><h1 id="title-h1">Apollo Client</h1><p>Apollo Client is the client-side counterpart to <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Apollo Server"><a href="Apollo_Server.html">Apollo Server</a></span></span>. We use it for managing queries and mutations from the frontend to our Apollo GraphQL server. It is specifically designed to work with React.</p><blockquote><p>We will be working with the <a href="Apollo_Server.md#example-schema">schema</a> we defined when working on the server</p></blockquote><h2 id="initializing-the-client">Initializing the client</h2><p>We initialise the client and set-up in memory caching to reduce network requests:</p><pre><code class="js language-js">const client = new ApolloClient({
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uri: "http://localhost:4000",
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cache: new InMemoryCache(),
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});</code></pre><blockquote><p>The <code>uri</code> property must match the location of our Apollo server.</p></blockquote><h2 id="utilising-the-provider">Utilising the provider</h2><p>Apollo provides a top level application context that we can wrap our React app in. This will provide access to the client object from anywhere within the app, eg:</p><pre><code class="jsx language-jsx">ReactDOM.render(
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<ApolloProvider client={client}>
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<GlobalStyles />
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<Pages />
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</ApolloProvider>,
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document.getElementById("root")
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);</code></pre><h2 id="running-a-query">Running a query</h2><h3 id="queries-as-entry-points">Queries as entry points</h3><p>From the client point of view, the queries in the schema are <em>entry points</em>. Although the queries exist in the schema, this alone is not sufficient for them to be entry points. Remember a schema is just a specification or contract between the frontend and the backend, it is not itself executable code.</p><p>Therefore, for each query in the schema we must write a frontend implementation. We do this with <strong>query constants</strong>. The frontend implementation has a backend analogue: the <a href="Apollo_Server.md#implementing-resolvers">resolver</a> that is invoked when the frontend issues a query. The schema standardises this relationship so that every query on the client must have a corresponding resolver on the backend.</p><h3 id="query-constants">Query constants</h3><p>To run a query against our server we must define a query contant first. We use a <code>gql</code> literal again:</p><pre><code class="js language-js">import { gql } from "@apollo/client";
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const TRACKS = gql`
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query GetTracks {
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tracksForHome {
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id
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title
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thumbnail
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length
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modulesCount
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author {
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name
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photo
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}
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}
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}
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`;</code></pre><p>The convention is to name the query constant in <code>ALL_CAPS</code>.</p><blockquote><p>Note that the name of the query on the client doesn’t have to match the query type defined in the schema (there is no <code>GetTracks</code> in the schema), this is just a client-side designator. However it should reference the schema on the second line (<code>tracksForHome</code>).</p></blockquote><h3 id="usequery-hook"><code>useQuery</code> hook</h3><p>The <code>useQuery</code> hook provides a straightforward wrapper for sending queries and receiving data back from the server.</p><p>When a component renders, <code>useQuery</code> returns an object from the Apollo Client that contains loading, error, and data properties.</p><pre><code class="jsx language-jsx">const { loading, error, data } = useQuery(TRACKS);
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const Tracks = () => {
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const { loading, error, data } = useQuery(TRACKS);
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if (loading) return "Loading...";
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if (error) return `Error! ${error.message}`;
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return <Layout grid>{JSON.stringify(data)}</Layout>;
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};</code></pre><ul><li>We destructure the <code>loading, error, data</code> variables that are returned from the hook</li><li>We pass in our query constant as an argument.</li><li>In the example we just render the serialized data but we could of course pass the data as a prop and map through it in an embedded child component.</li></ul></div></article><nav class="ui attached segment deemphasized bottomPane" id="neuron-tags-pane"><div><span class="ui basic label zettel-tag" title="Tag">graphql</span></div></nav><nav class="ui bottom attached icon compact inverted menu blue" id="neuron-nav-bar"><!--replace-start-9--><!--replace-end-9--><a class="right item" href="impulse.html" title="Open Impulse"><i class="wave square icon"></i></a></nav></div></div><!--replace-end-6--><!--replace-end-3--><!--replace-end-2--><div class="ui center aligned container footer-version"><div class="ui tiny image"><a href="https://neuron.zettel.page"><img alt="logo" src="https://raw.githubusercontent.com/srid/neuron/master/assets/neuron.svg" title="Generated by Neuron 1.9.35.3" /></a></div></div></div></body></html>
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