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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"><code>useContext</code></h1><p>We use React’s Context API to enable us to easily share stateful data between all levels of an application without having to use repetitious ‘prop-drilling’ at a component level.</p><p>This works best with data that is global by definition, such as theme parameters or access authorisation. Context should not be used when standard and proximate compositional methods and prop-passing are sufficient.</p><h2 id="providers-and-consumers">Providers and consumers</h2><p>The Context API preceded the introduction of hooks in React 16.8 and it still underpins context management in hooks, although the syntax is simplified.</p><p>The process is as follows:</p><ul><li>Initiate a context</li><li>Create a parent <code>Provider</code> component that owns the data that will be passed down</li><li>Create child <code>Consumer</code> components that receive the data of the parent.</li></ul><p>We will demonstrate using a set of styles as the context that we want to pass around our app.</p><pre><code class="jsx language-jsx">const style = {
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border: "2px solid dodgerblue",
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background: "lightblue",
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color: "dodgerblue",
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};</code></pre><p>Le’s initiate a context:</p><pre><code class="jsx language-jsx">const StyleContext = React.createContext();</code></pre><p>Now that we have our data and have initialized a context, we can apply it to our app components:</p><pre><code class="jsx language-jsx"><StyleContext.Provider value={style}>...</StyleContext.Provider></code></pre><p>This is the parent element of the context environment. This stores the contextual data as a prop. Next we need to make a component that acts as the consumer of this data. Let’s create this component and call it <code>Child</code> for simplicity:</p><pre><code class="jsx language-jsx">const Child = () => {
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return (
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<StyleContext.Consumer>
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{(value) => (
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<div style={value}>
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<p>
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I'm a <b>child</b>. I <b>consume</b> of the data my parent provides.
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</p>
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</div>
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)}
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</StyleContext.Consumer>
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);
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};</code></pre><p>The contents of this component are wrapped in tags that reference the specific context (<code>StyleContext</code>) and the role that the component plays: <code>Consumer</code>.</p><p>We are taking the <code>value</code> prop owned by <code>StyleContext.Provider</code> and passing it to the wrapping component of <code>Child</code> as a <code>style</code> tag. This means that <code>Child</code> will display these styles.</p><p>The final step is just to insert our <code><Child /></code> components beneath the parent component, e.g.:</p><pre><code class="jsx language-jsx"><StyleContext.Provider value={style}>
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<Child />
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<Child />
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</StyleContext.Provider></code></pre><h2 id="usecontext-hook"><code>useContext</code> hook</h2><p>The hook removes the need to explicitly declare <code>Provider</code> and <code>Consumer</code> components; their role becomes more implicit.</p><p>To demonstrate let’s use a different example. We are going to have a single integer as the data context, <code>42</code>.</p><p>First invoke and intialize the Context API, just as before:</p><pre><code class="jsx language-jsx">const NumberContext = React.createContext(42);</code></pre><p>Next, we just need to create our provider component. We don’t have to worry about a consumer since this is handled implicitly by the invocation of the hook</p><pre><code class="jsx language-jsx">const Context = () => {
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const data = useContext(NumberContext);
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return <Typography>{data}</Typography>;
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};</code></pre><p>Then, in our code we just insert the <code>Context</code> component:</p><pre><code class="jsx language-jsx"><Context>...</Context></code></pre><h2 id="updating-context-state">Updating context state</h2><p>In the examples above we have only been consuming state that is owned by the provider however in most scenarios you will also want to update the state from a consumer. This is best achieved by combining <code>useContext</code> with a reducer and is detailed in <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Application state management"><a href="Application_state_management_with_React_hooks.html">Application state management</a></span></span>.</p></div></article><nav class="ui attached segment deemphasized backlinksPane" id="neuron-backlinks-pane"><h3 class="ui header">Backlinks</h3><ul class="backlinks"><li><span class="zettel-link-container cf"><span class="zettel-link"><a href="Application_state_management_with_React_hooks.html">Application state management</a></span></span><ul class="context-list" style="zoom: 85%;"><li class="item"><div class="pandoc"><p>Although <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: useReducer"><a href="React_useReducer.html">React_useReducer</a></span></span> and <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: useContext"><a href="React_useContext.html">React_useContext</a></span></span> have many sound use cases by themselves, when they are combined they offer a way to acheive a system of global state management, without utilising third-party libraries like Redux.</p></div></li></ul></li></ul></nav><nav class="ui attached segment deemphasized bottomPane" id="neuron-tags-pane"><div><span class="ui basic label zettel-tag" title="Tag">javascript</span><span class="ui basic label zettel-tag" title="Tag">react</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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