88 lines
15 KiB
HTML
88 lines
15 KiB
HTML
|
<!DOCTYPE html><html><head><meta content="text/html; charset=utf-8" http-equiv="Content-Type" /><meta content="width=device-width, initial-scale=1" name="viewport" /><!--replace-start-0--><!--replace-start-5--><!--replace-start-8--><title>useReducer - My Zettelkasten</title><!--replace-end-8--><!--replace-end-5--><!--replace-end-0--><link href="https://cdn.jsdelivr.net/npm/fomantic-ui@2.8.7/dist/semantic.min.css" rel="stylesheet" /><link href="https://fonts.googleapis.com/css?family=Merriweather|Libre+Franklin|Roboto+Mono&display=swap" rel="stylesheet" /><!--replace-start-1--><!--replace-start-4--><!--replace-start-7--><link href="https://raw.githubusercontent.com/srid/neuron/master/assets/neuron.svg" rel="icon" /><meta content="The useReducer hook is best used in scenarios where you are manipulating state in a way that is too complex for the trivial React_useState use case. useState is best employed when you are updating a single value or toggling a boolean. If you are updating the state of an object or more complex data s" name="description" /><meta content="useReducer" property="og:title" /><meta content="My Zettelkasten" property="og:site_name" /><meta content="article" property="og:type" /><meta content="React_useReducer" property="neuron:zettel-id" /><meta content="React_useReducer" property="neuron:zettel-slug" /><meta content="javascript" property="neuron:zettel-tag" /><meta content="react" property="neuron:zettel-tag" /><script type="application/ld+json">[]</script><style type="text/css">body{background-color:#eeeeee !important;font-family:"Libre Franklin", serif !important}body .ui.container{font-family:"Libre Franklin", serif !important}body h1, h2, h3, h4, h5, h6, .ui.header, .headerFont{font-family:"Merriweather", sans-serif !important}body code, pre, tt, .monoFont{font-family:"Roboto Mono","SFMono-Regular","Menlo","Monaco","Consolas","Liberation Mono","Courier New", monospace !important}body div.z-index p.info{color:#808080}body div.z-index ul{list-style-type:square;padding-left:1.5em}body div.z-index .uplinks{margin-left:0.29999em}body .zettel-content h1#title-h1{background-color:rgba(33,133,208,0.1)}body nav.bottomPane{background-color:rgba(33,133,208,2.0e-2)}body div#footnotes{border-top-color:#2185d0}body p{line-height:150%}body img{max-width:100%}body .deemphasized{font-size:0.94999em}body .deemphasized:hover{opacity:1}body .deemphasized:not(:hover){opacity:0.69999}body .deemphasized:not(:hover) a{color:#808080 !important}body div.container.universe{padding-top:1em}body div.zettel-view ul{padding-left:1.5em;list-style-type:square}body div.zettel-view .pandoc .highlight{background-color:#ffff00}body div.zettel-view .pandoc .ui.disabled.fitted.checkbox{margin-right:0.29999em;vertical-align:middle}body div.zettel-view .zettel-content .metadata{margin-top:1em}body div.zettel-view .zettel-content .metadata div.date{text-align:center;color:#808080}body div.zettel-view .zettel-content h1{padding-top:0.2em;padding-bottom:0.2em;text-align:center}body div.zettel-view .zettel-content h2{border-bottom:solid 1px #4682b4;margin-bottom:0.5em}body div.zettel-view .zettel-content h3{margin:0px 0px 0.4em 0px}body div.zettel-view .zettel-content h4{opacity:0.8}body div.zettel-view .zettel-content div#footnotes{margin-top:4em;border-top-style:groove;border-top-width:2px;font-size:0.9em}body div.zettel-view .zettel-content div#footnotes ol > li > p:only-of-type{display:inline;margin-right:0.5em}body div.zettel-view .zettel-content aside.footnote-inline{width:30%;padding-left:15px;margin-left:15px;float:right;background-color:#d3d3d3}body div.zettel-view .zettel-content .overflows{overflow:auto}body div.zettel-view .zettel-content code{margin:auto auto auto auto;font-size:100%}body div.zettel-view .zettel-content p code, li code, ol code{padding:0.2em 0.2em 0.2em 0.2em;background-color:#f5f2f0}body div.zettel-view .zettel-content pre{overflow:auto}body div.zettel-view .zettel-content dl dt{font-weight:bold}body div.zettel-view .zettel-content blockquote{background-color:#f9f9f9;border-left:solid 10px #cccccc;margin:1.5
|
|||
|
|
async=""
|
|||
|
|
id="MathJax-script"
|
|||
|
|
src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"
|
|||
|
|
></script>
|
|||
|
|
<link
|
|||
|
|
href="https://cdnjs.cloudflare.com/ajax/libs/prism/1.23.0/themes/prism.min.css"
|
|||
|
|
rel="stylesheet"
|
|||
|
|
/><link rel="preconnect" href="https://fonts.googleapis.com" /><link
|
|||
|
|
rel="preconnect"
|
|||
|
|
href="https://fonts.gstatic.com"
|
|||
|
|
crossorigin
|
|||
|
|
/><link
|
|||
|
|
href="https://fonts.googleapis.com/css2?family=IBM+Plex+Mono:ital,wght@0,100;0,200;0,300;0,400;0,500;0,600;0,700;1,100;1,200;1,300;1,400;1,500;1,600;1,700&family=IBM+Plex+Sans+Condensed:ital,wght@0,100;0,200;0,300;0,400;0,500;0,600;0,700;1,100;1,200;1,300;1,400;1,500;1,600;1,700&family=IBM+Plex+Sans:ital,wght@0,100;0,200;0,300;0,400;0,500;0,600;0,700;1,100;1,200;1,300;1,400;1,500;1,600;1,700&family=IBM+Plex+Serif:ital,wght@0,100;0,200;0,300;0,400;0,500;0,600;0,700;1,100;1,200;1,300;1,400;1,500;1,600;1,700&display=swap"
|
|||
|
|
rel="stylesheet"
|
|||
|
|
/>
|
|||
|
|
<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.23.0/components/prism-core.min.js"></script>
|
|||
|
|
<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.23.0/plugins/autoloader/prism-autoloader.min.js"></script>
|
|||
|
|
<style>
|
|||
|
|
body .ui.container,
|
|||
|
|
body ul {
|
|||
|
|
font-family: "IBM Plex Sans" !important;
|
|||
|
|
}
|
|||
|
|
body blockquote {
|
|||
|
|
border-left-width: 3px !important;
|
|||
|
|
font-style: italic;
|
|||
|
|
}
|
|||
|
|
.headerFont,
|
|||
|
|
.ui.header,
|
|||
|
|
body h1,
|
|||
|
|
h2,
|
|||
|
|
h3,
|
|||
|
|
h4,
|
|||
|
|
h5,
|
|||
|
|
h6 {
|
|||
|
|
font-family: "IBM Plex Sans Condensed" !important;
|
|||
|
|
}
|
|||
|
|
body p {
|
|||
|
|
line-height: 1.4;
|
|||
|
|
}
|
|||
|
|
.monoFont,
|
|||
|
|
body code,
|
|||
|
|
pre,
|
|||
|
|
tt {
|
|||
|
|
font-family: "IBM Plex Mono" !important;
|
|||
|
|
font-size: 12px !important;
|
|||
|
|
line-height: 1.4 !important;
|
|||
|
|
}
|
|||
|
|
</style>
|
|||
|
|
<!--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>useReducer</code></h1><p>The <code>useReducer</code> hook is best used in scenarios where you are manipulating state in a way that is too complex for the trivial <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: useState"><a href="React_useState.html">React_useState</a></span></span> use case. <code>useState</code> is best employed when you are updating a single value or toggling a boolean. If you are updating the state of an object or more complex data structure, it is often more efficient to employ <code>useReducer</code>.</p><p>This makes the code more manageable and also helps with separating state management from rendering.</p><h2 id="syntax">Syntax</h2><pre><code class="jsx language-jsx">const [state, dispatch] = useReducer(reducer, initialState);</code></pre><ul><li><code>initialState</code><ul><li>The starting state, typically an object</li></ul></li><li><code>reducer</code><ul><li>A pure function that accepts two parameters:<ul><li>The current state</li><li>An action object</li></ul></li><li>The reducer function must update the current state (immutably) and return the new state</li><li>We can think of the reducer as working in the same manner as <code>state</code>/<code>setState</code> in the <code>useState</code> hook. The functional role is the same, it is just that the reducer offers more than one type of update.</li></ul></li></ul><h3 id="example-reducer">Example reducer</h3><pre><code class="js language-js">function reducer(state, action) {
|
|||
|
|
let newState;
|
|||
|
|
switch (action.type) {
|
|||
|
|
case "increase":
|
|||
|
|
newState = { counter: state.counter + 1 };
|
|||
|
|
break;
|
|||
|
|
case "descrease":
|
|||
|
|
newState = { counter: state.counter - 1 };
|
|||
|
|
break;
|
|||
|
|
default:
|
|||
|
|
throw new Error();
|
|||
|
|
}
|
|||
|
|
return newState;
|
|||
|
|
}</code></pre><p>In this example we are updating an object with the following shape:</p><pre><code class="js language-js">{
|
|||
|
|
counter: 0,
|
|||
|
|
}</code></pre><p>This would be the <code>initialState</code> that we pass to the <code>useReducer</code> hook along with a reference to <code>reducer</code> above.</p><p>To update the state we would invoke the <code>dispatch</code> function which applies one of the actions defined in the reducer. For example the following dispatch increments the counter by one:</p><pre><code class="js language-js">dispatch({ type: "increase" });</code></pre><p>To view the updated value:</p><pre><code class="js language-js">console.log(state.counter);</code></pre><h3 id="refining-the-syntax">Refining the syntax</h3><p>Because React doesn’t mutate state, the reducer doesn’t directly modify the current state in the <code>state</code> variable, it creates a new instance of the state object on each update.</p><p>In the reducer example above this is achieved by declaring a variable <code>newState</code> that is updated by each <code>action</code> type and then returned. There is a more elegant way of doing this using spread syntax:</p><pre><code class="js language-js">function reducer(state, action) {
|
|||
|
|
switch (action.type) {
|
|||
|
|
case "increase":
|
|||
|
|
return { ...state, counter: state.counter + 1 };
|
|||
|
|
break;
|
|||
|
|
case "decrease":
|
|||
|
|
return { ...state, counter: state.counter - 1 };
|
|||
|
|
break;
|
|||
|
|
default:
|
|||
|
|
throw new Error();
|
|||
|
|
}
|
|||
|
|
}</code></pre><h3 id="including-payloads">Including payloads</h3><p>In the examples so far, we have updated the the state directly via the action type however it is also possible to pass data along with the <code>action.type</code> as <code>action.payload</code>.</p><p>For example:</p><pre><code class="js language-js">dispatch(
|
|||
|
|
{
|
|||
|
|
type: 'increase_by_payload'
|
|||
|
|
payload: 3,
|
|||
|
|
});</code></pre><p>Then we would update our reducer to handle this case:</p><pre><code class="js language-js">function reducer(state, action) {
|
|||
|
|
switch (action.type) {
|
|||
|
|
...
|
|||
|
|
case 'increase_by_payload':
|
|||
|
|
return {...state, counter: state.counter + action.payload}
|
|||
|
|
default:
|
|||
|
|
throw new Error();
|
|||
|
|
}
|
|||
|
|
}</code></pre></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>
|