153 lines
6.5 KiB
Markdown
153 lines
6.5 KiB
Markdown
---
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id: 5a24c314108439a4d4036180
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title: Optimize Re-Renders with shouldComponentUpdate
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challengeType: 6
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isRequired: false
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---
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## Description
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<section id='description'>
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So far, if any component receives new <code>state</code> or new <code>props</code>, it re-renders itself and all its children. This is usually okay. But React provides a lifecycle method you can call when child components receive new <code>state</code> or <code>props</code>, and declare specifically if the components should update or not. The method is <code>shouldComponentUpdate()</code>, and it takes <code>nextProps</code> and <code>nextState</code> as parameters.
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This method is a useful way to optimize performance. For example, the default behavior is that your component re-renders when it receives new <code>props</code>, even if the <code>props</code> haven't changed. You can use <code>shouldComponentUpdate()</code> to prevent this by comparing the <code>props</code>. The method must return a <code>boolean</code> value that tells React whether or not to update the component. You can compare the current props (<code>this.props</code>) to the next props (<code>nextProps</code>) to determine if you need to update or not, and return <code>true</code> or <code>false</code> accordingly.
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</section>
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## Instructions
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<section id='instructions'>
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The <code>shouldComponentUpdate()</code> method is added in a component called <code>OnlyEvens</code>. Currently, this method returns <code>true</code> so <code>OnlyEvens</code> re-renders every time it receives new <code>props</code>. Modify the method so <code>OnlyEvens</code> updates only if the <code>value</code> of its new props is even. Click the <code>Add</code> button and watch the order of events in your browser's console as the other lifecycle hooks are triggered.
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</section>
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## Tests
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<section id='tests'>
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```yml
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tests:
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- text: The <code>Controller</code> component should render the <code>OnlyEvens</code> component as a child.
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testString: assert((() => { const mockedComponent = Enzyme.mount(React.createElement(Controller)); return mockedComponent.find('Controller').length === 1 && mockedComponent.find('OnlyEvens').length === 1; })(), 'The <code>Controller</code> component should render the <code>OnlyEvens</code> component as a child.');
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- text: The <code>shouldComponentUpdate</code> method should be defined on the <code>OnlyEvens</code> component.
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testString: assert((() => { const child = React.createElement(OnlyEvens).type.prototype.shouldComponentUpdate.toString().replace(/s/g,''); return child !== 'undefined'; })(), 'The <code>shouldComponentUpdate</code> method should be defined on the <code>OnlyEvens</code> component.');
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- text: The <code>OnlyEvens</code> component should return an <code>h1</code> tag which renders the value of <code>this.props.value</code>.
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testString: 'async () => { const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250)); const mockedComponent = Enzyme.mount(React.createElement(Controller)); const first = () => { mockedComponent.setState({ value: 1000 }); return waitForIt(() => mockedComponent.find(''h1'').html()); }; const second = () => { mockedComponent.setState({ value: 10 }); return waitForIt(() => mockedComponent.find(''h1'').html()); }; const firstValue = await first(); const secondValue = await second(); assert(firstValue === ''<h1>1000</h1>'' && secondValue === ''<h1>10</h1>'', ''The <code>OnlyEvens</code> component should return an <code>h1</code> tag which renders the value of <code>this.props.value</code>.''); }; '
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- text: <code>OnlyEvens</code> should re-render only when <code>nextProps.value</code> is even.
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testString: 'async () => { const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250)); const mockedComponent = Enzyme.mount(React.createElement(Controller)); const first = () => { mockedComponent.setState({ value: 8 }); return waitForIt(() => mockedComponent.find(''h1'').text()); }; const second = () => { mockedComponent.setState({ value: 7 }); return waitForIt(() => mockedComponent.find(''h1'').text()); }; const third = () => { mockedComponent.setState({ value: 42 }); return waitForIt(() => mockedComponent.find(''h1'').text()); }; const firstValue = await first(); const secondValue = await second(); const thirdValue = await third(); assert(firstValue === ''8'' && secondValue === ''8'' && thirdValue === ''42'', ''<code>OnlyEvens</code> should re-render only when <code>nextProps.value</code> is even.''); }; '
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```
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</section>
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## Challenge Seed
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<section id='challengeSeed'>
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<div id='jsx-seed'>
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```jsx
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class OnlyEvens extends React.Component {
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constructor(props) {
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super(props);
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}
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shouldComponentUpdate(nextProps, nextState) {
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console.log('Should I update?');
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// change code below this line
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return true;
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// change code above this line
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}
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componentWillReceiveProps(nextProps) {
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console.log('Receiving new props...');
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}
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componentDidUpdate() {
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console.log('Component re-rendered.');
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}
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render() {
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return <h1>{this.props.value}</h1>
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}
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};
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class Controller extends React.Component {
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constructor(props) {
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super(props);
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this.state = {
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value: 0
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};
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this.addValue = this.addValue.bind(this);
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}
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addValue() {
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this.setState({
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value: this.state.value + 1
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});
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}
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render() {
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return (
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<div>
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<button onClick={this.addValue}>Add</button>
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<OnlyEvens value={this.state.value}/>
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</div>
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);
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}
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};
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```
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</div>
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### After Test
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<div id='jsx-teardown'>
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```js
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ReactDOM.render(<Controller />, document.getElementById('root'))
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```
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</div>
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</section>
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## Solution
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<section id='solution'>
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```js
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class OnlyEvens extends React.Component {
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constructor(props) {
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super(props);
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}
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shouldComponentUpdate(nextProps, nextState) {
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console.log('Should I update?');
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// change code below this line
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return nextProps.value % 2 === 0;
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// change code above this line
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}
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componentWillReceiveProps(nextProps) {
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console.log('Receiving new props...');
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}
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componentDidUpdate() {
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console.log('Component re-rendered.');
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}
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render() {
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return <h1>{this.props.value}</h1>
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}
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};
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class Controller extends React.Component {
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constructor(props) {
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super(props);
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this.state = {
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value: 0
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};
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this.addValue = this.addValue.bind(this);
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}
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addValue() {
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this.setState({
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value: this.state.value + 1
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});
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}
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render() {
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return (
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<div>
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<button onClick={this.addValue}>Add</button>
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<OnlyEvens value={this.state.value}/>
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</div>
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);
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}
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};
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```
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</section>
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