188 lines
5.2 KiB
Markdown
188 lines
5.2 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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forumTopicId: 301398
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dashedName: optimize-re-renders-with-shouldcomponentupdate
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---
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# --description--
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So far, if any component receives new `state` or new `props`, 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 `state` or `props`, and declare specifically if the components should update or not. The method is `shouldComponentUpdate()`, and it takes `nextProps` and `nextState` 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 `props`, even if the `props` haven't changed. You can use `shouldComponentUpdate()` to prevent this by comparing the `props`. The method must return a `boolean` value that tells React whether or not to update the component. You can compare the current props (`this.props`) to the next props (`nextProps`) to determine if you need to update or not, and return `true` or `false` accordingly.
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# --instructions--
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The `shouldComponentUpdate()` method is added in a component called `OnlyEvens`. Currently, this method returns `true` so `OnlyEvens` re-renders every time it receives new `props`. Modify the method so `OnlyEvens` updates only if the `value` of its new props is even. Click the `Add` button and watch the order of events in your browser's console as the lifecycle hooks are triggered.
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# --hints--
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The `Controller` component should render the `OnlyEvens` component as a child.
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```js
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assert(
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(() => {
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const mockedComponent = Enzyme.mount(React.createElement(Controller));
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return (
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mockedComponent.find('Controller').length === 1 &&
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mockedComponent.find('OnlyEvens').length === 1
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);
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})()
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);
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```
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The `shouldComponentUpdate` method should be defined on the `OnlyEvens` component.
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```js
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assert(
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(() => {
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const child = React.createElement(OnlyEvens)
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.type.prototype.shouldComponentUpdate.toString()
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.replace(/s/g, '');
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return child !== 'undefined';
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})()
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);
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```
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The `OnlyEvens` component should return an `h1` tag which renders the value of `this.props.value`.
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```js
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(() => {
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const mockedComponent = Enzyme.mount(React.createElement(Controller));
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const first = () => {
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mockedComponent.setState({ value: 1000 });
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return mockedComponent.find('h1').html();
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};
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const second = () => {
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mockedComponent.setState({ value: 10 });
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return mockedComponent.find('h1').html();
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};
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const firstValue = first();
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const secondValue = second();
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assert(firstValue === '<h1>1000</h1>' && secondValue === '<h1>10</h1>');
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})();
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```
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`OnlyEvens` should re-render only when `nextProps.value` is even.
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```js
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(() => {
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const mockedComponent = Enzyme.mount(React.createElement(Controller));
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const first = () => {
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mockedComponent.setState({ value: 8 });
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return mockedComponent.find('h1').text();
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};
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const second = () => {
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mockedComponent.setState({ value: 7 });
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return mockedComponent.find('h1').text();
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};
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const third = () => {
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mockedComponent.setState({ value: 42 });
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return mockedComponent.find('h1').text();
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};
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const firstValue = first();
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const secondValue = second();
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const thirdValue = third();
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assert(firstValue === '8' && secondValue === '8' && thirdValue === '42');
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})();
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```
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# --seed--
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## --after-user-code--
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```jsx
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ReactDOM.render(<Controller />, document.getElementById('root'));
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```
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## --seed-contents--
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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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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(state => ({
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value: 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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# --solutions--
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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 nextProps.value % 2 === 0;
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// Change code above this line
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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(state => ({
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value: 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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