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id	title	challengeType	isRequired	videoUrl	localeTitle
5a24c314108439a4d4036180	Optimize Re-Renders with shouldComponentUpdate	6	false		使用shouldComponentUpdate优化重新渲染

Description

到目前为止，如果任何组件接收到新的state或新的props ，它会重新呈现自己及其所有子项。这通常没问题。但是React提供了一个生命周期方法，您可以在子组件接收新state或props时调用它，并特别声明组件是否应该更新。方法是shouldComponentUpdate() ，它将nextProps和nextState作为参数。此方法是优化性能的有用方法。例如，默认行为是您的组件在收到新props时重新渲染，即使props未更改。您可以使用shouldComponentUpdate()通过比较props来防止这种情况。该方法必须返回一个boolean值，告诉React是否更新组件。您可以将当前道具（ this.props ）与下一个道具（ nextProps ）进行比较，以确定是否需要更新，并相应地返回true或false 。

Instructions

shouldComponentUpdate()方法添加到名为OnlyEvens的组件中。目前，此方法返回true因此每次收到新props时， OnlyEvens重新渲染。修改方法，以便OnlyEvens更新仅当value的新道具的是偶数。单击“ Add按钮，在触发其他生命周期挂钩时，在浏览器控制台中查看事件的顺序。

Tests

tests:
  - text: <code>Controller</code>组件应将<code>OnlyEvens</code>组件呈现为<code>OnlyEvens</code>组件。
    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.");'
  - text: 应该在<code>OnlyEvens</code>组件上定义<code>shouldComponentUpdate</code>方法。
    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.");'
  - text: <code>OnlyEvens</code>组件应返回一个<code>h1</code>标记，该标记呈现<code>this.props.value</code>的值。
    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>."); }; '
  - text: <code>OnlyEvens</code>只有在<code>nextProps.value</code>为偶数<code>OnlyEvens</code>应该重新渲染。
    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."); }; '

Challenge Seed

class OnlyEvens extends React.Component {
  constructor(props) {
    super(props);
  }
  shouldComponentUpdate(nextProps, nextState) {
    console.log('Should I update?');
     // change code below this line
    return true;
     // change code above this line
  }
  componentWillReceiveProps(nextProps) {
    console.log('Receiving new props...');
  }
  componentDidUpdate() {
    console.log('Component re-rendered.');
  }
  render() {
    return <h1>{this.props.value}</h1>
  }
};

class Controller extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      value: 0
    };
    this.addValue = this.addValue.bind(this);
  }
  addValue() {
    this.setState({
      value: this.state.value + 1
    });
  }
  render() {
    return (
      <div>
        <button onClick={this.addValue}>Add</button>
        <OnlyEvens value={this.state.value}/>
      </div>
    );
  }
};

After Test

console.info('after the test');

Solution

// solution required

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