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