84 lines
3.3 KiB
Markdown
84 lines
3.3 KiB
Markdown
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---
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id: 5a24c314108439a4d403617d
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title: Use the Lifecycle Method componentDidMount
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challengeType: 6
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isRequired: false
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videoUrl: ''
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localeTitle: 使用生命周期方法componentDidMount
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---
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## Description
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<section id="description">大多数Web开发人员在某些时候需要调用API端点来检索数据。如果您正在使用React，那么知道执行此操作的位置非常重要。 React的最佳实践是在生命周期方法<code>componentDidMount()</code>中对服务器进行API调用或任何调用。将组件安装到DOM后调用此方法。此处对<code>setState()</code>任何调用都将触发组件的重新呈现。在此方法中调用API并使用API返回的数据设置状态时，一旦收到数据，它将自动触发更新。 </section>
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## Instructions
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<section id="instructions"> <code>componentDidMount()</code>有一个模拟API调用。它在2.5秒后设置状态以模拟调用服务器以检索数据。此示例请求站点的当前活动用户总数。在render方法中，在<code>h1</code>呈现<code>activeUsers</code>的值。观看预览中发生的事情，并随时更改超时以查看不同的效果。 </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: <code>MyComponent</code>应该呈现一个包含<code>h1</code>标记的<code>div</code>元素。
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testString: 'assert((() => { const mockedComponent = Enzyme.mount(React.createElement(MyComponent)); return (mockedComponent.find("div").length === 1 && mockedComponent.find("h1").length === 1); })(), "<code>MyComponent</code> should render a <code>div</code> element which wraps an <code>h1</code> tag.");'
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- text: 应使用<code>componentDidMount</code>的超时函数更新组件状态。
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testString: 'assert((() => { const mockedComponent = Enzyme.mount(React.createElement(MyComponent)); return new RegExp("setTimeout(.|\n)+setState(.|\n)+activeUsers").test(String(mockedComponent.instance().componentDidMount)); })(), "Component state should be updated with a timeout function in <code>componentDidMount</code>.");'
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- text: <code>h1</code>标记应该从<code>MyComponent</code>的状态呈现<code>activeUsers</code>值。
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testString: 'async () => { const mockedComponent = Enzyme.mount(React.createElement(MyComponent)); const first = () => { mockedComponent.setState({ activeUsers: 1237 }); return mockedComponent.find("h1").text(); }; const second = () => { mockedComponent.setState({ activeUsers: 1000 }); return mockedComponent.find("h1").text(); }; assert(new RegExp("1237").test(first()) && new RegExp("1000").test(second()), "The <code>h1</code> tag should render the <code>activeUsers</code> value from <code>MyComponent</code>'s state."); }; '
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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 MyComponent 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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activeUsers: null
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};
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}
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componentDidMount() {
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setTimeout( () => {
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this.setState({
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activeUsers: 1273
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});
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}, 2500);
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}
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render() {
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return (
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<div>
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<h1>Active Users: { /* change code here */ }</h1>
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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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console.info('after the test');
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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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// solution required
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```
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</section>
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