88 lines
4.6 KiB
Markdown
88 lines
4.6 KiB
Markdown
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---
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id: 5a24c314108439a4d4036174
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title: Bind 'this' to a Class Method
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challengeType: 6
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isRequired: false
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videoUrl: ''
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localeTitle: 将'this'绑定到类方法
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---
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## Description
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<section id="description">除了设置和更新<code>state</code> ，您还可以为组件类定义方法。类方法通常需要使用<code>this</code>关键字，以便它可以访问方法范围内的类（例如<code>state</code>和<code>props</code> ）上的属性。有几种方法可以让您的类方法访问<code>this</code> 。一个常用的方法是显式绑定<code>this</code>所以在构造<code>this</code>组件已初始化变为绑定到类方法。您可能已经注意到最后一个挑战使用<code>this.handleClick = this.handleClick.bind(this)</code>作为构造函数中的<code>handleClick</code>方法。然后，当您在类方法中调用类似<code>this.setState()</code>的函数时， <code>this</code>引用该类，并且不会被<code>undefined</code> 。 <strong>注意：</strong> <code>this</code>关键字是JavaScript中最令人困惑的方面之一，但它在React中起着重要作用。虽然这里的行为是完全正常的，但这些课程并不是<code>this</code>进行深入审查的地方，所以如果上述内容令人困惑，请参考其他课程！ </section>
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## Instructions
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<section id="instructions">代码编辑器具有与组件<code>state</code>保持跟踪的项目计数。它还有一个方法，允许您增加此项目计数。但是，该方法不起作用，因为它使用未定义的<code>this</code>关键字。通过明确地结合修复它<code>this</code>到<code>addItem()</code>在组件的构造方法。接下来，将单击处理程序添加到render方法中的<code>button</code>元素。当按钮收到click事件时，它应该触发<code>addItem()</code>方法。请记住，传递给<code>onClick</code>处理程序的方法需要花括号，因为它应该直接解释为JavaScript。完成上述步骤后，您应该可以单击按钮并查看HTML中的项目计数增量。 </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>应该返回一个<code>div</code>元素，它按顺序包装两个元素，一个按钮和一个<code>h1</code>元素。
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testString: 'assert(Enzyme.mount(React.createElement(MyComponent)).find("div").length === 1 && Enzyme.mount(React.createElement(MyComponent)).find("div").childAt(0).type() === "button" && Enzyme.mount(React.createElement(MyComponent)).find("div").childAt(1).type() === "h1", "<code>MyComponent</code> should return a <code>div</code> element which wraps two elements, a button and an <code>h1</code> element, in that order.");'
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- text: '<code>MyComponent</code>的状态应使用键值对<code>{ itemCount: 0 }</code>初始化。'
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testString: 'assert(Enzyme.mount(React.createElement(MyComponent)).state("itemCount") === 0, "The state of <code>MyComponent</code> should initialize with the key value pair <code>{ itemCount: 0 }</code>.");'
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- text: 单击<code>button</code>元素应该运行<code>addItem</code>方法并将状态<code>itemCount</code>递增<code>1</code> 。
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testString: 'async () => { const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250)); const mockedComponent = Enzyme.mount(React.createElement(MyComponent)); const first = () => { mockedComponent.setState({ itemCount: 0 }); return waitForIt(() => mockedComponent.state("itemCount")); }; const second = () => { mockedComponent.find("button").simulate("click"); return waitForIt(() => mockedComponent.state("itemCount")); }; const firstValue = await first(); const secondValue = await second(); assert(firstValue === 0 && secondValue === 1, "Clicking the <code>button</code> element should run the <code>addItem</code> method and increment the state <code>itemCount</code> by <code>1</code>."); };'
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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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itemCount: 0
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};
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// change code below this line
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// change code above this line
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}
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addItem() {
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this.setState({
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itemCount: this.state.itemCount + 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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{ /* change code below this line */ }
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<button>Click Me</button>
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{ /* change code above this line */ }
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<h1>Current Item Count: {this.state.itemCount}</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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