147 lines
3.9 KiB
Markdown
147 lines
3.9 KiB
Markdown
---
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id: 5a24c314108439a4d4036177
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title: 写一个简单的计数器
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challengeType: 6
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forumTopicId: 301425
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dashedName: write-a-simple-counter
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---
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# --description--
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你可以结合目前为止所涵盖的概念来设计更复杂的有状态组件。这包括初始化`state`，编写设置`state`的方法，以及指定单击处理程序来触发这些方法。
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# --instructions--
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`Counter`组件跟踪`state`中的`count`值。有两个按钮分别调用`increment()`和`decrement()`方法。编写这些方法，使计数器值在单击相应按钮时增加或减少 1。另外，创建一个`reset()`方法，当单击 reset 按钮时，把计数设置为 0。
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**注意：** 确保你没有修改按钮的`classNames`。另外，请记住在构造函数中为新创建的方法添加必要的绑定。
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# --hints--
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`Counter`应该返回一个`div`元素，它包含三个按钮，按钮内容依次是`Increment!`、`Decrement!`、`Reset`。
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```js
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assert(
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(() => {
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const mockedComponent = Enzyme.mount(React.createElement(Counter));
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return (
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mockedComponent.find('.inc').text() === 'Increment!' &&
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mockedComponent.find('.dec').text() === 'Decrement!' &&
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mockedComponent.find('.reset').text() === 'Reset'
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);
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})()
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);
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```
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`Counter`应该使用设置为`0`的`count`属性初始化 state。
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```js
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const mockedComponent = Enzyme.mount(React.createElement(Counter));
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assert(mockedComponent.find('h1').text() === 'Current Count: 0');
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```
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单击 increment 按钮应将计数增加`1`。
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```js
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const mockedComponent = Enzyme.mount(React.createElement(Counter));
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mockedComponent.find('.inc').simulate('click');
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assert(mockedComponent.find('h1').text() === 'Current Count: 1');
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```
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单击 decrement 按钮应将计数减少`1`。
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```js
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const mockedComponent = Enzyme.mount(React.createElement(Counter));
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mockedComponent.find('.dec').simulate('click');
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assert(mockedComponent.find('h1').text() === 'Current Count: -1');
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```
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单击 reset 按钮应将计数重置为`0`。
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```js
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const mockedComponent = Enzyme.mount(React.createElement(Counter));
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mockedComponent.setState({ count: 5 });
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const currentCountElement = mockedComponent.find('h1');
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assert(currentCountElement.text() === 'Current Count: 5');
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mockedComponent.find('.reset').simulate('click');
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assert(currentCountElement.text() === 'Current Count: 0');
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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(<Counter />, document.getElementById('root'))
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```
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## --seed-contents--
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```jsx
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class Counter 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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count: 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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// Change code below this line
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// Change code above this line
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render() {
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return (
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<div>
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<button className='inc' onClick={this.increment}>Increment!</button>
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<button className='dec' onClick={this.decrement}>Decrement!</button>
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<button className='reset' onClick={this.reset}>Reset</button>
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<h1>Current Count: {this.state.count}</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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# --solutions--
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```jsx
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class Counter 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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count: 0
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};
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this.increment = this.increment.bind(this);
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this.decrement = this.decrement.bind(this);
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this.reset = this.reset.bind(this);
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}
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reset() {
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this.setState({
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count: 0
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});
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}
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increment() {
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this.setState(state => ({
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count: state.count + 1
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}));
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}
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decrement() {
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this.setState(state => ({
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count: state.count - 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 className='inc' onClick={this.increment}>Increment!</button>
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<button className='dec' onClick={this.decrement}>Decrement!</button>
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<button className='reset' onClick={this.reset}>Reset</button>
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<h1>Current Count: {this.state.count}</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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