2018-10-10 22:03:03 +00:00
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id: 5a24c314108439a4d4036176
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2021-03-05 15:43:24 +00:00
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title: 使用 State 切换元素
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2018-10-10 22:03:03 +00:00
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challengeType: 6
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2020-09-17 16:13:42 +00:00
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forumTopicId: 301421
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2021-01-13 02:31:00 +00:00
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dashedName: use-state-to-toggle-an-element
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2018-10-10 22:03:03 +00:00
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---
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2020-12-16 07:37:30 +00:00
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# --description--
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2021-03-05 15:43:24 +00:00
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有时可能在更新状态的时候想知道上一个状态是什么。 但是状态更新是异步的,这意味着 React 可能会把多个 `setState()` 集中在一起批量更新。 所以计算下一个值时 `this.state` 或者 `this.props` 不能作为当前值。 所以最好不要写如下的代码:
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2020-09-17 16:13:42 +00:00
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2021-01-12 16:22:19 +00:00
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```jsx
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2020-09-17 16:13:42 +00:00
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this.setState({
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counter: this.state.counter + this.props.increment
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});
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```
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2021-03-05 15:43:24 +00:00
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正确的做法是,给 `setState` 传入一个函数,这个函数可以访问 state 和 props。 给 `setState` 传入函数可以保证 state 和 props 是正确的值。 代码可以重写为这样:
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2020-09-17 16:13:42 +00:00
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2021-01-12 16:22:19 +00:00
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```jsx
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2020-09-17 16:13:42 +00:00
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this.setState((state, props) => ({
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counter: state.counter + props.increment
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}));
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```
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2021-03-05 15:43:24 +00:00
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如果只需要 `state`,那么用下面没有 `props` 的格式也是可以的:
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2020-09-17 16:13:42 +00:00
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2021-01-12 16:22:19 +00:00
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```jsx
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2020-09-17 16:13:42 +00:00
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this.setState(state => ({
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counter: state.counter + 1
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}));
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```
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2021-03-05 15:43:24 +00:00
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注意一定要把 object 放在括号里,否则 JavaScript 会认为这只是代码片段。
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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# --instructions--
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2018-10-10 22:03:03 +00:00
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2021-03-05 15:43:24 +00:00
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`MyComponent` 有一个初始值为 `false` 的`visibility` 属性。 如果 `visibility` 的值为 true,render 方法返回一个视图,如果为 false,返回另一个视图。
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2020-12-16 07:37:30 +00:00
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2021-03-05 15:43:24 +00:00
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目前,无法更新组件 `state` 中的 `visibility` 属性, 该值应在 true 和 false 之间来回切换。 按钮上有一个单击处理程序,它触发一个名为 `toggleVisibility()` 的类方法。 给函数传入 `setState` 来定义此方法,以便 `visibility` 的 `state` 在调用方法时切换到相反的值。 如果 `visibility` 是 `false`,则该方法将其设置为`true`,反之亦然。
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2020-12-16 07:37:30 +00:00
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2021-03-05 15:43:24 +00:00
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最后,单击按钮以查看基于其 `state` 的组件的条件渲染。
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2018-10-10 22:03:03 +00:00
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2021-03-05 15:43:24 +00:00
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**提示:** 不要忘记将 `this` 关键字绑定到 `constructor` 中的方法上!
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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# --hints--
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2018-10-10 22:03:03 +00:00
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2021-03-05 15:43:24 +00:00
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`MyComponent` 应该返回一个 `div` 元素,其中包含一个 `button` 元素。
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2018-10-10 22:03:03 +00:00
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```js
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2020-12-16 07:37:30 +00:00
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assert.strictEqual(
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Enzyme.mount(React.createElement(MyComponent)).find('div').find('button')
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.length,
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1
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);
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2018-10-10 22:03:03 +00:00
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```
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2021-03-05 15:43:24 +00:00
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`MyComponent` 应该使用设置为 `false` 的 `visibility` 属性来初始化其 state。
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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```js
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assert.strictEqual(
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Enzyme.mount(React.createElement(MyComponent)).state('visibility'),
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false
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);
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```
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2018-10-10 22:03:03 +00:00
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2021-03-05 15:43:24 +00:00
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单击按钮元素应在 `true` 和 `false` 之间切换 `visibility` 属性的状态。
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2020-09-17 16:13:42 +00:00
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2018-10-10 22:03:03 +00:00
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```js
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2021-02-06 04:42:36 +00:00
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(() => {
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2020-12-16 07:37:30 +00:00
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const mockedComponent = Enzyme.mount(React.createElement(MyComponent));
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const first = () => {
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mockedComponent.setState({ visibility: false });
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2021-02-06 04:42:36 +00:00
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return mockedComponent.state('visibility');
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2020-12-16 07:37:30 +00:00
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};
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const second = () => {
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mockedComponent.find('button').simulate('click');
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2021-02-06 04:42:36 +00:00
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return mockedComponent.state('visibility');
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2020-12-16 07:37:30 +00:00
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};
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const third = () => {
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mockedComponent.find('button').simulate('click');
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return mockedComponent.state('visibility');
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2020-12-16 07:37:30 +00:00
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};
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2021-02-06 04:42:36 +00:00
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const firstValue = first();
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const secondValue = second();
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const thirdValue = third();
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2020-12-16 07:37:30 +00:00
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assert(!firstValue && secondValue && !thirdValue);
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2021-02-06 04:42:36 +00:00
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})();
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2018-10-10 22:03:03 +00:00
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```
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2020-08-13 15:24:35 +00:00
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2021-03-05 15:43:24 +00:00
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应该给 `setState` 传入一个匿名函数。
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2020-12-16 07:37:30 +00:00
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```js
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const paramRegex = '[a-zA-Z$_]\\w*(,[a-zA-Z$_]\\w*)?';
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assert(
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new RegExp(
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'this\\.setState\\((function\\(' +
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paramRegex +
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'\\){|([a-zA-Z$_]\\w*|\\(' +
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paramRegex +
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'\\))=>)'
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).test(__helpers.removeWhiteSpace(code))
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);
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```
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2021-03-05 15:43:24 +00:00
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不要在 `setState` 里面使用 `this`。
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2020-12-16 07:37:30 +00:00
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```js
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assert(!/this\.setState\([^}]*this/.test(code));
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```
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2021-01-13 02:31:00 +00:00
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# --seed--
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## --after-user-code--
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```jsx
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ReactDOM.render(<MyComponent />, document.getElementById('root'));
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```
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## --seed-contents--
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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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visibility: false
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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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if (this.state.visibility) {
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return (
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<div>
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<button onClick={this.toggleVisibility}>Click Me</button>
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<h1>Now you see me!</h1>
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</div>
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);
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} else {
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return (
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<div>
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<button onClick={this.toggleVisibility}>Click Me</button>
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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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```
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2020-12-16 07:37:30 +00:00
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# --solutions--
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2021-01-13 02:31:00 +00:00
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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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visibility: false
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};
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this.toggleVisibility = this.toggleVisibility.bind(this);
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}
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toggleVisibility() {
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this.setState(state => ({
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visibility: !state.visibility
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}));
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}
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render() {
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if (this.state.visibility) {
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return (
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<div>
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<button onClick={this.toggleVisibility}>Click Me</button>
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<h1>Now you see me!</h1>
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</div>
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);
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} else {
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return (
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<div>
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<button onClick={this.toggleVisibility}>Click Me</button>
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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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```
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