2021-05-05 10:13:49 -07:00
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---
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id: 5a24c314108439a4d4036173
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title: 用 this.setState 設置狀態
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challengeType: 6
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forumTopicId: 301412
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dashedName: set-state-with-this-setstate
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---
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# --description--
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2021-10-18 08:17:43 -07:00
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前面的挑戰涵蓋了組件的 `state` 以及如何在 `constructor` 中初始化 state。 還有一種方法可以更改組件的 `state`。 React 提供了 `setState` 方法來更新組件的 `state`。 在組件類中調用 `setState` 方法如下所示:`this.setState()`,傳入鍵值對的對象, 其中鍵是 state 屬性,值是更新後的 state 數據。 例如,如果我們在 state 中存儲 `username`,並想要更新它,代碼如下所示:
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2021-05-05 10:13:49 -07:00
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```jsx
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this.setState({
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username: 'Lewis'
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});
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```
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2021-10-18 08:17:43 -07:00
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React 要求永遠不要直接修改 `state`,而是在 state 發生改變時始終使用 `this.setState()`。 此外,應該注意,React 可以批量處理多個 state 更新以提高性能。 這意味着通過 `setState` 方法進行的 state 更新可以是異步的。 `setState` 方法有一種替代語法可以解決異步問題, 雖然這很少用到,但是最好還是記住它! 有關詳細信息,請參閱[React 文檔](https://reactjs.org/docs/state-and-lifecycle.html#state-updates-may-be-asynchronous)。
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2021-05-05 10:13:49 -07:00
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# --instructions--
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2021-10-18 08:17:43 -07:00
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代碼編輯器中有一個 `button` 元素,它有一個 `onClick()` handler。 當 `button` 在瀏覽器中接收到單擊事件時觸發此 handler,並運行 `MyComponent` 中定義的 `handleClick` 方法。 在 `handleClick` 方法中,使用 `this.setState()` 更新組件的 `state`。 設置 `state` 中的 `name` 屬性爲字符串 `React Rocks!`。
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2021-05-05 10:13:49 -07:00
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單擊按鈕查看渲染的 state 的更新。 如果不完全理解單擊處理程序代碼在此時的工作方式,請不要擔心。 在接下來的挑戰中會有講述。
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# --hints--
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`MyComponent` 的 state 應該使用鍵值對 `{ name: Initial State }` 來初始化。
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```js
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assert(
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Enzyme.mount(React.createElement(MyComponent)).state('name') ===
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'Initial State'
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);
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```
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2021-11-17 06:20:53 -08:00
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`MyComponent` 應該渲染一個 `h1` 標題元素。
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2021-05-05 10:13:49 -07:00
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```js
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assert(Enzyme.mount(React.createElement(MyComponent)).find('h1').length === 1);
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```
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2021-11-17 06:20:53 -08:00
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渲染的 `h1` 標題元素應包含從組件狀態渲染的文本。
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2021-05-05 10:13:49 -07:00
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```js
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async () => {
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const waitForIt = (fn) =>
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new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250));
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const mockedComponent = Enzyme.mount(React.createElement(MyComponent));
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const first = () => {
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mockedComponent.setState({ name: 'TestName' });
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return waitForIt(() => mockedComponent.html());
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};
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const firstValue = await first();
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assert(/<h1>TestName<\/h1>/.test(firstValue));
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};
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```
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調用 `MyComponent` 的 `handleClick` 方法應該將 state 的 name 屬性設置爲 `React Rocks!`。
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```js
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async () => {
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const waitForIt = (fn) =>
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new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250));
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const mockedComponent = Enzyme.mount(React.createElement(MyComponent));
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const first = () => {
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mockedComponent.setState({ name: 'Before' });
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return waitForIt(() => mockedComponent.state('name'));
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};
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const second = () => {
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mockedComponent.instance().handleClick();
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return waitForIt(() => mockedComponent.state('name'));
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};
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const firstValue = await first();
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const secondValue = await second();
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assert(firstValue === 'Before' && secondValue === 'React Rocks!');
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};
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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(<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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name: 'Initial State'
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};
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this.handleClick = this.handleClick.bind(this);
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}
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handleClick() {
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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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render() {
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return (
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<div>
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<button onClick={this.handleClick}>Click Me</button>
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<h1>{this.state.name}</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 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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name: 'Initial State'
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};
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this.handleClick = this.handleClick.bind(this);
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}
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handleClick() {
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// Change code below this line
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this.setState({
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name: 'React Rocks!'
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});
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// Change code above this line
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}
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render() {
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return (
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<div>
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<button onClick = {this.handleClick}>Click Me</button>
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<h1>{this.state.name}</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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