React 表单与事件
本章节我们将讨论如何在 React 中使用表单。
HTML 表单元素与 React 中的其他 DOM 元素有所不同,因为表单元素生来就保留一些内部状态。
在 HTML 当中，像 <input>, <textarea>, 和 <select> 这类表单元素会维持自身状态，并根据用户输入进行更新。但在React中，可变的状态通常保存在组件的状态属性中，并且只能用 setState() 方法进行更新。
一个简单的实例
在实例中我们设置了输入框 input 值value = {this.state.data}。在输入框值发生变化时我们可以更新 state。我们可以使用onChange事件来监听 input 的变化，并修改 state。
React 实例
class HelloMessage extends React.Component { constructor(props) { super(props); this.state = {value: 'Hello Runoob!'}; this.handleChange = this.handleChange.bind(this); } handleChange(event) { this.setState({value: event.target.value}); } render() { var value = this.state.value; return <div> <input type="text" value={value} onChange={this.handleChange} /> <h4>{value}</h4> </div>; } } const root = ReactDOM.createRoot(document.getElementById("root")); root.render( <HelloMessage /> );
上面的代码将渲染出一个值为 Hello Runoob! 的 input 元素，并通过 onChange 事件响应更新用户输入的值。
实例 2
在以下实例中我们将为大家演示如何在子组件上使用表单。onChange方法将触发 state 的更新并将更新的值传递到子组件的输入框的value上来重新渲染界面。
你需要在父组件通过创建事件句柄 (handleChange) ，并作为 prop (updateStateProp) 传递到你的子组件上。
React 实例
class Content extends React.Component { render() { return ( <div> <input type="text" value={this.props.myDataProp} onChange={this.props.updateStateProp} /> <h4>{this.props.myDataProp}</h4> </div> ); } } class HelloMessage extends React.Component { constructor(props) { super(props); this.state = { value: 'Hello Runoob!' }; this.handleChange = this.handleChange.bind(this); } handleChange(event) { this.setState({ value: event.target.value }); } render() { var value = this.state.value; return ( <div> <Content myDataProp={value} updateStateProp={this.handleChange} /> </div> ); } } const root = ReactDOM.createRoot(document.getElementById('root')); root.render(<HelloMessage />);
Select 下拉菜单
在 React 中，不使用 selected 属性，而在根 select 标签上用 value 属性来表示选中项。
React 实例
class FlavorForm extends React.Component { constructor(props) { super(props); this.state = {value: 'coconut'}; this.handleChange = this.handleChange.bind(this); this.handleSubmit = this.handleSubmit.bind(this); } handleChange(event) { this.setState({value: event.target.value}); } handleSubmit(event) { alert('Your favorite flavor is: ' + this.state.value); event.preventDefault(); } render() { return ( <form onSubmit={this.handleSubmit}> <label> 选择您最喜欢的网站 <select value={this.state.value} onChange={this.handleChange}> <option value="gg">Google</option> <option value="rn">Runoob</option> <option value="tb">Taobao</option> <option value="fb">Facebook</option> </select> </label> <input type="submit" value="提交" /> </form> ); } } const root = ReactDOM.createRoot(document.getElementById("root")); root.render( <FlavorForm /> );
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多个表单
当你有处理多个 input 元素时，你可以通过给每个元素添加一个 name 属性，来让处理函数根据 event.target.name 的值来选择做什么。
React 实例
class Reservation extends React.Component { constructor(props) { super(props); this.state = { isGoing: true, numberOfGuests: 2 }; this.handleInputChange = this.handleInputChange.bind(this); } handleInputChange(event) { const target = event.target; const value = target.type === 'checkbox' ? target.checked : target.value; const name = target.name; this.setState({ [name]: value }); } render() { return ( <form> <label> 是否离开: <input name="isGoing" type="checkbox" checked={this.state.isGoing} onChange={this.handleInputChange} /> </label> <br /> <label> 访客数: <input name="numberOfGuests" type="number" value={this.state.numberOfGuests} onChange={this.handleInputChange} /> </label> </form> ); } }
React 事件
以下实例演示通过 onClick 事件来修改数据：
React 实例
class HelloMessage extends React.Component { constructor(props) { super(props); this.state = {value: 'Hello Runoob!'}; this.handleChange = this.handleChange.bind(this); } handleChange(event) { this.setState({value: '菜鸟教程'}) } render() { var value = this.state.value; return <div> <button onClick={this.handleChange}>点我</button> <h4>{value}</h4> </div>; } } const root = ReactDOM.createRoot(document.getElementById("root")); root.render( <HelloMessage /> );
当你需要从子组件中更新父组件的state时，你需要在父组件通过创建事件句柄 (handleChange) ，并作为 prop (updateStateProp) 传递到你的子组件上。实例如下：
React 实例
class Content extends React.Component { render() { return <div> <button onClick = {this.props.updateStateProp}>点我</button> <h4>{this.props.myDataProp}</h4> </div> } } class HelloMessage extends React.Component { constructor(props) { super(props); this.state = {value: 'Hello Runoob!'}; this.handleChange = this.handleChange.bind(this); } handleChange(event) { this.setState({value: '菜鸟教程'}) } render() { var value = this.state.value; return <div> <Content myDataProp = {value} updateStateProp = {this.handleChange}></Content> </div>; } } const root = ReactDOM.createRoot(document.getElementById("root")); root.render( <HelloMessage /> );
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